Artists impression of CryoSat-2 in space. Image courtesy of ESA – P. Carril.
Successfully launched satellites are expected to operate for a certain period. This ends when the satellite can no longer able to perform its intended function, cannot maintain its operational altitude, or the fuel level is reaching the minimal point for a successful deorbit and disposal/return manoeuvres. Fuel is the most common reason for reaching this end-of-life point.
When launching satellites, the expected end-of-life is estimated–although this is always very conservative. There are many examples of satellites exceeding their expected life by years or, even, decades. However, operators always want to maximum the time their satellite is in operational orbit, which is understandable given the amount of money it costs to design, build and launch.
Last week the European Space Agency (ESA) faced this issue with CryoSat-2, which was launched on the 8th April 2010 and, originally, had an expected mission life of approximately four to five years. It is still operational today, having been in orbit for over thirteen years. However, its future was threatened because of an issue with its fuel usage.
The operational team at ESA’s European Space Operations Centre (ESOC) in Darmstadt, Germany, noticed in 2016 that the satellite was using up its nitrogen fuel faster than it expected. Working with Airbus, who built the satellite, they identified a leak in one of its smaller attitude thrusters and estimated it would reach the end of its life in 2025.
Following a launch failure for its predecessor Cryosat, Cryosat-2 has proved itself incredibly valuable in measuring polar ice – both the vast polar ice sheets and that floating on the sea in polar oceans. Its Synthetic Aperture Radar Interferometric Radar Altimeter (SIRAL) has provided datasets for:
- first complete assessment of Arctic sea-ice thickness
- measurements of the ice sheets covering Antarctica and Greenland
- showing how 200,000 mountain glaciers have succumbed to climate change
- monitoring the thickness of ice on lakes in winter
- producing bathymetric charts
Wanting to extend the mission as long as possible, on the 21st November, ESA swapped to its back-up fuel propulsion system. Back-ups are common throughout a satellite’s systems, as it is not possible to pop up and repair a faulty system. This switch was not without risk. The back-up system had never been used and, if it did not operate, there was a potential for the mission to come to an immediate end.
The team made the switch as the satellite was over Norway/Sweden and, despite a nervous twenty-five minutes as it circled the Earth, it reappeared fully operational. Over the next 24 hours, various procedures and tests were performed, but the new system successfully passed them. CryoSat-2 is now operating on its back-up propulsion system, which has the potential to extend the life of the satellite by between 5 and 10 years, although it will be several weeks before the team can be confident that there are no more leaks and everything is operating normally.
The successor of CryoSat-2 is likely to be the Copernicus Polar Ice and Snow Topography Altimeter (CRISTAL) mission, but it is not due for launch for a few years. It is hoped that this action will enable CryoSat-2 to remain operational until that time, and will ensure a continuous dataset of ice measurements is maintained, which will be invaluable for climate scientists.
