DeOrbitSail Nanosatellite Mission
The DeOrbitSail project is a low cost end-to-end space mission, which will demonstrate satellite deorbiting through the deployment of an ultra light 5 m x5 m sail, which will fit in a 3U CubeSat nanosatellite platform. The DeOrbitSail is an EU-funded (European Union, FP7 project) space mission to demonstrate the ability to deorbit space junk using a low cost, ultra light solar sail as a drag sail, bringing debris closer to Earth so they can eventually burn up through friction in Earth’s atmosphere. 1) 2) 3) 4) 5) 6) 7)
Table 1: 25 Year deorbiting requirement
The Deorbitsail project is a collaboration to build a 3U CubeSat sized satellite with a deployable sail that will demonstrate rapid deorbiting. The deorbiting capability of the Deorbitsail satellite is due to solar radiation pressure and to increased aerodynamic drag from the large surface area of the deployed sail in LEO (Low Earth Orbit).
DeorbitSail is a demonstrator mission: its purpose is to raise the technology readiness level of a deployable sail system. The Deorbitsail team hopes that an increase in TRL will make this low-mass, low-energy deployable sail an attractive deorbiting subsystem for future spacecraft, and therefore mitigate the problem of space debris.
It is estimated that the Deorbitsail system will be suitable for deorbiting satellites of 20 -500 kg from orbits below 1000 km within the guideline time of 25 years.
Table 2: Overview of the collaborative team (DeOrbitSail consortium)
Objectives of the mission:
• Demonstrate deorbiting
• Demonstrate deployment of 25 m2 solar sail
• Implement a 3-axis active ADCS to align the sail to the orbit plane ⇒ minimum drag in LEO
• Measure solar force over a minimum 1 year period
• Based on COTS, 3U Cubesat
• Funded by the European Commission (2011)
• Budget of 2.8 million Euros including launch
• Framework 7 program –FP7
• Difference to CubeSAIL: Carbon Fiber booms/deployment system designed by DLR
• Fit in a 3U Cubesat nanosatellite, launch in 2014.
Research goals: DeorbitSail is not exclusively a technology demonstration mission; there are a number of academic and analytic research goals. Among these is the validation of models for deorbiting effectiveness. Analytic tools that consider the shape of the spacecraft and sail, functioning of the attitude control system, and probable interactions with the existing space debris environment have been developed at Stellenbosch University and the Surrey Space Centre.
Figure 1: Artist's rendition of the Deorbitsail with deployed drag sail (image credit: SSC)
Deorbitsail is a small satellite mission that will launch and deploy a 5 m x 5 m, four-quadrant sail to demonstrate deorbiting. The spacecraft will be based on a 3U CubeSat standard structure, with dimensions close to 10 cm x 10 cm x 34 cm. Deorbitsail will be a satellite with a mass of <6 kg, but with power, communications, attitude control and data handling components, in addition to its densely packed sail and deployment system. The sail will be able to change the satellite’s orbit through drag and solar radiation pressure.
Figure 2: Left: The satellite immediately after antenna deployment, triggered on a timer after separation from the launch vehicle. Right: The satellite after solar panel deployment. The solar panels now lie in a plane at the bottom of the image (image credit: SSC)
Legend to Figure 2: Left: At the base of the spacecraft is a small stand to separate the magnetometer from the rest of the spacecraft. Right: From bottom to top, the satellite has three sections: The electronics bus (Fig. 6), the sail storage area (with packed sails, which will be clear yellow Kapton membranes), and the boom deployer.
The bottom 1U of DeorbitSail houses the majority of the electronics necessary for functioning as a satellite. Four deployable solar panels are connected at the base of the structure; these will each support six solar cells. The panels will initially lie flat on the long sides of the satellite, as depicted in Figure 3 (left), to improve the power profile during the initial tumbling phase. The solar panels will later deploy to lie in a single plane.
