HSO (Herschel Space Observatory)
HSO is an infrared astronomy mission, the fourth and final “Cornerstone” mission of ESA's (European Space Agency) Horizon 2000 program. In 1997, the Scientific Program Committee of ESA confirmed the Herschel (FIRST) mission. In December 2000, the former FIRST (Far Infrared and Submillimeter Telescope) mission was renamed to “Herschel” in honor of Britain's pioneering astronomers William and his sister Caroline Herschel. In 1800, Sir William Herschel (1738-1822) demonstrated that the electromagnetic spectrum extends beyond the visible range of light that is now known as the “infrared” region. 1) 2)
The primary objectives of the mission are to:
• Study the formation of galaxies in the early universe and their subsequent evolution
• Investigate the creation of stars and their interaction with the interstellar medium
• Observe the chemical composition of the atmospheres and surfaces of comets, planets and satellites
• Examine the molecular chemistry of the universe.
With a primary mirror 3.5 m in diameter, Herschel is the largest aperture infrared telescope sent into space so far. With its ability to observe across the far infrared and submillimeter wavelengths (55 - 672 µm, entire FIR region), Herschel will furnish observation data that has previously been unobtainable. In particular, HSO will have an unprecedented view of the cold universe, bridging the gap between what can be observed from ground and earlier space missions of this kind. Infrared radiation can penetrate the gas and dust clouds that hide objects from optical telescopes, looking deep into star-forming regions, galactic centers and planetary systems. Also cooler objects, such as tiny stars and molecular clouds, and even galaxies enshrouded in dust, which are barely emitting optical light, can be visible in the infrared. 3) 4) 5) 6) 7) 8)
A lot of cutting-edge technology was introduced to make this ambitious mission possible.
Background: The main scientific goals, mission requirements, and technological needs for Herschel were discussed for the first time already back in the 1980s at a time when the United States-Dutch-British IRAS (InfraRed Astronomical Satellite, launch Jan. 25, 1983) satellite inaugurated infrared space astronomy by mapping 250,000 cosmic infrared sources and large areas of extended emission. On November 19, 1995, ESA launched its ISO (Infrared Space Observatory) which has allowed a more detailed insight and new discoveries of the infrared sky. In August 2003, NASA launched SIRTF (Space Infrared Telescope Facility - SIRTF was renamed to Spitzer Space Telescope in Dec. 2003), a spaceborne, cryogenically cooled infrared observatory. As ESA's fourth key science mission (Cornerstone), Herschel is building on the success of these earlier missions by being the first to extend the spectral coverage into the FIR (Far-Infrared) and submillimeter wavelengths (Ref. 9).
Figure 1: Artist's rendition of the Herschel spacecraft (image credit: ESA)
ESA's prime contractor for Herschel is TAS (Thales Alenia Space), Cannes, France, which led a consortium of industrial partners with EADS Astrium (Friedrichshafen, Germany) responsible for the payload module, and the TAS industry branch (Turin, Italy) responsible for the service module. EADS Astrium (Toulouse, France) provided the telescope. There is also a host of subcontractors spread throughout Europe. - The kick-off for industry was in April 2001 and the Preliminary Design Review (PDR) was closed out at the end of 2002, while the Critical Design Review (CDR) was completed by mid 2004. 9) 10) 11) 12)
The Herschel satellite is a tall cylinder, about 7.5 m high and 4.0 m wide. The spacecraft features a modular design, consisting of the EPLM (Extended Payload Module) and the SVM (Service Module).
• The EPLM consists of the PLM ‘proper’ with the superfluid helium cryostat – based on the proven successful ISO (International Space Observatory, a 60 cm infrared telescope of ESA with a launch in 1995) technology – housing the HOB (Herschel Optical Bench) with the instrument FPUs (Focal Plane Units), and supporting the telescope, the SS (Sunshield/Sunshade), and payload associated equipment.
• The SVM houses ‘warm’ payload electronics, and provides the necessary ‘infrastructure’ for the satellite such as power, attitude and orbit control, the on-board data handling and command execution, communications, and safety.
SVM (Service Module):
The SVM is formed by an octagonal box built around a conical tube (cone):
- The SVM houses the equipment of the avionics and servicing subsystems, the payload "warm" units (WU) for HIFI, PACS and SPIRE instruments. The Herschel SVM is designed to provide the required mechanical and thermal environments during launch and in-orbit phases to the various equipment and instruments installed in it.
- The SVM supports the PLM cryostat support truss on the top of the cone. It supports other PLM items as the SVM shield for radiative de-coupling between SVM and cryostat, and SS (Sunshield and Sunshade) support truss (both on the top of the cone and upper closure panels).
- The SVM ensures the mechanical link with the launcher through the interface ring, and therefore ensures the main load path during launch.
Figure 2: The SVM accommodation of warm units (TAS, ESA)
The ACMS (Attitude Control and Measurement Subsystem) uses a star tracker (STR) for attitude sensing. An absolute pointing error of < 3.7 arcsec is required. The star tracker is supplemented by the CRS (Coarse Rate Sensor), SAS (Sun Acquisition Sensor), and the AAD (Attitude Anomaly Detector). The attitude is estimated from the measurements of the star tracker and of an accurate gyroscope and is controlled by the reaction wheels of the RWS (Reaction Wheel Subsystem). Due to a typical duration of observations with an instrument of less than a few hours, momentum dumping is not expected to interfere with the scientific observations. All ACMS devices are controlled by the ACC (Attitude Control Computer) using the ACMS bus or discrete interfaces (Figure 3).
The STR device used on-board the Herschel spacecraft was built by Selex Galileo (former Galileo Avionica), and is basically a video camera with a FOV of 16.4º x 16.4º and an image processing unit that interprets star field images in order to determine spacecraft attitude information, measured with respect to the J2000 inertial reference system. Two identical units are mounted together on the base of the cryostat and are operated in cold redundancy in order to minimize any thermal distortions that may disturb the relative alignment between their bore sight and that of the telescope, which is aligned along the same spacecraft axis but looks in the diametrically opposite direction (i.e. –X for the STR and +X for the telescope). At the heart of the STR is a thermoelectrically cooled CCD (512 x 512 pixels) and an ASIC that provides all of its low-level real-time digital functions, along with some pre-processing operations. All high level functions are managed by software permanently stored in PROM and EEPROM memories and run within an ERC32 microprocessor, which outputs data at 4 Hz onto the ACC’s 1553 data bus. 13)
The gyroscope (GYR), built by Northrop Grumman, consists of four HRG (Hemispherical Resonator Gyro) units, each sensitive to rates about one axis, integrated into a single gyro package that is mounted directly onto a service module shear wall panel. The four HRGs are arranged in redundant octahedral tetrad configuration such that any three of four axes provides observability to three orthogonal axes. Each HRG has its own set of buffer electronics that transfers its signals to a signal processor via one of two redundant gyro interface electronics units (GYR-E). Each gyro rate signal is then computed by the signal processors and is output at 4 Hz as an accumulated angle and a time tag by the GYR-E, onto the ACC’s 1553 data bus. The signal processor also provides precise thermal control for each HRG, via dedicated heater control channels, while the spacecraft’s service module provides active thermal control for the GYR’s immediate environment, via thermistors located on the unit’s mounting plate.
Figure 3: Overview of the avionics subsystem (image credit: TAS)
Legend to Figure 3: CRS (Coarse Rate Sensor Assembly), SAS (Sun Acquisition Sensor), AAD (Attitude Anomaly Detector)
Herschel science observations are made via three distinct attitude modes:
- fine pointing, for staring at fixed targets
- line scans, for mapping large areas of the sky
- raster scans, for imaging large areas of the sky via a sequence of fine pointings (rarely used).
After verification of the in-orbit performance of the ACMS during the Service Module commissioning phase, which revealed and corrected for interference from the thermal control of the Gyro Unit (Scalable Inertial Reference Unit), the pointing accuracy was monitored and improved during various campaigns (Ref. 13).
In-flight experience during Herschel operations allowed optimization of the overall pointing performance of the ACMS (Attitude Control and Monitoring System) to a factor 4.5 better than the requirements. After identification and correction of interference of the external heaters of the GYR box, which indeed had caused a violation of the AME (Attitude Measurement Error), further stepwise improvement of mainly the APE (Absolute Pointing Error) was achieved through the removal of warm pixels on the CCD of the Star Tracker, through the correction of the on-board knowledge of the focal length and the CCD characteristics, and finally through a clean-up of the on-board “star catalog” (Ref. 13).
Herschel is now (2012) observing with an accuracy in the order of < 1 arcsec for the APE (Absolute Pointing Error). The present absolute pointing performance is measured as APE ~ 0.8-0.9 arcsec (1σ) since OD1011 (Operational Day 1011). 14)
RCS (Reaction Control System): The RCS on the Herschel spacecraft is responsible for providing all the necessary forces and torques needed to change the angular momentum of the spacecraft. This is used to perform all in-orbit maneuvers, including those for orbit transfer, insertion and maintenance, and for attitude control. The Herschel RCS includes two propellant storage tanks, two latch valves and a pressure transducer, and 12 thrusters (6 nominal plus 6 redundant). The fuel of choice for the thrusters is hydrazine.
The CDMU (Command and Data Management Unit) is the main OBC of the CDMS (Command and Data Management System) using the 1553 bus protocol and a dedicated packet structure for interfacing with all subsystems. The data interfaces with the various subsystems are provided by: the DPUs (Data Processing Units) for the sensor complement, the CCU (Cryostat Control Unit), the PCDU (Power Control and Data Unit), and the XPND (Transponder). The mass memory on the CDMU has a capacity of 25 GByte (EOL) and is compatible with the needs of the mission requirement calling for autonomous operations of up to 48 hours. The CDMU and the ACC house the on-board software. Both are based on an ERC-32 microprocessor connected via the 1553 B interface.
Figure 4: Photo of the CDMU (left) and the ACC (right), image credit: Saab Ericson Space, Austrian Aerospace
RF communications: The subsystem is a full X-band system in downlink and uplink (i.e. TT&C and payload data) with the ground stations, providing a high downlink rate of up to 1.5 Mbit/s at L2 to download the scientific data during the 3 hours daily telecommunication period (the average data rate of the sensor complement is 130 kbit/s). The subsystem consists of two X-band transponders supporting GMSK modulation, 2 TWTA (Travelling Wave Tubes Assemblies) with 35 W power each, the MGA (Medium Gain Antenna) for high data rate transmission, LGAs (Low Gain Antennas) to ensure omni-directional coverage (2 antennas on Herschel). The RFDN (Radio Frequency Switching Network) permits to connect nominal and redundant RF chains to the various antennas. 15)
Nominal contact periods between a ground station and the Herschel spacecraft are planned for 3 hours per day, referred to as DTCP (Daily Telecommunication Period). To ensure autonomous operations, two main mechanisms have been implemented:
- An operation scheduling mechanism in charge of the programmed operations of the spacecraft including the instruments
- A fault protection system – mainly the FDIR (Failure Detection, Identification and Recovery) system – to guarantee safe operation of the spacecraft including the instruments in case of failure.
The spacecraft’s housekeeping and scientific data are downloaded from the spacecraft and routed from the receiving station to the MOC (Mission Operations Center) in ESOC (Darmstadt, Germany), and from there to the HSC (Herschel Science Center) at ESAC (ESA/European Space Astronomy Centre) in Villafranca, Spain. Here the data are processed, archived in the Herschel Science Archive, and made available to the observers.
Relevant data are distributed to the instrument control centers, where they are used to monitor and optimize instrument performances. The Herschel instrument control centers are:
• PACS: MPE (Max Planck Institute for Extraterrestrial Physics), Garching, Germany
• SPIRE: RAL (Rutherford Appleton Laboratory, Didcot, UK
• HIFI: SRON Netherlands Institute for Space Research, Groningen, the Netherlands.
• An additional center is the NASA Herschel Science Center, located at the California Institute of Technology Infrared Processing and Analysis Center, Pasadena, California, USA.
During LEOP (Launch and Early Orbit Phase), the satellites (Herschel and Planck) are being monitored 22 hours a day using ESA’s Deep Space Stations at New Norcia, near Perth, Australia (primary ground station), Cebreros, Spain, Perth, Australia and Kourou, French Guiana. 16) 17)
Table 1: Overview of key spacecraft parameters
Figure 5: Schematic view of the Herschel spacecraft and its major components (image credit: ESA) 18)
EPLM (Extended Payload Module):
The EPLM is mounted on top of the satellite bus, the SVM (Service Module) and consists of the cryostat containing the instruments' focal plane units (FPU) and the Herschel telescope.
