Space is getting crowded. Old satellites, failed spacecraft and debris are crowding orbit. This space junk means there is little space for new satellites and that the risk of collision in space is high.
The space debris environment, especially in low earth orbit (LEO), is an increasing risk for all spaceflight missions and ESA is already pioneering an eco-friendly approach to space activities. On the ground, that means adopting greener materials, processes and technologies. But in space it means preserving Earth’s orbital environment as a safe zone, free of debris.
One of the ways the space industry aims to solve the debris problem is through de-orbiting – pushing this junk out of orbit and into the Earth’s atmosphere where it can burn up. The most common approach, is to opt for a controlled re-entry. This solution is quite heavy and expensive, as it requires additional fuel. Other spacecraft use de-orbit subsystems. Currently most of these are clunky, weigh too much and need their own guidance and navigation system to make sure the space craft leaves orbit safely.
But an ongoing activity with ESA’s General Support Technology Programme (GSTP) and HPS, Germany, intends to change that. The activity is developing a subsystem that will recognise as soon as the satellite has come to the end of its mission or has failed, for example if it has had no signal for a month. It will then slowly unfurl a large aluminium-coated polyamide membrane, attached to four carbon-fibre reinforced booms. This membrane acts as a sail, to create a drag effect causing the spacecraft to decrease its orbit much faster, catching at the atmosphere to slow the worn-out spacecraft enough that it will burn up entirely. A process that can take quite long, depending on the spacecraft – up to 25 years.
Originally proposed in 2012, based on previous developments by DLR, the German space agency, the activity planned to build a passive de-orbit subsystem that held to just a few strict limitations. The system had to be extremely light, it had to be scalable so that it could be used on a variety of different spacecraft and it needed to be entirely passive – meaning no control system was needed.
In 2017, the first iteration of this subsystem, ADEO, was completed and survived extensive environmental tests. Since 2018, a consortium of twelve partners (including many small and medium-sized enterprises) have been working on ADEO-2.
At just 3.5 per cent of the spacecraft’s entire mass, ADEO-2 can be scaled up or down to meet requirements but will always be very light and cost-effective compared with the fuel needed for a controlled deorbit manoeuvre. Although at the end of the mission, or after a failure, the spacecraft can be tumbling, several aerothermodynamic calculations have shown that it is safe to deploy the sail and that the satellite will slowly self stabilise, without any need for an expensive guidance, navigation and control system added. Another advantage is that the entire subsystem, especially the sail membrane, burns up very easily on re-entry into the atmosphere.
The sail can work even if the satellite is completely unresponsive. If the satellite has functional problems then this subsystem can detect it and deploy the sail autonomously, starting the process of deorbiting.
Right now, the activity has started to build a prototype flight model sail of 25m2, which would be suitable for a spacecraft of around 700kg.
So far, a small version of ADEO (named ADEO-N) has already flown on the upper stage of the New Zealand launcher, Electron, and the sail was successfully deployed and deorbit achieved.The activity plans to be tested on ground by the beginning of 2021 and has already been committed to an in orbit verification flight on an EU mission next year.
Eventually, the activity plans to be used across thousands of spacecraft, from small cubesats to entire constellations, without the capacity for an active or controlled re-entry system, to help ensure that orbit can be a safe and useable place for many generations to come.
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