Figure 3: The satellite with deployed sail (image credit: DeorbitSail consortium)
Above the electronics stack is the sail storage area, where the folded sail will be stowed. A double Z-fold will be used to pack the sail membrane into this limited space. The sail is pulled from the sail storage area by four carbon fiber booms; their deployed appearance is shown in Figure 3. Above this storage area is the boom deployer. The deployer, which contains the four booms that will deploy and support the sail, occupies approximately 1U of volume. 8)
Figure 4 shows the DeorbitSail electronics stack. The majority of the space for electronics is occupied by the ADCS, a three-axis-stabilized system designed by Stellenbosch University and customized for DeorbitSail. Using a combination of coarse sensors and cameras, this system will determine the satellite attitude and tumbling rates. Attitude is controlled with three types of actuators: a set of three magnetorquers, a single momentum wheel controlling pitch, and a translation stage. The translation stage, located between the electronics bus and the sail storage area (Figure 2 right) is an experimental technology on this mission and not strictly required to stabilize the spacecraft. By shifting the alignment of the center of mass (located in the electronics bus) relative to the center of pressure (located on the deployable sail), small adjustments to the pitch and yaw of the spacecraft can be made without a propulsion system or additional momentum wheels.
While the ADCS (Attitude Determination and Control Subsystem) has been developed as part of the DeorbitSail project, the remainder of the DeorbitSail bus electronics is composed of off-the-shelf Cubesat parts. These are summarized in Table 3.
Table 3: COTS parts on the DeorbitSail
Figure 4: The DeorbitSail electronics stack (image credit: DeorbitSail consortium)
ADCS (Attitude Determination and Control Subsystem): The ADCS processing will be carried out by a software task running on the OBC . The estimation and control loop will execute at a rate of 1 Hz. It will make use of sensor data from the following sensors: 9)
• 3-axis resistive magnetometer
• optical sun- and nadir sensors
• 3-axis MEMS gyroscope
• coarse sun sensing using 6 photodiodes.
Use of a combination of magnetic torquer rods and sail center-of-pressure to center-of-mass control for actuation. The latter makes use of a translation stage between the satellite bus and the sail attachment. Translating the sail center-of-pressure with respect to the satellite bus changes the torque generated by the sail (both aerodynamic and solar radiation pressure) and can thus be used to control the attitude of the sail.
Figure 5: CubeSense attitude determination system (image credit: University of Stellenbosch)
Figure 6: Sail deployment mechanism (image credit: DLR)
Launch: The DeorbitSail project, funded by the European Union, is scheduled for launch in 2014. 10)
Orbit: Sun-synchronous orbit, altitude of ~ 600 km.
The nanosatellite will be deployed from ISIPOD ((Innovative Systems in Space Cubesat deployer). 11)
On-orbit operations and success milestones:
Like most Cubesats, DeorbitSail does not include any substantial redundant systems. The operational lifetime is expected to be a few months. It is also true that many Cubesats carry a generally higher risk of failure at every stage than conventionally built spacecraft. It is therefore valuable to have a concept of operations that gathers data as early as possible and recognizes many intermediate stages of success.
DeorbitSail has a substantial advantage in that the most direct proof of its success will be encoded in its TLEs (Two-Line Elements), which are a type of orbital data generated and provided by NORAD. The TLEs will indicate the altitude of the orbit as it is gradually lowered. Complete mission success will be achieved when the TLEs indicate that the orbit has rapidly decayed to demise in the atmosphere.
Between launch and demise, there are many measurable milestones of partial success. Unlike the long-term altitude decay, which is confirmed by third-party observations, all these intermediate measures will require a fully functional communications system. The success milestones begin with the functioning of the ADCS.
The first phase of operations is to identify the tumbling rates of the spacecraft based on readings from the six coarse sun sensors and one magnetometer. Detumbling will begin after confirmation that the ADCS estimator is functioning as expected. DeorbitSail will then detumble to a Y-Thompson spin and ADCS readings will confirm that the system has functioned correctly for this phase, as well as provide data to validate the ADCS models.
After achieving a Y-Thompson spin, the solar panels will deploy and the fine attitude determination system will be activated. Fine attitude determination is performed with a sun camera and a horizon camera. A new mode of attitude control will despin from the Y-Thompson spin, finally producing a stably pointed spacecraft.
At this point, the ADCS system will have successfully stabilized DeorbitSail in an orientation suitable for sail deployment. Reaching this state is a significant milestone and success for the ADCS design. Depending on the launch order of certain upcoming Cubesats (e.g., SSC’s CubeSail), it is likely to be the first in-orbit demonstration of the CubeComputer with this ADCS algorithm on a Cubesat, the first demonstration of the CubeAIM system, and the second use of CubeSense after STRaND-1. 12) Elements of the ADCS hardware have flown before in various forms, including in the ADCS of South Africa’s SumbandilaSat 13), but DeorbitSail will be the first demonstration of this Cubesat-specific ADCS.