Herschel relies on the successful ISO cryostat technology with a cryostat lifetime requirement of 3.5 years. From L2, both the Sun and the Earth always will be in the same general direction in the sky making it possible to design the spacecraft to have a ‘warm’ and a ‘cold’ side. This fact has enabled optimization of the thermal design including equipping the CVV (Cryostat Vacuum Vessel) with radiators on the ‘cold’ side, significantly lowering the outside temperature of the cryostat. The resulting helium boil-off rate for Herschel, just over 2 mg s-1, is only approximately half that of ISO, while the fraction of the total cryogen heat load contributed by the science payload has doubled to ~20%.
Herschel’s superfluid helium tank, where the helium is kept at its boiling temperature (1.65 K or –271.5ºC). The helium cools the focal plane unit of the scientific instruments and the three thermal shields. The liquid boils and produces gas that slowly flows from the tank into pipes around the payload, cooling it to between 1.7 K (– 271.4ºC) and 4 K (– 269ºC).
Figure 6: Photo of the cryostat vacuum vessel (left) and the cryostat (right) at ESA/ESTEC (image credit: ESA)
The detector FPUs (Folcal Pane Units) of all three science instruments accommodated in the CVV, mounted on the optical bench on top of the the superfluid helium tank, and are being cooled by the cryostat. This sophisticated vacuum flask was filled with 2367 liter of superfluid helium at launch, at temperatures below – 271ºC. Further cooling – down to 0.3 K – is necessary for the SPIRE and PACS bolometric detectors (a bolometer is a device capable of detecting and measuring small amounts of thermal radiation).
The cryostat, with the largest spaceborne dewar so far, plays a key role in the Herschel mission because it determines the lifetime of the observatory. The helium evaporates at a constant rate, gradually emptying the tank. When it has all gone, the temperature of the instruments will start to rise and Herschel will no longer be able to perform observations. However, its data will keep astronomers busy for decades.
Figure 7: Top view of the Herschel focal plane, with the focal plane units (FPUs) of the three scientific instruments (image credit: ESA)
The Herschel telescope is constructed of innovative materials representing major technological achievements. The requirements called for a stiff (optical and mechanical stability) and lightweight primary mirror of 3.5 m in diameter with an areal density of < 30 kg/m2. In addition, a diffraction-limited performance was required for the shortest wavelength. The total WFE (Wave Front Error) of the telescope was set to 6 µm rms. The size of Herschel’s primary mirror meant that it could not be built in a single piece but instead had to be constructed from 12 separate petals, thus becoming the first “segmented” space mirror as well as the largest to date. 19) 20) 21)
The telescope diameter is only limited by the size of the fairing on the Ariane 5-ECA rocket. ESA opted for a monolithic telescope built in 12 large segments of a ceramic material called Silicon Carbide (SiC). This meant segments of the mirror were 'baked’ like pottery in an oven rather than traditionally cast before the segments then were brazed together – forming the largest ceramic object ever built. The resulting mirror dish has a mass of about 300 kg, 1/3 the mass of the Hubble Space Telescope's primary reflector of 2.4 m aperture and ~ 900 kg mass, despite being almost two times larger in area.
Table 2: Herschel telescope characteristics
The Herschel telescope is a classic Cassegrain design with a 3.5 m parabolic primary mirror and the secondary mirror. The primary makes a perfect image of an on axis object point at infinity and the secondary re-images this point in a perfect manner in the focal plane.
The most complex element is obviously the 3.5 m primary reflector that is composed of 12 SiC segments brazed together and coated with a thin aluminum reflective layer. The segment is open-back lightweighted with triangular cells of inner diameter ~ 120 mm. The rib heights and thickness are optimized for minimizing mass while meeting frequency requirement.
The secondary mirror (M2), with 308 mm diameter, has been manufactured in a single SiC piece. It is adjusted on the SiC barrel by tilt and focus adjustment shims. In order to avoid the Narcissus effect on the detectors, the central part of the secondary mirror is shaped in such a way that no parasitic reflected beam can enter the focal plane.
The hexapod structure (also made of SiC) supports M2 in a stable position with respect to M1. Finally, three quasi-isostatic bipods, made of titanium, support the primary mirror and interface with the cryostat. The focus is approximately 1 m below the vertex of M1, inside the cryostat.
Table 3: Herschel telescope optical parameters
The telescope is located outside the cryostat and protected by the sunshade from direct radiation from the Sun. The expected orbital temperature is below 80 K. At this temperature, even given a low emissivity, the source contribution is almost always only a small fraction of the telescope background. For comparison, the telescope background ‘flux’ is of the order of 1000 Jy (“Jansky”), while that of Uranus is ~ 250 Jy and Neptune ~ 100 Jy. Therefore, a precise characterization of its behavior is of critical importance. The telescope background depends primarily on:
- The average temperature: The telescope temperature will be in the 60-90 K range, but the actual value will only be known in space. Efforts are being made to lower the value and to narrow the range of uncertainly as much as possible. The telescope temperature depends critically on the temperatures and emissivities of the thermal interfaces, the sunshade/shield and the CVV topside.
- The effective emissivity: beyond 100 µm, it has a stronger influence on the telescope background level than the temperature. It has been observed that a 1% reduction in emissivity gives a greater improvement than a 5 K reduction in temperature.
- The straylight.
The telescope was built in the timeframe 2001-2006. The manufacture, alignment and optical characterization down to operational temperatures (~70 K) was completed in September 2006. The telescope was developed and manufactured under the prime contractorship of EADS Astrium SAS (Toulouse, France) involving a consortium of subcontractors throughout Europe.
The SiC technology itself was supplied by Boostec Industries (Tarbes, France), polishing of the primary mirror was performed by Opteon of Finland, while the secondary was polished by Zeiss (Germany). Coating of the reflectors was done using the facility available in the Calar Alto observatory (Spain). Ultimately optical testing of the assembled telescope was carried out in a custom built vacuum chamber and metrology setup at the specialist facilities available at Centre Spatiale de Liege, at the University of Liège, Belgium.
Figure 8: Petals of the Herschel telescope after sintering at Boostec (image credit: ESA)
Figure 9: Inspection of the Herschel telescope flight model at ESTEC in January 2008 with its 3.5 m diameter primary mirror (image credit: ESA)
Launch: The Herschel spacecraft was launched in tandem with the Planck spacecraft of ESA on May 14, 2009. The launch site was Kourou and launch vehicle was Ariane-5 ECA.
Orbit: The two spacecraft separated after launch and were directly injected towards the second Lagrange point of the Sun-Earth system, L2. About 60 days after launch, Herschel will be injected into a large Lissajous orbit around the L2 point at a distance of around 1.5 million km from Earth, on Earth's nightside (Figure 10).
Figure 10: Lagrangian points of the Sun-Earth system (image credit: ESA)
Figure 11: Schematic view of Herschel and Planck orbits around L2 (image credit: ESA) 22)
Lissajous orbits are the natural motion of a satellite around a collinear libration point in a two-body system and require less momentum change to be expended for station keeping than halo orbits, where the satellite follows a simple circular or elliptical path about the libration point.
The chosen orbit will take Herschel about 500 000 km above and below the plane of the ecliptic with a maximum azimuthal excursion of around 800 000 km to either side of L2. The Earth to spacecraft distance will vary from approximately 1.2 to 1.8 million km. No insertion maneuver is needed to achieve this orbit.
The L1 and L2 libration points of the Sun-Earth system offer the particular quality of providing stable thermal as well as illumination environments (solar aspect angle: 60-120º). Hence, the vicinity of L2 is considered as a very attractive region for solving astronomy problems with an unobstructed view into the universe. Spacecraft flown to either L1 or to L2 are generally considered to be in the class of “deep-space” missions.
Figure 12: Illustration of the HSO in orbit at L2 (ESA/ AOES Medialab)
• March 18. 2014: Before concluding its observations in April 2013, Herschel provided the largest survey of cosmic dust, spanning a wide range of nearby galaxies located 50–80 million light-years from Earth. The catalog contains 323 galaxies with varying star formation activity and different chemical compositions, observed by Herschel’s instruments across far-infrared and submillimeter wavelengths.
This census of dust in local galaxies has been completed using data from ESA’s HSO (Herschel Space Observatory), providing a huge legacy to the scientific community. Cosmic dust grains are a minor but fundamental ingredient in the recipe of gas and dust for creating stars and planets. But despite its importance, there is an incomplete picture of the dust properties in galaxies beyond our own Milky Way. 23)
Figure 13: Herschel completes the largest survey in the infrared and visible range of cosmic dust in the local universe [image credit: ESA/Herschel/HRS-SAG2 and HeViCS (Herschel Virgo Cluster Survey) Key Programs/Sloan Digital Sky Survey/ L. Cortese (Swinburne University)]
Legend to Figure 13: A sample collage of galaxies in the Herschel Reference Survey at infrared/submillimeter wavelengths by Herschel (left) and at visible wavelengths from the Sloan Digital Sky Survey (SDSS, right). The Herschel image is colored with blue, representing cold dust, and red representing warm dust; the SDSS image shows young stars in blue and old stars in red. Together, the observations plot young, dust-rich spiral/irregular galaxies in the top left, with giant dust-poor elliptical galaxies in the bottom right.
• March 3, 2014: The billowing clouds portrayed in Figure 14 from ESA’s Herschel observatory are part of NGC 7538, a stellar nursery for massive stars. Located around 9000 light-years away, this is one of the few regions of massive-star formation that are relatively close to us, allowing astronomers to investigate this process in great detail. 24) 25)
Star factories like NGC 7538 consist mainly of hydrogen gas, but they also contain small amounts of cosmic dust. It was through this minor – but crucial – component that Herschel could image these star-forming regions, because dust shines brightly at the far-infrared wavelengths that were probed by the observatory.
With a total mass of almost 400 000 Suns, NGC 7538 is an active factory where stars come to life – especially huge ones that are over eight times more massive than the Sun. Hundreds of seeds of future stellar generations nestle in the mixture of surrounding gas and dust scattered across the image. Once they reach a critical mass, they will ignite as stars. Thirteen of these proto-stars have masses greater than 40 Suns, and are also extremely cold, less than –250ºC.
One group of stellar seeds seem to trace a ring-like structure, visible in the left part of the image. The ring may be the edge of a bubble carved by previous stellar explosions – as stars reach the end of their lives and explode as dramatic supernovas – but astronomers are still investigating the origin of this peculiar arrangement.
Figure 14: Star factory NGC 7538 (image credit: ESA/Herschel/PACS/SPIRE)
Legend to Figure 14: The image is a composite of the wavelengths of 70 µm (blue), 160 µm (green) and 250 µm (red) and spans about 50 x 50 arc minutes. North is up and east is to the left.
• January 22, 2014: The Herschel team of researchers has discovered water vapor around Ceres in the spacecraft data, the first unambiguous detection of water vapor around an object in the asteroid belt. With a diameter of 950 km, Ceres is the largest object in the asteroid belt, which lies between the orbits of Mars and Jupiter. But unlike most asteroids, Ceres is almost spherical and belongs to the category of ‘dwarf planets’, which also includes Pluto. 26) 27) 28)
It is theorized that Ceres is layered, perhaps with a rocky core and an icy outer mantle. This is important, because the water-ice content of the asteroid belt has significant implications for our understanding of the evolution of the Solar System.
Figure 15: Schematic view of the variability of water absorption on Ceres (image credit: ESA)
Legend to Figure 15: This graph shows variability in the intensity of the water absorption signal detected at Ceres by the HIFI (Heterodyne Instrument for the Far-Infrared) instrument of HSO on March 6, 2013. The most intense readings correspond to two dark regions on the surface known as Piazzi and Region A, identified in the ground-based image of Ceres by the W. M. Keck Observatory on Mauna Kea, Hawaii. The two data points at 110º longitude were taken in a time interval of about 9 hours — equal to the Ceres rotation period — showing that variability in the water vapor production is possible even over short periods (Ref. 27).
Although Herschel was not able to make a resolved image of Ceres, the astronomers were able to derive the distribution of water sources on the surface by observing variations in the water signal during the dwarf planet’s 9-hour rotation period. Almost all of the water vapor was seen to be coming from just two spots on the surface.
