Old satellites and debris pose a danger to space travel. An international research team with participation from Würzburg is working on a satellite that cleans up space - inspired by the animal kingdom.
In 1957, the former Soviet Union sent the first satellite into space. Since then, Sputnik 1 has been followed by around 20,000 others. While the pioneer burned up when it re-entered the Earth's atmosphere just a few months later, many of its successors remain in Earth's orbit as space debris long after the end of their service life.
It was not until the mid-1990s that awareness of the problem arose, and initial tests to clean up the debris followed in the 2000s. These did not yet lead to practical mission successes in orbit, though. An international research consortium now wants to advance the clean-up of space through innovative technologies. Mohamed Khalil Ben-Larbi, Professor for Space Informatics and Satellite Systems at Julius-Maximilian-Universität Würzburg (JMU), and his team are also involved.
The European Union is funding the project "gEICko: GEcko based Innovative Capture Kit for uncooperative and unprepared Orbital assets" as part of EIC Pathfinder with a total of four million euros. Of this, just under 700,000 euros will go to Würzburg.
gEICKo is completed by the Technical University of Berlin, the University of Padua (Italy), the Tecnico in Lisbon (Portugal), the Fraunhofer Institute for High-SpeedDynamics (EMI), and the Spanish solar company DHV Technology.
Space debris as a danger to space travel
There are currently over 50,000 pieces of debris measuring at least ten centimetres in space. These are mainly disused satellites and rocket upper stages and fragments from explosions and collisions. In addition to intact satellites, these objects also endanger space projects. The International Space Station (ISS) has had to perform several evasive manoeuvres to avoid collisions.
"The commercialization of space travel means that more and more satellites are entering orbit. Explosions and collisions would also generate an increasing number of debris, even if we were to stop space travel completely tomorrow," says Khalil Ben-Larbi.
However, cleaning up the space debris is a highly complex and expensive undertaking. To date, it has not been possible to perform a so-called docking with an uncooperative object - such as a disused and no longer controllable satellite - in orbit. Initial tests show, however, that the technology is within reach.
Potential targets for a clean-up operation are prioritized according to two main criteria: mass and orbit. Even in space, there are main traffic arteries where the risk of collision is correspondingly high. If a collision occurs there, the consequences are particularly serious, as the resulting debris is highly likely to endanger other operational satellites.
Complicated rendezvous
In space travel, rendezvous and docking refer to the process of two objects approaching each other in space and ultimately establishing a physical connection. Such manoeuvres are complex even when both parties cooperate and communicate with each other, such as when docking space capsules to the ISS. This becomes much more difficult when the target object is drifting and tumbling in space and was never designed for such a manoeuvre in the first place.
"The task is to successfully dock with an object whose state, position, and orientation we can only determine using our own instruments, which may be tumbling uncontrollably and has no components specifically designed for docking," explains Ben-Larbi.
So how can docking be achieved? With nets or harpoons, there would only be a limited number of attempts. A gripper arm would first have to find a suitable spot; it would also be complex and expensive.
The solution: gecko materials.
Inspiration from the animal kingdom
The team wants to develop a docking system that can later be used in a satellite and equip it with a special contact surface. The contact surface is coated with synthetic gecko materials.
These are silicones with specially structured surfaces. If they hit a smooth counterpart, such as the solar panels of a discarded satellite, at the right speed and angle, they stick. This is based on what are known as van der Waals forces. Geckos also use these molecular forces of attraction to stick to walls or ceilings.
The chaser, i.e. the cleanup satellite, docks onto the target object via this contact surface and then changes its orbit so that, over time, it either re-enters the Earth's atmosphere and burns up or ends up in a graveyard orbit where it no longer poses a threat.
The brain of the satellite is being developed in Würzburg
The Würzburg researchers will work with the gecko materials and adapt them to the special requirements of space. They will test surface properties and different arrangements and shapes of microstructures within the materials to achieve a stable balance between adhesive force and mechanical resilience - even on dusty or aged target surfaces.
In addition, Khalil Ben-Larbi's team is working on the guidance, navigation, and control of the satellite - its brain, so to speak.
"First, we must locate the target object, then approach it, and finally dock with it. The final approach is particularly complex. The angle and speed must be precisely coordinated for a planned and controlled 'collision'; after all, we don't want to create any more debris," says Ben-Larbi.
If direct docking proves impossible, the satellite has another ace up its sleeve: gecko materials are also to be attached to a line. Like a chameleon's tongue, this could be thrown at the target project to establish the connection.
Gecko satellite saves costs
A major advantage of gecko satellites is their comparatively low cost. The technology could be used on small satellites, which saves money in both development and rocket launch.
"The key to the low cost lies in the simple design and low complexity. This means fewer components are needed. This not only reduces development costs, but also the overall mass of the system," explains Ben-Larbi.
It will be several years before the Geckos clean up space. At the end of the three-year funding period, a functional prototype of the docking mechanism should be ready. Further funding could follow. "I am optimistic that the Gecko satellites with this mechanism will be ready for use in about ten years," says Khalil Ben-Larbi.