In the next phase of operations, the sail will be deployed. The horizon camera will take a black-and-white photograph of the deployed sail. As a result of the camera location, this picture will include somewhat less than half of the sail. Incomplete deployment (which may be correctable) and some forms of sail failure will be diagnosable from this image. Receipt of an image showing a completely deployed sail is the second major success milestone. This will be the measure of success for the deployment system.
With the sail deployed, the satellite will move into its extended operations phase. In this phase, there are three substantial demonstration tasks remaining:
1) the effectiveness of the COM-COG offset system in attitude control of a sail
2) the long-term operations of the sail
3) the effectiveness of a drag sail and its agreement with models in deorbiting of a satellite.
Effectiveness of the translation stage can be evaluated with the outputs of the ADCS, and as long as the camera and communications systems remain operational, more pictures can be taken of the sail. If communications are lost at any point after sail deployment, the effectiveness of the sail (and perhaps the continued functioning of the ADCS) can be evaluated by the TLEs.
The following data products are anticipated, and will be logged in this order:
• Detumbling and stabilization phase:
- Initial tumbling rates
- Progression of tumbling rates to Y-Thompson spin
- Progression from Y-Thompson spin to stable pointing.
• Sail deployment phase:
- Deployed sail image (Figure 7).
• Extended operations:
- Progression of attitude with actuation of the translation stage
- TLEs during deorbiting.
Figure 7: Horizon camera sample image quality. The diameter of the visible circle is approximately 460 pixels (image credit: Stellenbosch University)
2) Vaios Lappas, S. Pelleggrino, H. Guenat, M. Straubel, H. Steyn, V. Kostopoulos, E. Sarris, O. Takinalp, S. Wokes, A. Bonnema, “DeOrbitSail: De-Orbiting of Satellites using Solar Sails,” , 2nd International Conference on Space Technology (ICST), Athens, Greece, Sept. 15-17, 2011
3) Vaios Lappas, “Gossamer Systems for Satellite Deorbiting: The CubeSail and DeorbitSail Space Missions,” AIAA SDM (Structural Dynamics and Materials) conference, Honolulu, HI, USA, April 23-26, 2012, URL: http://www.deorbitsail.com/files/GSF%20Keynote%2027.4.12%20Final%202%20no%20clips.pdf
4) Vaios J. Lappas, Olive R. Stohlman, Lourens Visagie, Theodoros Theodorou, Juan M. Fernandez, George Prassinos, “Deorbitsail: flight-testing a deorbiting system,” Proceedings of the 64th International Astronautical Congress (IAC 2013), Beijing, China, Sept. 23-27, 2013, paper: IAC-13-A63.4.6
5) W. H. Steyn, “FP7: Deorbit Sail Project,” April 18, 2011, URL: http://www.esastap.org.za/download/space01_2011_pres01.pdf
7) Olive R. Stohlman, Vaios Lappas, “ Deorbitsail: a deployable sail for de-orbiting ,” 54th AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics, and Materials Conference, Boston, MA, USA, April 8-11, 2013
8) Olive R. Stohlman, Juan M. Fernandez, Vaios J. Lappas, Martin Hillebrandt, Christian Hühne, Marco Straubel, “Testing of the Deorbitsail drag sail subsystem,” 54th AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics, and Materials Conference, Boston, MA, USA, April 8-11, 2013
10) Tereza Pultarova, “University of Surrey’s sail space debris remover to launch in 2014,” University of Surrey, July 2, 2013, URL: http://eandt.theiet.org/news/2013/jul/surrey-debris-sail.cfm
12) C. P. Bridges, S. Kenyon, C. I. Underwood, M. N. Sweeting, “STRaND: Surrey Training Research and Nanosatellite Demonstrator,” 1st IAA Conference on University Satellite Mission and CubeSat Workshop, Rome, Italy, January 24-29, 2011
13) Sias Mostert, Herman Steyn, Hendrik Burger, Helena Bosman, “Sumbandilasat—An operational technology demonstrator,” Acta Astronautica, 2008, Vol. 63, pp. 1273-1282, URL: http://staff.ee.sun.ac.za/whsteyn/Papers2/ActaAstro_3141.pdf
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.