The strength of the signal also varied over hours, weeks and months, because of the water vapor plumes rotating in and out of Herschel's views as the object spun on its axis. This enabled the scientists to localize the source of water to two darker spots on the surface of Ceres, previously seen by NASA's Hubble Space Telescope and ground-based telescopes. The dark spots might be more likely to outgas because dark material warms faster than light material. When NASA's Dawn spacecraft arrives at Ceres in 2015, it will be able to investigate these features.
Here is what scientists think is happening: when Ceres swings through the part of its orbit that is closer to the sun, a portion of its icy surface becomes warm enough to cause water vapor to escape in plumes at a rate of about 6 kg/s. When Ceres is in the colder part of its orbit, no water escapes.
• December 2013: Using ESA's Herschel Space Observatory, a team of astronomers has found first evidence of a noble-gas based molecule in space. A compound of argon, the molecule was detected in the gaseous filaments of the Crab Nebula, one of the most famous supernova remnants in our Galaxy. While argon is a product of supernova explosions, the formation and survival of argon-based molecules in the harsh environment of a supernova remnant is an unforeseen surprise. 29) 30)
Figure 16 shows the Crab Nebula, an iconic supernova remnant in our Galaxy, as viewed by ESA's HSO (Herschel Space Observatory). A wispy and filamentary cloud of gas and dust, the Crab Nebula is the remnant of a supernova explosion that was observed by Chinese astronomers in the year 1054.
A new study, led by Michael Barlow from UCL (University College London), UK, and based on data from ESA's Herschel Space Observatory, has found the first evidence of such a compound in space. The results are published in the journal Science. 31)
Argon hydride is produced when ions of argon (Ar+) react with hydrogen molecules (H2), but these two species are usually found in different regions of a nebula. While ions form in the most energetic regions, where radiation from a star or stellar remnant ionizes the gas, molecules take shape in the denser, colder pockets of gas that are shielded from this powerful radiation.
But soon, the study team realized that even in the Crab Nebula, there are places where the conditions are just right for a noble gas to react and combine with other elements. There, in the transition regions between ionized and molecular gas, argon hydride can form and survive.
This new picture was supported by the comparison of the Herschel data with observations of the Crab Nebula performed at other wavelengths, which revealed that the regions where they had found ArH+ also exhibit higher concentrations of both Ar+ and H2. There, argon ions can react with hydrogen molecules forming argon hydride and atomic hydrogen.
Figure 16: Herschel image of the Crab Nebula with emission lines from argon hydride in its spectrum (image credit: ESA/Herschel/PACS, SPIRE/MESS Key Program Supernova Remnant Team)
Legend to Figure 16: This image is based on data taken with the PACS instrument on board Herschel, at a wavelength of 70 µm. At these long wavelengths, astronomers can detect the glow from cosmic dust present in the nebula.
Below the image, a spectrum of the far-infrared light from the Crab Nebula is shown. The spectrum was taken with the SPIRE instrument on board Herschel at frequencies ranging from 450 GHz (corresponding to a wavelength of about 660 µm) to 1400 GHz (corresponding to a wavelength of about 200 µm).
The spectrum, which was originally taken to study the dust content of this supernova remnant, enabled the serendipitous discovery of the first noble-gas based compound found in space: argon hydride (ArH+).
On top of the emission from dust, which dominates the continuum emission seen in the spectrum, the team of astronomers found two emission lines that had never been seen before. These are the two lines seen at the left-hand and right-hand side of the spectrum.
The astronomers identified these lines as the first two rotational transitions of ArH+ at frequencies of 617.5 GHz and 1234.6 GHz, respectively. The investigation was performed with the help of two extensive databases of molecular spectra, and through comparison with another, well studied emission line that was found in the Crab Nebula's spectrum – that from the molecular ion OH+, visible near the center of the spectrum, at a frequency of 971.8 GHz.
Figure 17: Herschel and Hubble composite image of the Crab Nebula [image credit: ESA/Herschel/PACS/MESS Key Program Supernova Remnant Team; NASA, ESA and Allison Loll/Jeff Hester (Arizona State University)] 32)
Legend to Figure 17: The image combines Hubble's view of the nebula at visible wavelengths, which was obtained using three different filters sensitive to the emission from oxygen and sulphur ions and is shown here in blue, with Herschel's far-infrared image, which reveals the emission from dust in the nebula and is shown here in red. The Hubble image is based on archival data from the Wide Field and Planetary Camera 2 (WFPC2).
• June 2013: A survey from Herschel has revealed that the reservoir of molecular gas in the Milky Way is hugely underestimated - almost by one third - when it is traced with traditional methods. Monitoring the emission from ionized carbon, the new study identified molecular gas in the intermediate evolutionary stage between diffuse, atomic gas and the densest star-forming molecular clouds. The discovery not only indicates that there is more raw material for the formation of new stars in the Galaxy, but also that it extends farther than astronomers knew. 33)
In the Milky Way, as well as in other galaxies, stars are born from the collapse of the densest and coldest clumps of matter in a molecular cloud. These clouds are gigantic star-forming complexes consisting mainly of molecular hydrogen (H2), a gas that does not emit any light at the low temperatures found in molecular clouds.
Astronomers investigating the early stages of star formation are not only interested in how molecular clouds fragment to form stars, but also in the processes that take place even earlier and initially cause molecular clouds to take shape from diffuse, atomic hydrogen gas. For this purpose, astronomers study the distribution and properties of H2 across the Galaxy – but without the benefit of direct observations, they must resort to alternative methods to trace it.
The most widely used proxy to track down molecular gas in star-forming regions is carbon monoxide (CO). A mere contaminant in molecular clouds, CO radiates much more efficiently than H2 and can be detected easily. However, such indirect tracers can be biased, since there is no guarantee that all portions of a cloud containing H2 also contain CO, in which case observations of CO would miss these regions entirely.
To achieve a more complete picture of the Milky Way's molecular content, astronomers in the past decades have combined observations of CO with other tracers of H2. These include the emission from dust – another contaminant in molecular clouds – and the gamma rays that are produced when cosmic ray particles interact with atomic and molecular hydrogen in the ISM (Interstellar Medium).
Figure 18: Artist's impression of molecular gas across the Milky Way's plane (image credit: ESA - C. Carreau)
The [C II] 158 µm line is an important tool for understanding the life cycle of interstellar matter. Ionized carbon is present in a variety of phases of the ISM (Interstellar Medium), including the diffuse ionized medium, warm and cold atomic clouds, clouds in transition from atomic to molecular, and dense and warm PDRs (Photon Dominated Regions). 34)
The study is based on observations performed with the HIFI (Heterodyne Instrument for the Far-Infrared) on board ESA's Herschel Space Observatory. The observations were performed within the Herschel Open Time Key Program "State of the Diffuse ISM: Galactic Observations of the THz CII Line – GOT C+" (Principal Investigator: William D. Langer, JPL, Caltech, USA).
• On June 17, 2013 mission controllers sent the final command to the Herschel spacecraft marking the end of operations for ESA’s hugely successful space observatory. Herschel’s science mission had already ended in April upon exhaustion of the crucial liquid helium that cooled the observatory’s instruments close to absolute zero. However, the spacecraft had to be kept active for a few more weeks, during which the final maneuvers and passivation activities were to be performed. 35)
Almost immediately after helium exhaustion, engineers at ESA/ESOC in Darmstadt also seized the rare opportunity to conduct a series of technology tests on the satellite, which remained fully functional although no longer capable of scientific observation. A series of thruster burns moved the spacecraft from its orbit around the L2 point 1.5 million km from the Earth, and into a heliocentric orbit. Finally, in June, the spacecraft was switched off. Herschel’s new orbit will send it around the Sun, coming back into Earth’s neighbourhood around 13 years from now.
• April 29, 2013: ESA’s Herschel space observatory has exhausted its supply of liquid helium coolant, ending more than three years of pioneering observations of the cool Universe. The confirmation that the helium was finally exhausted came at the beginning of the spacecraft's daily communication session on April 29 with its ground station in Western Australia. A clear rise in temperatures was measured in all of Herschel's instruments.
The event was not unexpected: the mission began with over 2300 liter of liquid helium (a mass of 335 kg), which has been slowly evaporating since the final top-up the day before Herschel’s launch on 14 May 2009. The liquid helium was essential to cool the observatory’s instruments close to absolute zero, allowing Herschel to make highly sensitive observations of the cold Universe until today. 36)
• April 23, 2013: Astronomers have finally found direct proof that almost all water present in Jupiter's stratosphere was delivered by comet Shoemaker-Levy 9, which struck the planet in 1994. The result is based on new data from Herschel that revealed more water in Jupiter's southern hemisphere, where the impacts occurred, than in the north as well as probing the vertical distribution of water in the planet's stratosphere. 39) 40)
Herschel’s observations found that there was 2–3 times more water in the southern hemisphere of Jupiter than in the northern hemisphere, with most of it concentrated around the sites of the 1994 comet impact. Additionally, it is only found at high altitudes. Herschel has a unique combination of instruments that allowed the project team to perform a three-dimensional reconstruction of water in Jupiter's stratosphere: using PACS, the team could map the distribution of water across the entire disc of the planet; the vertical profile of water in the stratosphere was observed using HIFI. The combination of both data sets was crucial to linking the source of water to the famous cometary impact of almost 19 years ago.
Figure 19: Distribution of water in Jupiter's stratosphere (ESA/Herschel/T. Cavalié et al.,NASA/ESA/Reta Beebe)
Legend to Figure 19: This map shows the distribution of water in the stratosphere of Jupiter as measured with ESA's Herschel space observatory. White and cyan indicate highest concentration of water, and blue indicates lesser amounts. The map has been superimposed over an image of Jupiter taken at visible wavelengths with the NASA/ESA Hubble Space Telescope.
The distribution of water clearly shows an asymmetric distribution across the planet's disc: water is more abundant in the southern hemisphere. Based on this and other clues collected with Herschel, astronomers have established that at least 95 per cent of the water currently present in Jupiter's stratosphere has been supplied by comet Shoemaker-Levy 9, which famously impacted the planet at intermediate southern latitudes in 1994.
The map is based on spectrometric data collected with the PACS (Photodetector Array Camera and Spectrometer) and HIFI (Heterodyne Instrument for the Far Infrared) instruments on board Herschel around 66.4 µm (PACS), a wavelength that corresponds to one of water's many spectral signatures. The HIFI observations were around 179.5 µm.
• April 19, 2013: New views of the Horsehead Nebula and its turbulent environment have been unveiled by ESA’s Herschel space observatory and the NASA/ESA Hubble space telescope. The Horsehead Nebula lies in the constellation Orion, about 1300 light years away, and is a popular target for amateur and professional astronomers alike. It sits just to the south of star Alnitak, the easternmost of Orion’s famous three-star belt, and is part of the vast Orion Molecular Cloud complex. 41)
The new far-infrared Herschel view shows in spectacular detail the scene playing out around the Horsehead Nebula at the right-hand side of the image, where it seems to surf like a ‘white horse’ in the waves of turbulent star-forming clouds.
Legend of Figure 20: A stunning new view from ESA’s Herschel space observatory of the iconic Horsehead Nebula in the context of its surroundings. The image is a composite of the wavelengths of 70 µm (blue), 160 µm (green) and 250 µm (red), and covers 4.5º x 1.5º . The image is oriented with northeast towards the left of the image and southwest towards the right.
The Horsehead appears to rise above the surrounding gas and dust in the far right-hand side of this scene, and points towards the bright Flame Nebula. Intense radiation streaming away from newborn stars heats up the surrounding dust and gas, making it shine brightly to Herschel’s infrared-sensitive eyes (shown in pink and white in this image).
To the left, the panoramic view also covers two other prominent sites where massive stars are forming, NGC 2068 and NGC 2071. Extensive networks of cool gas and dust weave throughout the scene in the form of red and yellow filaments, some of which may host newly forming low-mass stars.
- April 19, 2013: Astronomers have used NASA's Hubble Space Telescope to photograph the iconic Horsehead Nebula in a new, infrared light to mark the 23rd anniversary of the famous observatory's launch aboard the space shuttle Discovery on April 24, 1990.
