China’s demonstration of its capability to destroy an aging satellite in 2007, and the collision between the American satellite Iridium and the Russian Cosmos in 2009 brought a new emphasis on the orbital debris problem. Although most of the work had been concentrated on avoidance prediction and debris monitoring, all major space agencies are now claiming the need for active removal of debris [R1, R2]. In 2011, about 14’000 debris above 10 cm were catalogued (see Figure 1). About 2’000 of these are remains of launch vehicles and 10’000 originate from non-operational satellites.
In addition, over the years, many studies have been performed to actively remove debris from LEO and GEO, and there is a vast pool of information on this topic. The debris of interest in LEO are of two types: spent rocket upper stages and dead satellites. The masses of the earlier go from 1.4 to 8.9 tons with dimensions from 2 to 4 meters in diameter and from 6 to 12 meters in length. The satellites masses are in the same range but their dimensions and shapes can change a lot from one object to another. According to the Inter-Agency Space Debris Coordination Committee (IADC), the debris to remove in priority are the ones with the highest masses and greatest probability of collision [R3]. These high-priority objects can be found in well-defined altitude and inclination regions. The most numerous family is the one composed by SL-8 rocket bodies (more than 150 individuals) but the heaviest objects are for instance the SL-16 rocket bodies and Envisat.
In Europe, several missions have made demonstrations of rendezvous technologies, which can be reused for Active Debris Removal (ADR). The ESA Automated Transfer vehicle (ATV) performs cooperative rendezvous with International Space Station (ISS). GRACE (Gravity Recovery and Climate Experiment, NASA JPL), launched in 2002, are twin satellites in formation flying. TerraSAR-X (DLR and EADS Astrium, launched in 2007) was joined by TanDEM-X in 2010. Both satellites perform formation flying at distances as short as 250 m. Prisma (SSC) has demonstrated inspection and rendezvous with a cooperative object. DLR’ DEOS should demonstrate the robotic arm for servicing missions. Proba-3 (ESA, to be launched in 2015-2016) will consist in two satellites of 350 and 200 kg that can autonomously perform formation flying to distance from 250 down to 25 m. All these missions are stepping stones toward ADR. However, key elements remain to be validated for rendezvous with a non-cooperative object. These elements will require further technology development and demonstration, especially to minimize mission risks.
Although the “greatest-threat” debris are large rockets bodies (R/B) and large satellites, some of the technologies needed for ADR can be scaled down and demonstrated with a nano/micro-satellite (20-100 kg). This observation has been one of the founding rationale for the CleanSpace One project.
Figure 1: Increase in the number of space debris since 1957 (Credit: NASA, J.C. Liou, 2011 [R3]).
The project’s objectives
The motivation behind the CleanSpace One (CSO) project is to advance TRL levels and start mitigating the impact on the space environment by acting responsibly and removing our “debris” from orbit. As a non-commercial entity, EPFL holds a unique position ensuring that ecological goals and disruptive research are not affected by the need to generate profit.
The objectives of the CleanSpace One (CSO) project are to:
- Increase awareness, responsibility in regard to orbital debris and educate aerospace students;
- Demonstrate technologies related to Active Debris Removal;
- De-orbit SwissCube.
The project was officially initiated in 2012 while under the responsibility of the Swiss Space Center and was transferred to the EPFL Space Engineering Center (eSpace) at the end of 2014.
SwissCube status and rendezvous/capture challenges
SwissCube is the first Swiss-owned and student-designed satellite. It was launched on September 23, 2009, and has been operational since. The operations have been transferred in 2012 to Swiss Radio Amateurs (HB9MFL), and all subsystems of the satellite are still performing nominally.
SwissCube is a 1-Unit CubeSat (100 x 100 x 113.5 mm3), and it weighs 820 g. It features two deployed antennas (180 and 610 mm long) for downlink and uplink of data (see Figure 2). It is on a Sun synchronous orbit at about 720 km altitude and 98.4o inclination.
Figure 2:SwissCube picture before launch and during Thermal Vacuum testing. Note the two antennas sticking out of the satellite, perpendicular to each other.
The project typically receives 5-6 collision warnings per year. That is currently its highest probability of failure.
Challenges to the capture of SwissCube
While the rendezvous (proximity operations) and capture with a large debris is a great challenge (the debris is not cooperative, so no position information, radio link or visual markers are available to the chaser). Furthermore the capture of a large tumbling object requires technologies that have not yet been demonstrated in flight. In addition to these challenges, CSO will face two specificities: detection and high rate tumbling capture.
Because of its very small size, especially compared to typical large debris (several to tens of meters long), SwissCube will be a challenge to detect. We estimate the error on the knowledge of its orbital position to be on the order of 5 km. Thus, a specific detection sequence on CleanSpace One will have to be performed to find SwissCube in space. Special detection instruments will be carried on-board and will be turned on at about 8 km away from the estimated position of SwissCube. In addition, tracking from the ground will help reduce the position uncertainties. Once the detection has taken place, it will be tracked and further manoeuvres will be performed to get closer.
Furthermore, over the last years of flight, several sudden and sharp increases in the rotation rates of SwissCube have been observed. Figure 3 shows these rotation rates. They have been measured up to 50 deg/s. SwissCube has two antennas, one of them is 61 cm long, and final capture (grasping) could occur at conditions where Swisscube is rotating at these rates. Thus the capture system will have to operate reliably under severe tumbling conditions that are for the most part unknown before launch.
Figure 3:SwissCube’s angular rotations as a function of time. This graph shows the unpredictable and sudden increase in rotation speeds that the satellite takes. Currently, these rotations are actively controlled and reduced to a few deg/s. (Credit: S. Rossi, EPFL)
CleanSpace One status
The project is currently in a funding consolidation phase. Nevertheless, technology developments are on-going and have been focused on the critical technologies: the detection strategy, the rendezvous sensors and on-board signal processing (to bring smarts and autonomy to the satellite), and the capture system.
During the last quarters, to design the most efficient capture system, the engineers benefited from the collaboration of microengineering students from the University of Applied Science in Geneva (HES-SO HEPIA). The students came up with various solutions, from articulated arms with claws, to throw-nets, to a system of tentacles. They finally opted for the so-called “PacMan” solution. The prototype resembles a net in the form of a cone that unfolds and then closes back down once it has captured the small satellite. “This system is more reliable and offers a larger margin for maneuvering than a claw or an articulated hand,” says Michel Lauria, professor of industrial technology at HEPIA. Similarly, students and professors of the University of Applied Science Valais (HES-Valais) and HE-ARC lent their efforts to the signal processing (vision) of the project by ensuring that the high-performance processors are prepared for operation in space.