Docking CubeSats to enable innovative missions

How to successfully dock Cubesats in space? This is the issue Camille Pirat, researcher at the Swiss Space Center, is working for several months. His PhD on this topic is supported through ESA’s Networking and Partnering Initiative. It is now highlighted by a video and an article on the Agency’s website.

CubeSats are nanosatellites of standardised dimensions based on multiple 10-cm-sided cubes, which ESA is employing for both educational and technology-demonstration purposes. In future they could also serve as the building blocks of other, larger missions by being docked together in orbit. Separate CubeSat segments could for instance be used to construct a very large telescope mirror or radio antenna for astronomy, getting around size limitations set by current rocket fairings.

“My interest in this topic came out of a previous R&D project with ESA, designing a CubeSat mission to test out active space debris removal technologies, such as those that will be needed for ESA’s proposed e.Deorbit mission, to capture and deorbit an entire large derelict satellite from orbit”, explains Camille Pirat.

“The idea would be to demonstrate the pre-capture approach and synchronisation of attitude between the chaser spacecraft and the tumbling target at the CubeSat scale, to prepare for a full-scale mission. It was that work that gave rise to this very interesting question: how can we perform rendezvous and docking between CubeSats?

High accuracy

The challenge, outlines the researcher, is that CubeSats obviously have tight mass, propellant and power constraints, needing a positioning accuracy of something like 1cm, previously achieved by ESA’s ATV supply spacecraft when docking with the International Space Station. But obviously the ATV was orders of magnitude bigger. 

“A CubeSat docking would be more like placing a needle into a 1cm-diameter hole, employing a limited number of sensors and a strictly limited amount of propellant. A high level of onboard autonomy would also be desirable”, continued the young man.

The two nanosatellites would begin by using GPS navigation for the control system to bring them into closer range, with inter-satellite links established at about 20 km from each other. Closer in, they would then rely on camera-based navigation, with LED beacons fitted to the CubeSats to help measure the relative range and attitude between chaser and target. Camille is now looking at how changes in lighting conditions – sunglare for exemple - might impact this solution.

Electric propulsion

Cold-gas thrusters are currently being baselined, although electric propulsion would offer a way of squeezing extra efficiency out of scarce onboard fuel for longer-range rendezvous operations – with knock-on effects for the size and capacity of solar arrays.

“I’m doing the work in Switzerland, but with regular visits to ESA’s ESTEC technical centre in the Netherlands,” adds Camille Pirat. “This gives me the chance to confer with veterans of ESA’s ATV spacecraft – it was such a great programme, it’s very useful to be able to learn from their experience.”