Hubble has been producing ground-breaking science for two decades. During that time, it has benefited from a slew of upgrades from space shuttle missions, including the 2009 addition of a new imaging workhorse, the high-resolution Wide Field Camera 3 that took the new portrait of the Horsehead. 43)
Legend to Figure 21: The Horsehead Nebula as viewed at near-infrared wavelengths [1.1 µm (blue/cyan) and 1.6 µm (red/orange)] with the Wide Field Camera 3 on the NASA/ESA Hubble Space Telescope. The image is approximately 6 arcmin across and is oriented with north to the left and east down.
This thick pillar of gas and dust is sculpted by powerful stellar winds blowing from clusters of massive stars located beyond the field of this image. The bright source at the top left edge of the nebula is a young star whose radiation is already eroding the surrounding interstellar material.
• March 27, 2013: In this new view of a vast star-forming cloud called W3 (Figure 22), ESA’s Herschel space observatory tells the story of how massive stars are born. W3 is a giant molecular cloud containing an enormous stellar nursery, some 6200 light-years away in the Perseus Arm, one of our Milky Way Galaxy’s main spiral arms. Spanning almost 200 light-years, W3 is one of the largest star-formation complexes in the outer Milky Way, hosting the formation of both low- and high-mass stars. The distinction is drawn at eight times the mass of our own Sun: above this limit, stars end their lives as supernovas.
Dense, bright blue knots of hot dust marking massive star formation dominate the upper left of the image in the two youngest regions in the scene: W3 Main and W3 (OH). Intense radiation streaming away from the stellar infants heats up the surrounding dust and gas, making it shine brightly in Herschel’s infrared-sensitive eyes. Older high-mass stars are also seen to be heating up dust in their environments, appearing as the blue regions labelled AFGL 333 in the lower left of the annotated version of the image, and the loop of KR 140, at bottom right. 44)
Figure 22: Annotated image of the W3 giant molecular cloud combining Herschel bands at 70 µm (blue), 160 µm (green) and 250 µm (red). The image spans 2 x 2 degrees. North is up and east is to the left, (image credit: ESA/PACS & SPIRE consortia)
• March 2013: ESA's Herschel space observatory is expected to exhaust its supply of liquid helium coolant in the coming weeks after spending more than three exciting years studying the cool Universe. 45)
A pioneering mission, it is the first to cover the entire wavelength range from the far-infrared to submillimeter, making it possible to study previously invisible cool regions of gas and dust in the cosmos, and providing new insights into the origin and evolution of stars and galaxies. - To make such sensitive far-infrared observations, the detectors of the three science instruments – two cameras/imaging spectrometers and a very high-resolution spectrometer – must be cooled to a frigid -271°C, close to absolute zero. They sit on top of a tank filled with superfluid liquid helium, inside a giant thermos flask known as a cryostat.
• December 2012: By combining the observing powers of ESA’s Herschel space observatory and the ground-based Keck telescopes, astronomers have characterized hundreds of previously unseen starburst galaxies, revealing extraordinary high star-formation rates across the history of the Universe. 46)
Many of the brightest, most actively star-forming galaxies in the Universe were actually undetectable by Earth-based observatories, hidden from view by thick clouds of opaque dust and gas. Thanks to ESA’s Herschel space observatory, which views the Universe in infrared, an enormous amount of these “starburst” galaxies have recently been uncovered, allowing astronomers to measure their distances with the twin telescopes of Hawaii’s W. M. Keck Observatory (on the summit of Mauna Kea) . What they found is quite surprising: at least 767 previously unknown galaxies, many of them generating new stars at incredible rates. 47)
By gathering literally hundreds of hours of spectral data (redshifts) on the galaxies with the Keck telescopes, estimates of their distances could be determined as well as their temperatures and how often new stars are born within them. - The galaxies, many of them observed as they were during the early stages of their formation, are producing new stars at a rate of 100 to 500 a year — with a mass equivalent of several thousand Suns — hence the moniker “starburst” galaxy. By comparison the Milky Way galaxy only births one or two Sun-mass stars per year. 48)
Figure 23: A representation of the distribution of nearly 300 starbursts in one 1.4º x 1.4º field of view (image credit: ESA)
Legend to Figure 23: This 3D projection of almost 300 galaxies in the census in the same part of the sky. The third dimension shows how many billions of years back in time we are seeing each galaxy, determined by observations from the Keck Observatory. At top are images from the Hubble Space Telescope of five galaxies in the census.
• November 2012: Astronomers using ESA's Herschel Space Observatory have detected massive debris discs around 61 Virginis (28 light years away) and M-type Gliese 581 (20 light years away), two nearby stars that are known to host super-Earth planets. The study also reveals that debris discs are preferentially found in planetary systems with low-mass planets rather than in those hosting high-mass planets. This suggests that debris discs may survive more easily in the absence of very massive planets, and highlights the importance of debris discs in the study of planet formation. 49) 50) 51)
Figure 24: Artist's rendition of the debris disc and planets around G-type star 61 Virginis , superimposed on Herschel PACS images at 70, 100 and 160 µm (image credit: ESA/Herschel/PACS/Mark Wyatt, University of Cambridge, UK)
Legend to Figure 24: 61 Virginis is a G-type star that is known to host at least two planets, which have masses equivalent to about five and 18 times the mass of Earth and orbit their parent star at 0.05 and 0.22 AU, respectively, – much closer than Mercury is to the Sun. The debris disc discovered with Herschel extends between 30 and 100 AU from the star – well beyond the orbits of its known planets.
A sketch of the debris disc and the orbits of the two known planets is superimposed on the image. The relative sizes of the disc and planetary orbits are not drawn to scale: the disc is about 100 times larger than the orbit of the outermost planet.
The data for this study were gathered as part of the DEBRIS (Disc Emission via a Bias-free Reconnaissance in the Infrared/Submillimeter) Open Time Key Program, a volume-limited survey to detect and characterize dusty debris discs around nearby main-sequence stars with the PACS and SPIRE instruments on board Herschel. The survey targets the nearest ~90 stars to the Sun of each of the following spectral types: A, F, G, K, M. — DEBRIS is an international collaboration of over 30 researchers from Canada, the USA, the UK., Spain, Germany, France, Switzerland, and Chile. The project is led by Brenda Matthews (Herzberg Institute of Astrophysics, National Research Council of Canada, and Department of Physics & Astronomy, University of Victoria, Victoria, BC, Canada).
• August 2012: The prediction for ‘end-of-helium’ date is regularly refined and has been updated to March 2013, providing a couple of more weeks of observing. 52)
• July 2012: Mission operations of the Herschel space observatory and of the payload continued nominally. 53)
• The Herschel spacecraft and its payload are operating nominally in early 2012. The end of the mission is expected sometime in late 2012 when the coolant (helium) will be depleted. Once boil off has occurred Herschel's instruments will no longer operate. 54)
Herschel is well into what is expected to be its last year of observing. In-flight operations will continue until all superfluid helium has been exhausted, which is predicted to occur around February 2013. All Key program observations and the vast majority of the observations selected in late 2010 in the first in-flight call for observing proposals have been performed. Current observing is dominated by programs selected in late 2011. The spacecraft and ground segment are working well. 55)
• In January 2012, a new composite image of the Eagle Nebula (Figure 26), observed by Herschel and XMM Newton, was released by ESA. — In 1995, the Hubble Space Telescope's 'Pillars of Creation' image of the Eagle Nebula (Figure 25) became one of the most iconic images of the 20th century. Now (January 2012), two of ESA's orbiting observatories have shed new light on this enigmatic star-forming region. 56)
The Eagle Nebula is 6500 light-years away from Earth in the constellation of Serpens. It contains a young hot star cluster, NGC6611, visible with modest back-garden telescopes, that is sculpting and illuminating the surrounding gas and dust, resulting in a huge hollowed-out cavity and pillars, each several light-years long. - The Hubble image hinted at new stars being born within the pillars, deeply inside small clumps known as 'evaporating gaseous globules' or EGGs. Owing to obscuring dust, Hubble's visible light picture was unable to see inside and prove that young stars were indeed forming.
Figure 26: New Herschel and XMM-Newton image observed in far infrared and X-rays of the Eagle Nebula in January 2012 (image credit: far-infrared: ESA/Herschel/PACS/SPIRE/Hill, Motte, HOBYS Key Programme Consortium; X-ray: ESA/XMM-Newton/EPIC/XMM-Newton-SOC/Boulanger, Ref. 56)
Legend to Figure 26: Combining almost opposite ends of the electromagnetic spectrum, this composite of the Herschel in far-infrared and XMM-Newton’s X-ray images shows how the hot young stars detected by the X-ray observations are sculpting and interacting with the surrounding ultra-cool gas and dust, which, at only a few degrees above absolute zero, is the critical material for star formation itself. Both wavelengths would be blocked by Earth’s atmosphere, so are critical to our understanding of the lifecycle of stars.
The ESA Herschel Space Observatory's new image shows the pillars and the wide field of gas and dust around them. Captured in far-infrared wavelengths, the image allows astronomers to see inside the pillars and structures in the region. - In parallel, a new multi-energy X-ray image from ESA's XMM-Newton telescope shows those hot young stars responsible for carving the pillars.
Figure 27: XMM-Newton: hot stars in X-rays (image credit: ESA/XMM-Newton/EPIC/XMM-Newton-SOC/Boulanger, Ref. 56)
XMM-Newton’s images of the Eagle Nebula region in X-rays, which here is color-coded to show different energy levels (red: 0.3–1 keV, green: 1–2 keV and blue: 2–8 keV) is helping astronomers to investigate a theory that the Eagle Nebula is being powered by a hidden supernova remnant. The researchers are looking for signs of very diffuse emission and how far this extends around the region. They believe that an absence of this X-ray emission beyond that found by previous orbiting space telescopes (Chandra and Spitzer) would support the supernova remnant theory. The work on this is continuing (Ref. 56).
• July 2011: New data from HSO have revealed surprisingly large amounts of cold dust in the remnant of the famous supernova SN1987A, which exploded 24 years ago (1987) in the Large Magellanic Cloud, a neighboring galaxy of the Milky Way. With this discovery, the astronomers confirm that supernovae are able to produce significant quantities of dust over very short time scales. This may help explain previous observations, by Herschel and other observatories, of abundant dust in the early Universe as seen in high-redshift galaxies. The results are published online today in Science Express. 57) 58)
• Herschel continues to work well, which was the main conclusion from the 2nd In-Orbit Performance Review carried out in ESOC on 26 May, 2011. The review was preceded by the third measurement of the remaining superfluid helium in the cryostat. The measurements to date together with modelling indicate that Herschel could continue performing observations until some time in the spring of 2013. 59)
• The Herschel spacecraft and its payload are operating nominally in 2011. Herschel is carrying out routine science observations with its three instruments (PACS, SPIRE and HIFI) and continues to generate excellent science data. Well over half of the observing time allocated to the 42 Key Programs before the launch has been used. In the first ‘in-flight’ call to the worldwide astronomical community for open time proposals, 241 proposals were given observing time on the recommendation of the Herschel Observing Time Allocation Committee in October 2010. 60) 61) 62)
• During the Christmas holidays 2010, ESA's Herschel and XMM-Newton space observatories have combined forces to show the Andromeda Galaxy in a new light. Herschel sees rings of star formation in this, the most detailed image of the Andromeda Galaxy ever taken at infrared wavelengths, and XMM-Newton shows dying stars shining X-rays into space. 63)
Figure 28: Images of the Andromeda Galaxy taken by the Herschel and XMM-Newton S/C instruments in various spectral ranges (image credit: ESA)
Legend to Figure 28: The Andromeda Galaxy is our nearest large galactic neighbor (at ~ 2.1 million light years away from the Milky Way), containing several hundred billion stars (some estimates are up to 1012 stars). Combined, these images (Figure 28) show all stages of the stellar life cycle. The infrared image from Herschel shows areas of cool dust that trace reservoirs of gas in which forming stars are embedded. The optical image shows adult stars. XMM-Newton’s X-ray image shows the violent endpoints of stellar evolution, in which individual stars explode or pairs of stars pull each other to pieces. - The images Herschel instruments were from PACS and SPIRE, while the EPIC instrument was used on XMM-Newton.
- Attitude and orbit control: Herschel is a 3-axis stabilized platform with pointing accuracy in the arcsec range: 3.7 arcsec for APE (Absolute Pointing Error), 0.3 arcsec for RPE (Relative Pointing Error). The scientific mode of observation is based on attitude determination by a Star Tracker (STR) and a high precision gyro, while control is performed by reaction wheels. The in-orbit determination of attitude control performance shows that the requirements are met.
Table 5: Herschel spacecraft pointing performances
To achieve the required scientific observations, Herschel attitude control allows performing preprogrammed pointing modes such as: a) Raster pointing: matrix of fixed pointing; b) Line-scanning: scanning of a portion of the sky by a series of lines; c) Tracking of solar system objects.
- The satellite data handling is performed in the CDMU (Command and Data Management Unit) which communicates with the instruments and with the ACMS (Attitude Control and Measurement Subsystem) via 2 separate 1553 buses. It also includes a 32 Gbit mass memory able to store 48 hours of instrument data.
Since beginning of mission, the only CDMU reconfiguration was a PM reset which occurred on Herschel: this was due to an HIFI anomaly which saturated the 1553 bus and led to PM reset and bus reconfiguration in-accordance with the onboard FDIR (Failure Detection Isolation and Recovery).
- Propulsion subsystem: The propulsion subsystem is based on a monopropellant system. The hydrazine stored in 2 bladder tanks is fed to 20 N thrusters (2 branches of 6 thrusters) and to 1N thrusters.
Figure 29: Herschel fuel consumption since launch (image credit: ESA)
- TT&C (Telemetry Tracking & Command): The subsystem is a full X-X band link with the ground stations. It allows a high telemetry data rate of 1.5 Mbit/s at L2 to download the scientific data during the 3 hours daily telecommunication period. A fixed medium gain antenna is used for high rate data transmission while low gain antennas ensure omnidirectional coverage for emergency cases. - For this high data rate, GMSK (Gaussian Minimum Shift Keying) modulation was used to limit the bandwidth occupation. This is the first application of GMSK in space and was fully successful. For the time being, 100% data return has been achieved.
- PLM (Payload Module): Figure 30 shows the in-flight cool-down of the Herschel cryostat exterior part. An anti-contamination phase of 15 days has been implemented directly after launch during which the telescope primary and secondary mirrors were maintained at 170 K. This was to prevent outgassing products of the satellite from contaminating these sensitive surfaces. A slightly higher than predicted cryostat outer vessel temperature has been found in orbit with a difference of 9 K w.r.t. prediction. This has an influence on the cryogenic lifetime: however the 3.5 years specification can still be met.
Figure 30: Herschel cryostat cool-down (exterior part), image credit: ESA
Figure 31 shows the cool-down profile for the cryostat interior: the red curve represents the temperature of the He II bath. The temperature peak corresponds to the cool-down of the cryostat in the early phase after launch when the exterior is cooling down but it still at high temperature with high energy input into the He tank. The maximum temperature of the He II is 1.97 K, remaining well below the transition temperature to superfluid Helium at 2.1 K.
Figure 31: Herschel cool-down (Helium tank temperature during the first 3 months compared with prediction), image credit: ESA
Figure 32 shows the helium temperature for the first year of mission. The temperature has remained stable. The variations are due to instrument activities including PACS and SPIRE sorption cooler recycling.
Figure 32: Helium temperature for 1 year (image credit: ESA)
- The Herschel 1st results symposium took place in ESTEC on May 4-6 2010. This corresponds to 8 months of observation, as Herschel instruments are operational since September 2009. The overall performances by far meet the astronomer expectations. In particular, the telescope size allows performing nice and sharp images in the far infrared range.
The first year in-orbit has been extremely successful with almost flawless operations of the Herschel satellite and performances as expected. This has allowed to already bring a wealth of scientific results. It proves that the complex development, mainly in the fields of cryogenics and optics at IR and submilimetric wave lengths, has been rigorously conducted (Ref. 64).
• On January 19, 2010, ESA's Herschel observatory is back to full operation following the reactivation of its HIFI instrument. 69)
Figure 33: Infrared image of the Rosette molecular cloud 5000 light years from Earth as seen by HSO (image credit: ESA/PACS & SPIRE Consortium/HOBYS Key Programme Consortia) 75)
• In mid-January 2010, the HIFI instrument has been successfully switched on, after being inactive for more than 160 days due to an unexpected anomaly in the electronic system. This achievement brings Herschel back to its full observing capacity. HIFI was built specifically to observe water in a variety of celestial objects. Over the next three years HIFI spectra will be used to probe stellar environments and to study the role of gas and dust in the formation of stars and planets and the evolution of galaxies. 76)
On 3 August 2009, early in the performance verification phase, it was discovered that HIFI was no longer operating normally. This was later traced to an unexpected voltage peak in the electronics of HIFI’s LCU (Local Oscillator Control Unit) having shut it down.
• On Sept. 28, 2009, Herschel reached another important milestone: the first set of observations corresponding to the so-called 'Science Demonstration Phase' were made available to their owners. This delivery took place several weeks ahead of the originally planned schedule and marks the start of the transition from the Performance Verification Phase to the Routine Phase of operations. 77)
• In early September 2009, during the Herschel performance verification phase, the SPIRE and PACS photometers were used in 'parallel mode' to observe a 2º x 2º field in an area near the Galactic Plane, 60º from the Galactic Center of the Milky Way, in the constellation of the Southern Cross. This region is considered to be a good test case for verification and demonstration purposes as it is typical of crowded fields where there may be many molecular clouds along the line-of-sight. Studying the earliest phases of star formation in molecular clouds is an area of scientific investigation to which Herschel is particularly well-suited, and the ability of the SPIRE and PACS instruments to map regions of massive cloud complexes provides the capability to detect stages of star formation which have not been found with previous infrared missions. 78) 79)
• HIFI anomaly: On August 3, 2009, HIFI was found with an unknown LO status '14' (Ref. 74).
Figure 34: SPIRE/PACS image of region of the Milky Way near the Galactic Plane (image credit: ESA and the SPIRE & PACS Consortia)
• On July 24, 2009, Herschel has successfully completed its in-orbit commissioning review. The spacecraft is in excellent condition, and its instruments have been fully commissioned and checked out. Herschel is now certified ready to begin the performance verification phase. This is where the scientists will "tweak" the observatory to attain optimum performance for scientific observations. 80) 81)
• On June 14, 2009, Herschel opened its 'eyes' and the PACS (Photodetector Array Camera and Spectrometer) obtained images of M51, ('the whirlpool galaxy', first observed by Charles Messier in 1773, who provided the designation Messier 51) for a first test observation. Scientists obtained images in three colors which clearly demonstrate the superiority of Herschel, the largest infrared space telescope ever flown. The image is a composite of three observations taken at 70, 100 and 160 µm with PACS on June 14 and 15, immediately after the the satellite’s cryocover was opened on June 14, 2009. The objective was to produce a very early image that gives a glimpse of things to come. 82)
Figure 35: Herschel/PACS images of M51 (“Whirlpool Galaxy”), image credit: ESA and the PACS Consortium
• A few days after the perfect injection of the Herschel spacecraft, ESA/ESOC concluded the LEOP phase coverage (releasing the additional ground stations that enabled near-continuous contact between mission controllers and Herschel). The nominally assigned stations are now New Norcia (Australia) and Cebreros (Spain). The commissioning activities of the spacecraft's service module began on May 15, 2009. All of the satellite's subsystems have shown nominal performance, and commissioning continues. 83)
• On May 16, 2009, two days after launch, the Herschel spacecraft used its mobile phone technology (the GMSK-modulated High Data Rate downlink) to radio back to Earth. Herschel was using the same technology implemented in GSM (Global System for Mobile communications) mobile phone networks to send test data to ESA’s deep space tracking station. Herschel’s 1.5 Mbit/s test transmission - roughly the same data rate provided by a home broadband Internet connection - was picked up at ESA’s ESTRACK (ESA Tracking stations network) station at New Norcia, Australia, as the satellite was travelling some 280,000 km from Earth.
This event marks the first-ever use of GMSK (Gaussian Minimum Shift Keying) modulation in a spaceborne mission. GMSK is commonly used in GSM mobile phone networks due to its very efficient use of bandwidth and power. During the mission, the GMSK-based radio links will be used by the spacecraft to transfer data gathered by the scientific instruments and on-board subsystems, providing information on flight status and overall health. 84) 85)
Sensor complement: (HIFI, PACS, SPIRE)
With the biggest mirror yet flown in space, Europe's new Herschel Space Observatory will peer through a new wavelength window at the cool and diffuse regions, such as interstellar and circumstellar dust and gas, in the Galaxy as well as in extragalactic objects. With the wide wavelength coverage of its scientific instruments and highly sensitive detectors Herschel will provide crucial data for understanding the formation of galaxies and stars and the dynamics of late stages of stellar evolution. (Ref. 9). 86) 87) 88)
The Science Team was formed in 1998, its composition is derived from ESA, the instrument consortia, and a number of Mission Scientists representing the astronomical community `at large'. In 2001 the Herschel Optical System Scientist was added. It is chaired by the Herschel Project Scientist.
The Herschel science payload comprises three instruments which have been conceived and optimized with the prime science goals in mind. The instruments offer a wide range of capabilities to cater for a wide and diverse range of observing programs. The instruments were selected from the response to an Announcement of Opportunity (AO) issued in October 1997.
The science instruments are 'nationally' funded by the ESA member states. In addition, there is international collaboration with NASA, CSA (Canadian Space Agency), and Poland.
Figure 36: Herschel observes in the spectral range of 55-670 µm, the far-infrared and submillimeter region (image credit: University of Victoria) 89)
Table 6: Overview of instrument spectral ranges
HIFI (Heterodyne Instrument for the Far-Infrared)
HIFI was designed and developed by an international consortium led by SRON. PI: Thijs de Graauw, SRON (Space Research Organization Netherlands), Groningen, The Netherlands. Co-PIs: Tom Phillips, Caltech, USA; Emmanuel Caux, CESR, France; Jürgen Stutzki, University of Cologne, Germany. Major funding of HIFI was provided by SRON (Netherlands), DLR (Germany), NASA (USA), CNES and CNRS (France). The consortium includes institutes from France, Germany, USA, Canada, Ireland, Italy, Poland, Russia, Spain, Sweden, Switzerland and Taiwan. 90) 91) 92) 93)
The main objectives of HIFI are:
- Probing the physics, kinematics, and energetics of star forming regions through their atomic and molecular cooling lines
- Surveying the molecular inventory of such diverse regions as shocked molecular clouds, PDRs, diffuse atomic clouds, hot cores, proto-planetary disks around new stars, winds of dying stars, and the toroids around AGNs (Active Galactic Nuclei)
- Measuring the out-gassing of comets and the vertical distribution in the giant planets of molecules such as H2O
- Measuring the mass loss regulating post main-sequence stellar evolution and the gas/dust ISM (Inter Stellar Medium) replenishment
- Measuring the intense galactic [CII] emission so as to probe the ionized and warm neutral components of the ISM.
The HIFI science objectives center around understanding the cyclic interrelation between the stars and the interstellar medium in galaxies. The goal is to detect many molecular rotational line transitions and fine-structure transitions of atoms, ions and isotopes as the cool ISM reprocesses essentially all central-source radiation to the FIR and sub-mm regime and gives clear indications of its composition and physical conditions.
HIFI instrument consists of five major subsystems, shown in the block diagram of Figure 38.
1) The FPU, located inside the cryostat, contains the relay optics, diplexers for LO injection, a focal-plane chopper, mixers, low-noise IF pre-amplifiers, and calibration sources. The FCU (FPU Control Unit), located at the SVM (Service Module), supplies the bias voltages for mixers and IF preamplifiers in the FPU and controls the LO diplexers, the focal plane chopper mechanism and the calibration source.
2) The local oscillator subsystem comprises the LOU (Local Oscillator Unit) mounted on the outside of the cryostat. The LOU contains 7 LOAs (Local Oscillator Assemblies), each contain two LO multiplier chains and their feeding power amplifiers/triplers. These chains are fed by a common LSU (LO Source Unit) and generate the LO signals which are coupled into the FPU via 7 windows in the cryostat wall. The LSU and a LCU (Local Oscillator Control Unit) are located SVM and contain the reference frequency source and the bias supplies and controls of the local oscillator.
3) The WBS (Wide-Band Spectrometer) consists of a pair of 4 GHz wide AOS (Acousto-Optical Spectrometers) with a frequency resolution of about 1 MHz and a bandwidth of 4 GHz for each of the two polarizations. They are located in the SVM.
4) A High-Resolution Spectrometer (HRS)  consists of a pair of auto-correlator spectrometers and will provide several combinations of bandwidth and frequency resolutions. The HRS is divided into 4 sub-bands, each of which can be placed anywhere within the full 4 GHz IF band. The HRS modules will also be located in the SVM.
5) An ICU (Instrument Control Unit) within the SVM interprets commands from the satellite telecommand system, controls the operation of the instrument, and returns science and housekeeping data to the satellite telemetry system.
The HIFI instrument combines the high spectral resolution of the radio heterodyne technique with the low noise detection. The FPU contains seven mixer units, each being equipped with two orthogonally polarized mixers. Five mixers, covering the lower frequency range from 480 GHz up to 1250 GHz, are realized in SIS (Superconductor-Insulator-Superconductor) technology, the remaining two use HEB (Hot Electron Bolometer) technology optimized to cover the range between 1.41 THz and 1.91 THz. One mixer at a time multiplies the optical signal seen through the Herschel telescope with a single frequency quasi-optical input signal, provided by the LOU.
Low noise pre-amplifiers, realized in HEMT (High Electron Mobility Transistor) technology, provide the required amplitudes for the IF (Intermediate Frequency) band, that is transmitted via coaxial cables to two pairs of spectrometers located on the service module panels: Two wide band spectrometers optimized to achieve 1 MHz resolution over 4 GHz bandwidth, and two high resolution spectrometers consisting of a pair of auto-correlator spectrometers with several combinations of resolution and bandwidth.
Figure 37: General HIFI component diagram (image credit: SRON, ESAC)
Figure 38: Block diagram of the HIFI instrument showing the various subsystems and their interconnections (image credit: SRON)
The ICU (also referred to as DPU) is the only HIFI subsystem that interfaces electrically with the spacecraft for telemetry and telecommand. It distributes electrical power to the FCU, it takes care of the command execution and synchronization, it packages the telemetry and provides the health-autonomous mode. The ICU electronics consists of a single box, positioned in the warm part of the S/C and as close as possible to the FCU, LCU, WBS-V, WBS-H, HRS-V and HRS-H subsystems.
The interface with the spacecraft handles the baseline data rate of ~100 kbit/s and is compliant with the MIL-STD-1553B bus standard. The ICU is acting as a Remote Terminal and the CDMS (Command and Data Management Subsystem) as the bus controller. The ICU design concept and hardware has a very high degree of commonality with the data processing units of PACS and SPIRE.
Figure 39: ICU general block diagram and interfaces with S/C and other HIFI subsystems (image credit: SRON)
Figure 40: Block diagram of the main internal components of ICU (image credit: SRON)
General instrument features:
• The HIFI instrument provides continuous frequency coverage over the range 480-1250 GHz (625-240 µm) in five bands with approximately equal tuning range. An additional pair of bands provide coverage of the frequency range 1410-1910 GHz (213-157 µm). The instrument operates at only one local oscillator frequency at a time.
• In all mixer bands two independent mixers receive both horizontal and vertical polarizations of the astronomical signal, although in some cases reduced bandwidth or use of a single polarization is required to stay within the data rate available to the instrument.
• The user has the choice of using only a single polarization if he/she chooses.
• The first 5 mixer bands use SIS (superconductor-insulator-superconductor) mixers; bands 6 and 7, use Hot-Electron Bolometers (HEBs).
• The instantaneous bandwidth of the instrument will be 4 GHz.
Table 7: HIFI frequency coverage and band allocation
In Table 7, the values presented are Local Oscillator frequencies. Each band is further split in two ("a" and "b") due to the use of two Local Oscillator chains for the lower and upper portions of the frequency range for each band. A further 8 GHz is available at each end of the frequency range due to the frequency placement of the upper and lower sidebands in HIFI.
The HIFI Focal Plane Subsystem consists of three hardware units: 94)
1) FPU (Focal Plane Unit) which is located on the optical bench in the Herschel cryostat
2) Up-converter and 3 dB Coupler, contained in the satellite's service module (SVM)
3) FCU (FPU Control Unit), also contained in the satellite's SVM.
Figure 41: Overview of the HIFI FPU, its components and signal chain (image credit: SRON, ESAC)
In reality, the HIFI signal chain is a virtual unit, since its elements are physically distributed throughout the FPU. The complexity of the FPU has necessitated a modular design in which the FPU is divided into six major assemblies: the Common Optics Assembly; the Diplexer (beam combiner) Assembly; the Mixer Sub-Assemblies (of which there are 14); the second-stage IF amplifier box; the Focal Plane Chopper; and the Calibration Source Assembly.
The optics assembly relay the instrument's 7 signal beams from the telescope's focal plane into a diplexer box. This is done with 6 common mirrors (the telescope relay optics (Figure 42) and 7 sets of 3 mirrors (the channel-splitting optics). Together, these optics have three primary functions:
• They produce an image of the telescope secondary on the fourth mirror in the chain after the secondary (M6), enabling the implementation of a Focal Plane Chopper.
• They produce an image of the telescope focal plane on the first mirrors in the Channel-Splitting Optics, allowing the beams to be split by seven mirrors with different orientations.
• In each channel, they create an image of the telescope secondary within the beam combiner assembly. This image has a large Gaussian beam waist, to minimize diffraction losses, and a frequency independent size, to simplify visible-light alignment.
The seven local oscillator beams from the LOU (Local Oscillator Unit) and enter the FPU through windows in the cryostat. Using 7 sets of five mirrors, the Cold Local Oscillator Optics re-image the LO beam waists at the FPU input to waists in the diplexer box that match those produced by the channel-splitting optics.
Figure 42: Schematic view of the HIFI telescope relay optics (image credit: SRON, ESAC)
Within the beam combining assembly, each of the 7 signal beams is combined with its corresponding local oscillator beam, creating two linearly polarized beams per channel (referred to as Horizontal, H, and Vertical, V, beams). Each of these 14 beams is then directed into a Mixer Sub-Assembly. At low frequencies, where significant LO powers are available, the combining is done with polarizing beamsplitters (Figure 43).
At high frequencies, where LO power is scarcer, a Martin-Puplett diplexer is used for LO injection. As in the beamsplitter channels, the first beamsplitter creates two beams containing LO and signal power in orthogonal polarizations. However, in this case, the second beamsplitter is replaced with a polarizing Michelson interferometer that rotates the LO beam polarization relative to that of the signal beam, creating a linearly polarized output. In this manner, the coupling of both the LO and signal powers to the mixers is high (95%, or better), although diplexer scanning mechanisms are needed for frequency tuning.
Figure 43: Beamsplitter and diplexer mixing with sample diplexer unit (SRON, ESAC)
HIFI mixers: The mixers at the heart of the FPU largely determine the instrument's sensitivity. For this reason, the mixer technologies used in each band have been selected to yield the best possible sensitivity. In particular, a range of Superconductor-Insulator-Superconductor (SIS) mixer technologies are being used in the lowest 5 frequency bands (covering 480-1250 GHz) while the top two bands (covering 1410-1910 GHz) incorporate Hot Electron Bolometer mixers (HEB mixers).
Each of the 14 linearly polarized outputs from the diplexer/beam combiner box enters a Mixer Sub-Assembly (MSA) that includes:
- a set of three mirrors that focus the optical beam into the mixer
- a mixer unit where the incoming signal and LO signal are combined
- a low-noise IF amplifier (plus two IF isolators that suppress reflections in the cable between the mixer and the amplifier)
- low-frequency filtering for the mixers DC bias lines
- a mechanical structure that thermally isolates the mixer unit (at 2 K) from the FPU structure (at 10 K).
Figure 44: Illustration of a MSA device (image credit: SRON, ESAC)
The FPC (Focal Plane Chopper) is the sixth mirror of the telescope relay optics. The chopper mirror is able to rotate (in one direction) around the center of its optical surface. Tilting the chopper is equivalent to tilting the telescope secondary, which moves the beam on the sky. The primary uses of the chopper are to steer the beam on the sky, and to redirect the instrument's optical beam into the on-board calibration sources.
Figure 45: Photo of the FPC device (image credit: SRON, ESAC)
CSA (Calibration Source Assembly): Mounted on the side of the Common Optics Assembly, the CSA includes two blackbody signal loads that are used to calibrate the instrument's sensitivity (the first is an absorber at the FPU temperature around 10 K, while the second is a lightweight blackbody cavity that can be heated to 100 K), plus mirrors that focus the FPU's optical beam into the loads. Temperature sensors are available to read out the actual temperature of both calibration loads. The HIFI optical beam is steered towards the calibration sources by the use of extreme positions of the Focal Plane Chopper.
Figure 46: Photo of the FPU (Focal Plane Unit) of HIFI, image credit: SRON, ESA)
Table 8: Primary instrument characteristics of HIFI
1) The time needed to observe a weak spectral line scales inversely with the square of the receiver noise temperature.
2) The bandwidth is only 2.4 GHz in bands 6 and 7 (due to a bandwidth limitation in the state-of-the-art HEB mixers that are used at these high frequencies).
Figure 47: Schematic view of the HIFI instrument measurement paths (image credit: SRON, ESA)
HIFI observing modes: For HIFI, three ATOs (Astronomical Observing Templates) are available:
• AOT I: Single Point, for observing science targets at one position on the sky
• AOT II: Mapping, for covering extended regions
• AOT III: Spectral Scanning, for surveying a single position on the sky over a continuous range of frequencies selected within the same LO band by the user.
Each AOT can be used in a variety of different modes of operation, providing the widest range of options for performing spectroscopic science observations in different astronomical that HIFI and the Observatory will allow, in terms of reference measurements and calibration. In other words, the three AOTs come with Observing Modes where the user may select from different calibration modes, choosing the mode best suited to the observing situation and science goals.
Observations created in one of the three AOTs may be performed in a number of different observing modes, which differ mainly in the selection of the reference measurements during the course of observing. All observations consist of source measurements, reference measurements and a set of calibration measurements that may be used to fully calibrate the spectra in both frequency and intensity. Observing mode design is intended to supply an optimum balance between observing efficiency and self-contained calibrations timed by instrumental performance and stability metrics. The currently designed observing modes and their relation to the AOTs is given in Figure 48.
Figure 48: Overview of available AOT observing modes (SRON)
PACS (Photodetector Array Camera and Spectrometer)
The PACS instrumentation was designed and built by a consortium of institutes and university departments from across Europe under the leadership of Albrecht Poglitsch (PI: Principal Investigator) at MPE (Max-Planck-Institute for Extraterrestrial Physics), Garching, Germany. The Co-PI is Christoffel Waelkens of KU Leuven, Belgium. The consortium members are: Austria: UVIE (University of Vienna); Belgium: IMEC, KUL (Katholieke Universiteit Leuven), CSL (Centre Spatiale de Liege); France: CEA (Commissariat a l'Energie Atomique), OAMP; Germany: MPE, MPIA (Max-Planck-Institut für Astronomie); Italy: IFSI (Istituto di Fisica dello Spazio Interplanetario), OAP/OAT, OAA/CAISMI, LENS, SISSA; Spain: IAC (Instituto de Astrofísica de Canarias). Major funding was provided from from DLR and MPG (Germany), BELSPO/PRODEX (Belgium), CNES and CEA (France), ASI (Italy), Ministerio de Ciencia y Tecnologia (Spain), and the Ministry of Science and Research (Austria).
PACS consists of a color camera and an imaging spectrometer. Within its wavelength range (55–210 µm), the PACS camera is the first instrument capable of obtaining the complete image of a target at once. The PACS spectrometer has a lower resolution than that of HIFI, but it is perfectly suited to seeing young galaxies and the gas clouds from which stars form. 95) 96) 97) 98) 99)
PACS operates either as a special camera (photometer), in two colors simultaneously, or as a spectrometer, using either its bolometer or its photoconductor array detectors.
PACS provides two basic operational modes in the wavelength band 57 - 210 µm:
• Imaging dual-band photometry (60-85 µm or 85-125 µm and 125-210 µ) over a FOV (Field of View) of 1.75 arcmin x 3.50 arcmin, with full sampling of the telescope point spread function (diffraction/wavefront error limited)
• Integral-field line spectroscopy between 55 and 210 µm with a resolution of ~ 75-300 km/s and an instantaneous coverage of ~ 1500 km/s, over a FOV of 47 arcsec x 47 arcsec (or 47” x 47'').
Both modes allow spatially chopped observations by means of an instrument-internal chopper mirror with variable throw; this chopper is also used to alternatively switch two calibration sources into the FOV.
Figure 49 shows an optical circuit block diagram of the major functional parts of PACS. At the top, the entrance and calibration optics is common to all optical paths through the instrument. On the right, the spectrometer serves both, the short-wavelength (“blue”), and long-wavelength (“red”) photoconductor arrays. A fixed dichroic beam splitter separates blue from red spectrometer light at the very end of the optical path. On the left, the bolometer fixed dichroic beam splitter comes before the blue and red imaging branches since they require different magnification. Directly in front of their baffle enclosures the blue detectors have filter wheel mechanisms which contain the bandpass filters for short wavelength photometry, and the order selection band passes for 2nd and 3rd order operation of the grating spectrometer, respectively.
Figure 49: Functional block diagram of PACS overall optics (image credit: MPE)
The focal plane sharing of the instrument channels is shown in Figure 50. The photometric bands, which can be observed simultaneously, cover the same FOV. The FOV of the spectrometer is offset from the photometer field. However, this has no effect on the observing efficiency.
The focal plane unit provides photometric and spectroscopic capabilities through five functional units :
- common input optics with the chopper, calibration sources and a focal plane splitter
- a photometer optical train with a dichroic beam splitter and separate re-imaging optics for the two short-wavelength bands (60-85 µm/ 85-130 µm) selectable via a filter wheel and the long-wavelength band (130-210 µm), respectively
- two bolometer arrays with cryogenic buffers/multiplexers and a common 0.3 K sorption cooler
- a spectrometer optical train with an image slicer unit for integral field spectroscopy, an anamorphic collimator, a movable diffraction grating in Littrow mount, anamorphic re-imaging optics, and a dichroic beam splitter for separation of diffraction orders. The blue channel contains an additional filter wheel for selecting its short or long wavelength part
- two photoconductor arrays with attached cryogenic readout electronics (CRE).
Figure 50: Optical layout of the PACS instrument (image credit: ESA, MPE)
Figure 51: Schematic of the PACS focal plane usage (image credit: MPE)
Legend: to Figure 51: Long-wavelength and short-wavelength photometry bands cover identical FOVs. The spectrometer FOV is offset in the -z direction. Chopping is done along the y axis (left-right in this view) and also allows observation of the internal calibrators on both sides of the used area in the telescope focal plane. The chopper throw for sky observations is ±1/2 the width of the photometer field such that object and reference fields can be completely separated (photometer field 1 and 2).
The photometric bands, which can be observed simultaneously, cover the same FOV, while the FOV of the spectrometer is offset from the photometer field. Since photometry and spectroscopy operation are mutually exclusive this has no effect on the observing efficiency.
Table 9: PACS photometer overall characteristics/performances
Bolometer arrays: The PACS bolometers are filled arrays of square pixels which allow instantaneous beam sampling. 4 x 2 monolithic sub-arrays of 16 x 16 pixels are tiled together to form the short-wave focal plane array (Figure 52). The subarrays are mounted on a 0.3 K carrier which is thermally isolated from the surrounding 2 K structure. The buffer/multiplexer electronics is split in two levels; a first stage is part of the indium-bump bonded back plane of the focal plane arrays, operating at 0.3 K, and a buffer stage running at 2 K. The multiplexing readout samples each pixel at a rate of 40 Hz or 20 Hz. Both array assemblies are mounted in a subunit of the FPU (Focal Plane Unit) together with the 0.3 K cooler which provides uninterrupted operation for two days.
Figure 52: The PACS photometer subunit (left) is assembled with 4 x 2 subarrays of 16 x 16 pixels each forming the focal plane FM short-wave bolometer (image credit: MPE)
Legend to Figure 52: The 0.3 K focal plane is suspended from its 2 K enclosure by Kevlar strings. Close to the right edge of the left picture, the thermal interface to the 0.3 K cooling bar is visible. The complete photometer subunit with the two bolometer assemblies (short-wave, baffle cone removed / long-wave, with baffle cone) and the 0.3 K sorption cooler is shown in the right panel.
Figure 53: Illustration of the PACS FPU (Focal Plane Unit) instrument (image credit: MPE, ESA)
The spectrometer covers the wavelength range from 55 µm to 210 µm, in two channels that operate simultaneously in the blue (55-98 µm) and red (102-210 µm) band. It provides a resolving power between 1000 and 5000 (i.e. a spectral resolution of ~75-300km/s), depending on wavelength for a fixed grating position The instantaneous coverage is ~1500km/s. It allows simultaneous imaging of a 47" x 47" FOV, resolved in 5 x 5 pixels. An image slicer employing reflective optics is used to re-arrange the 2-D FOV along a 1 x 25 pixels entrance slit for the grating.
This integral-field concept allows efficient detection of weak individual spectral lines with sufficient baseline coverage and high tolerance to pointing errors without compromising spatial resolution, as well as for spectral mapping of extended sources regardless of their intrinsic velocity structure.
Image slicer: The image slicer's main function is to transform the 5 x 5 pixel image at its focal plane into a linear 1 x 25 pixel entrance slit for the grating spectrometer. The slicer assembly consists of 3 set of mirrors:
- The slicer stack: 5 identical spherical field mirrors, individually tilted, which forms separate pupil images for each "slice" on the set of 5 capture mirrors.
- The capture mirrors re-combine the separate beams into the desired linear image on the set of 5 spherical mirrors at the exit of the slicer assembly.
- The field mirrors at the exit re-combine the pupils separated in the slicer into a common virtual pupil. The collimators of the spectrometer will later form an (anamorphic) image of this virtual pupil onto the grating. At the same time, the field mirror apertures serve as the entrance slit of the grating spectrometer.
Figure 54: Projection of focal plane onto spectrometer arrays (image credit: MPE, ESA)
Table 10: PACS grating/pixel spectral characterization
Photoconductor arrays: The 25 x 16 pixels Ge:Ga photoconductor arrays (Figure 55) are a completely modular design. 25 linear modules of 16 pixels each are stacked together to form a contiguous, 2-dimensional array. The light cones in front of the actual detector block provide for area-filling light collection in the focal plane. Each linear module of 16 detectors is read out by a cryogenic amplifier/multiplexer circuit [CRE (Cryogenic Readout Electronics)] in CMOS technology.
Figure 55: Fully assembled 25 x 16 stressed (back) and unstressed (front) Ge:Ga photoconductor arrays with integrated cryogenic readout electronics (image credit: MPE)
PACS photometer AOTs:
Three generic observing modes are offered with the AOT (Astronomical Observing Template) photometer:
• Point-source photometry: This mode is devoted to target a source which is completely isolated and point-like or smaller than one blue matrix. A typical use of this mode is for point-source photometry. It uses chopping and nodding, both with amplitude of 1 blue matrix, and dithering with a 1 pixel amplitude, keeping the source on the array at all times.
• "Small source" photometry: This mode is devoted to target sources that are smaller than the array size, yet larger than a single matrix. To be orientation independent, this means sources that fit in circle of 1.75 arcmin diameter. This mode uses also chopping and nodding, but this time the source cannot be kept on the array at all times.
• Large area or extended source mapping: This mode is necessary to map sources larger than the array size, or to cover large contiguous areas of the sky, e.g. extragalactic surveys. There are two ways to perform these kinds of observations:
- raster mapping with chopping
- scan mapping without chopping
In all photometer observing modes, dual-band imaging observations are performed, either in the blue (70 µm) and red (160 µm) bands or in the green (100 µm) and red (160 µm) bands, via the a selection button in the main AOT panel.
PACS spectrometer AOTs:
Two different observation schemes are offered with the PACS spectrometer: line and range spectroscopy.
• Line spectroscopy mode: A limited number of relatively narrow emission/absorption lines can be observed for either a single spectroscopic FOV (0.78' x 0.78') or for a larger map. Background subtraction is achieved either through standard chopping/nodding, for faint/compact sources, or through 'wavelength-switching' techniques for line measurement of the grating mechanism of bright extended sources.
• Range spectroscopy mode: This is a more flexible and extended version of the line spectroscopy mode, where a freely defined wavelength range is scanned by stepping through the relevant angles of the grating, synchronized with the chopper. Both arrays are used at a time.
The full wavelength ranges covered by the scan and the ranges covered to the highest sensitivity, i.e. the wavelength seen by all 16 spectral pixels are shown in Table 11 and compared with respective FWHM of the spectrometer at these wavelengths, for an unresolved line.
Table 11: PACS spectral coverage in line scan
PACS calibration framework:
The calibration of the PACS observing modes is addressed centrally by the Observatory. About 5-7% of the observing time will be spent on calibrating the PACS instrument. The PACS Instrument Control Centre Team in collaboration with the Herschel Calibration Scientists will plan, work out, execute and analyze dedicated observations on celestial standards, according to the In-flight Calibration Plan, in order to consistently and thoroughly characterize all instrumental effects. They will generate the calibration files needed for the Standard Product generation, as indicated in the flow diagrams in chapter 7, as well as for more sophisticated Interactive Analysis steps.
SPIRE (Spectral and Photometric Imaging Receiver)
The SPIRE project represents a large international collaboration. The University of Wales, Cardiff (School of Physics and Astronomy), is the lead institute, with Matthew Griffin as the Principal Investigator (PI). The SPIRE instrument was built at Rutherford Appleton Laboratory (RAL), UK. The RAL Space Science and Technology Division provides the overall project management and system engineering for the SPIRE hardware and ICC (Instrument Control Center) teams. The Co-PI is Laurent Vigroux of IAP (Institut D'Astrophysique de Paris), France. The Consortium includes more than 150 scientists, engineers and managers from eight countries (Canada, China, France, Italy, Spain, Sweden, UK, USA). Major funding was provided from STFC (Science and Technology Facilities Council), UK; CNES, CEA and CNRS (France), NASA (USA), and other contributions such as those from ASI (Italy), CSA (Canada), Ministerio de Educación y Ciencia (Spain), Stockholm Observatory (Sweden), and the National Astronomical Observatories (China). 100) 101) 102) 103) 104) 105) 106) 107)
SPIRE is a three-band imaging photometer and an imaging FTS (Fourier Transform Spectrometer), which provides medium spectral resolution over a broad range. Both instruments use bolometer arrays operating at 0.3 K and are coupled to the telescope by hexagonally close-packed conical feedhorns. Three bolometer arrays are used for broadband photometry (λ/Δλ ~3) in spectral bands centered on approximately 250, 350 and 500 µm. The same 4 arcmin x 8 arcmin FOV is being observed simultaneously in these three bands through the use of two fixed dichroic beamsplitters. Signal modulation can be provided either by SPIRE's two-axis BSM (Beam Steering Mirror) or by scanning the telescope across the sky. An internal thermal source is available to provide a repeatable calibration signal for the detectors (and can also be seen by the FTS detectors).
The compact optical layout with dual input and output ports follows a Mach-Zehnder configuration using two high-efficiency intensity beamsplitters. The servo-controlled scanning mirror mechanism is based on a double-pendulum design. A mechanical displacement leads to a four-fold change in the optical path difference. A thermal calibration source is placed at the entrance of the second input port of the FTS to compensate for the emission from the warm telescope.
The FTS has spatially separated input and output ports. One input port views a 2.6 arcmin diameter FOV on the sky and the other is fed by an on-board reference source. Two detector arrays at the output ports cover overlapping bands of 194-324 µm and 316-672 µm. The FTS spectral resolution is set by the total optical path difference, and can be adjusted between 0.04 and 1 cm-1 (corresponding to λ/Δλ = 1000 - 40 at 250 µm).
Figure 56: Illustration of the SPIRE FPU (image credit: SPIRE consortium)
The photometer and spectrometer both have cold pupil stops conjugate with the Herschel secondary mirror, which is the system pupil for the telescope and defines a 3.29 m diameter used portion of the primary. Feedhorns provide a roughly Gaussian illumination of the pupil, with an edge taper of around 8 dB in the case of the photometer arrays.
The cold FPU (Focal Plane Unit) of SPIRE has a size of about 700 mm x 400 mm x 400 mm; it is supported from the 10 K cryostat optical bench by thermally insulating mounts. The inside is divided into two compartments, on either side of a central panel. One side contains common input optics and the photometer, and the other side contains the FTS..
The FPU has three temperature stages: the Herschel cryostat provides temperatures of 4.5 K and 1.7 K via high thermal conductance straps to the instrument, and an internal 3He refrigerator cools all five detector arrays to approximately 0.3 K. Two sets of JFET (Junction-gate Field Effect Transistor) preamplifiers are used to read out the bolometer signals, one for the photometer and one for the spectrometer. The JFET units are attached to the 10 K optical bench next to the 4.5 K enclosure, with the JFETs heated internally to their optimum operating temperature of ~120 K.
There are three SPIRE warm electronics units: the DCU (Detector Control Unit) provides the bias and signal conditioning for the arrays and cold electronics, and demodulates and digitizes the detector signals; the FCU (FPU Control Unit) controls the 3He cooler and the two FPU mechanisms, and reads out all the FPU thermometers; and the DPU (Digital Processing Unit) runs the on-board software and interfaces with the spacecraft for commanding and telemetry. The 130 kbit/s available data rate allows all photometer or spectrometer detectors to be sampled and the data transmitted to the ground with no on-board processing.
The relative position of the SPIRE focal plane with respect to the other two instruments, HIFI and PACS, is shown in Figure 57.
Figure 57: SPIRE location on sky with respect to the other two instrument sharing the Herschel focal plane (ESA)
Legend to Figure 57: The center of the SPIRE photometer is offset by ~11 arcmin from the centre of the highly curved focal surface of the Herschel telescope, shown by the large shaded circle.
Table 12: Key parameters of the SPIRE instrument
The photometer is implemented as a three-band camera with a single FOV common to three detector arrays. The optical scheme brings the telescope focal plane first to an intermediate pupil, at which a Beam Steering Mirror is placed, and thence to an intermediate focal plane (at which the FTS field of view is picked off). This is in turn re-imaged by a one-to-one optical relay onto the detectors. The highly tilted and curved focal surface of the telescope is corrected by the optics so as to produce flat and undistorted focal planes at the arrays.
The layout of the common input optics and the photometer side of the instrument is shown in Figures 58 and 59. The optics are all-reflective except for the dichroics used to direct the three bands onto the bolometer arrays, and the filters used to define the passbands. The image is diffraction-limited over the 4 x 8 arcmin. field of view, which is offset by 11 arcmin. from the centre of the Herschel telescope's highly curved focal surface.
The input mirror M3 lies below the telescope focus, and receives the f/8.7 Herschel telescope beam and forms an image of the secondary at the flat beam steering mirror, M4. Mirror M5 converts the focal ratio to f/5 and provides an intermediate focus at M6. The three mirrors M7, M8 and M9 (inside the 1.7 K box) form an optical relay to bring the focal plane to the detector arrays. The beam is spectrally divided and directed onto the three arrays a by a combination of two fixed dichroics and flat folding mirrors (also inside the 1.7 K box). M3-M8 are at 4.5 K and all subsequent optical elements are at 1.7 K.
Figure 58: Schematic view of the SPIRE photometer layout (SPIRE consortium)
Figure 59: Optical design of the photometer (SPIRE consortium)
BSM (Beam Steering Mirror): The BSM (M4) is located in the optical path before any subdivision of the incident radiation into photometer and spectrometer optical chains, and is used both for photometer and FTS observations. For photometric observations of point sources, the BSM is used to make a small map around the nominal position to eliminate pointing or source position uncertainties. For small maps using the photometer (and also the FTS), it is used to create a fully sampled image. It can chop up to ±2 arcmin along the long axis of the 4 x 8 arcmin FOV. The nominal chop frequency for the photometer is 2 Hz. It can simultaneously chop at up to 1 Hz in the orthogonal direction by up to 30 arcsec. This two-axis motion allows "jiggling" of the pointing to create a fully sampled image of the sky.
Filters and passbands: The spectral passbands are defined by a sequence of metal mesh filters at various locations (the instrument input aperture, directly in front of the detector, and at intermediate locations in the optical train), the reflection/transmission edges of the dichroics, and the cutoff wavelengths of the feedhorn output waveguides. The three bands are centered at approximately 250, 350 and 500 µm, with λ/Δλ of 3.3, 3.4, and 2.5 respectively.
Bolometer arrays: The SPIRE detectors are spider-web bolometers using NTD (Neutron Transmutation Doped) germanium thermometers, which are coupled to the telescope by hexagonally close-packed 2Fl-diameter single-mode conical feedhorns, giving diffraction limited beams of FWHM 18, 25 and 36" for the 250, 360 and 520 µm bands respectively.
In contrast to HIFI's heterodyne technique, SPIRE will detect photons directly, by means of five arrays of bolometers. Bolometers can detect very small amounts of energy and convert them to electrical signals. They are currently the most sensitive direct detectors for radiation in the far-infrared to millimeter range. - SPIRE's bolometers are of a "spider web" design developed by JPL (James Bock). Each consists of a weblike mesh of silicon nitride, which absorbs light and conducts the energy to the tiny thermistor that sits at the center of the web. 108)
Figure 60: (a) Layout of the photometer arrays. The shaded detectors are those for which there is exact overlap on the sky for the three bands; (b) SPIRE PLW detector array module (SPIRE consortium)
Photometer calibration source: An internal thermal source is used to provide a repeatable signal for the bolometers. It is not designed to provide an absolute calibration - that will be done by observing standard astronomical sources. It radiates through a 2.8 mm hole in the center of M4, occupying an area contained within the region of the pupil obscured by the hole in the primary. The source can produce a power at the detector of 1-2% of the telescope background. The latter is typically a few pW, so the signal level is a few x 10-14 W. With a detector NEP of a few x 10-17 W Hz-1/2, this gives a large instantaneous S/N. Mirror M4 is at a pupil, so the illumination is close to uniform over the arrays.
Figure 61: View of the SPIRE photometer detector assembly (image credit: SPIRE Consortium)
The FTS uses two broadband intensity beam splitters in a Mach-Zehnder configuration. All four ports of the interferometer are independently accessible as in the Martin-Puplett (M-P) polarizing FTS. But the throughput is a factor of two higher than for the M-P as none of the incoming radiation is rejected, and there is no sensitivity to the polarization of the incident radiation. A thermal source at the second input port allows the background power from the telescope to be matched. The amplitude of the interferogram central maximum is proportional to the difference in the radiant power from the two ports, so this allows the large telescope background to be nulled, reducing the dynamic range requirements for the detector sampling. Detector arrays are placed in the two output ports, with overlapping bands of 200-325 µm and 315-670 µm. A single back-to-back scanning roof-top mirror serves both interferometer arms. It has a frictionless mechanism using double parallelogram linkage and flex pivots, and a Moiré fringe sensing system.
Figure 62: Schematic view of the FTS instrument layout (SPIRE consortium)
FTS optics: The focal plane layout of the FTS is shown in Figure 62 and the optical design is illustrated in Figure 63. The spectrometer beam enters through a hole in the panel into the FTS side of the instrument. A pupil stop is located between the pick-off mirror and the input fold mirror. The input relay mirror brings the beam to an intermediate focus just after the first beam divider, after which the beam is collimated and sent to the moving roof-top mirror. The roof-top shifts the beam and sends it towards the camera mirror, which produces an image just before the output beam divider. A pupil is located near the final fold mirror, making this a convenient location for the entrance to the 1.7 K enclosure. This pupil moves when the optical path difference changes, so it is not suitable for a limiting cold stop. Instead, the limiting aperture is located at the 4.5 K pupil between the pick-off mirror and the input fold mirror. The output relay mirror focuses the beam onto the detector arrays. Each array has a lens incorporated in its 0.3 K filter stack to correct for the non-telecentric FTS optics and provide more uniform fringe contrast and efficiency across the field.
Figure 63: Optical design of the FTS (image credit: SPIRE consortium)
FTS detector arrays: The two spectrometer arrays contain 37 hexagonally closely-packed detectors in the shortwavelength array and 19 in the long-wavelength array. The array modules are similar to those used for the photometer, with an identical interface to the 1.7 K enclosure. The detectors on the periphery are partly vignetted by the 2.6 arcmin FOV admitted by the instrument optics (shown by the large circles in Figure ). The feedhorn and detector cavity designs are carefully optimized to provide good sensitivity across the whole wavelength range of the FTS.
Figure 64: Schematic view of the FTS detector arrays (the shaded detectors are coaligned on the sky in the two bands), image credit: SPIRE consortium
3He cooler and 300 mK thermal strap system:
The same 3He cooler design is being used for both the SPIRE and PACS instruments. Gas gap heat switches control the cooler and there are no moving parts. Liquid confinement in zero g is achieved by a porous material which holds the liquid by capillary attraction. A Kevlar wire suspension supports the cooler during launch, whilst minimizing the parasitic heat load.
The cooler contains 6 STP (Standard Temperature Pressure) liter of 3He, fits in a 200 mm x 100 mm x 100 mm envelope and has a mass of about 1.7 kg. Operating from 1.7 K, it achieves a temperature of 274 mK with a 10 mW load and a 46 hr hold time, and a total time-averaged power load on the 1.7 K heat sink of ~ 3 mW. Copper straps connect the 0.3 K stage to the five detector arrays, and are held rigidly at various points by Kevlar support modules. The supports at the entries to the 1.7 K boxes are also designed to be light-tight. The cooler will be recycled during periods when the Herschel telemetry antenna is pointed towards Earth for data downlink and command uplink. In these DTCPs (Daily Telecommunications Periods) of ~ 3 hours, operation of the science instruments is not planned due to possible interference from the transponder operation and the highly restricted pointing.
The operations of the HSO (Herschel Space Observatory) are conducted in a decentralized manner. The ground segment comprises the following elements: 109)
• HSC (Herschel Science Center), provided by ESA, located at ESAC (ESA/European Space Astronomy Centre) in Villafranca near Madrid, Spain. The HSC is the prime interface between Herschel and the science community. - The HSC, supported by the NHSC (NASA Herschel Science Center) and located at IPAC (Infrared Processing and Analysis Center) in California, acts as the point of interface to the science community and the outside world in general (primarily to the US-based users). The HSC is supported by the Herschel Science Team, for the maximization of the scientific return of the mission, and by the Herschel Observing Time Allocation Committee (HOTAC) for the selection of observing proposals. 110) 111)
• Three dedicated ICCs (Instrument Control Centers), one for each instrument, provided by the respective PI institution. Each ICC is responsible for enabling the operation and for the calibration of its instrument.
• MOC (Mission Operations Center), provided by ESA, located at ESOC, Darmstadt, which is responsible for the execution of all in-orbit operations.
- Mission Control based on SCOS-2000 (Satellite Control and Operation System 2000 - the generic mission control system software of ESA) including the NCTRS (Network Controller and Telemetry Router System).
- Simulator based on SIMSAT (Simulation Infrastructure for the Modeling of SATellites)
- Flight Dynamics based on ORATOS (Orbit and Attitude Operations. System) infrastructure
- Ground Stations based on TMTCS (Telemetry and Telecommand System).
These applications require communication services within the MOC and remote locations:
- Spacecraft Integration sites (Cannes (France), Torino (Italy), ESTEC (European Space Research and Technology Centre) at Noordwijk (Netherlands), CSL (Centre Spatial de Liège), Belgium.
- ESA Ground Stations CSG (Guiana Space Centre) at Kourou (French Guyana), New Norcia and Perth (Australia), Cebreros (Spain), Maspalomas (Gran Canaria)
- ICCs (Instrument Control & Science Centers): PACS ICC, HIFI ICC, SPIRE ICC, HSC, Planck: PSO, HFI DPC, LFI DPC.
Herschel and Planck are the first ESA missions launched using novel computer solutions based on Linux operating systems and hosted in the modernized ESOC data centers.
Figure 65: Overview of the HSO ground segment (image credit: ESA)
MOC (Mission Control Center):
SCOS 2000 is the generic mission control system built by ESA following the CCSDS telemetry and telecommand packet standards and PUS (Packet Utilization Standard) and used by the agency to support its missions.
The MOC architecture follows the client/server logic, as a distributed system running across a network of workstations grouped in several independent chains (3 for each spacecraft in the case of Herschel and Planck), each chain composed by a main server and several clients attached to it and with only one chain being allowed to have the operational control at a given time. This control can be transferred from one chain to the other wither explicitly or as a result of a fail-over. 112)
Figure 66: Ground segment architecture at MOC in support of the Herschel and Planck missions (image credit: ESA)
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The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: ”Observation of the Earth and Its Environment: Survey of Missions and Sensors” (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates.