Two and a half years after being lofted into low-Earth polar orbit in April 2016, Microscope is preparing to bow out. Designed to test the equivalence principle to an unprecedented degree of precision using two proof masses in free fall, the CNES satellite successfully completed its science mission in February this year. “From a science standpoint, teams have until the end of next year to exploit all of the data from the satellite and publish their results,” says Pierre-Yves Guidotti. “That’s the final stage for Microscope. From a technical standpoint, we used the last six months of the mission to conduct a number of technology experiments that enabled us to better characterize our instruments and the satellite’s behaviour in orbit, which is very useful for refining the error margins on measurements. It has also laid the groundwork for possible similar missions in the future.”
“The first results from Microscope have already helped to constrain certain alternative theories aiming to unify quantum physics and general relativity,” notes Isabelle Petitbon. “Whether they confirm the equivalence principle or detect a violation of it, the final results will further advance science in this quest, but the satellite has reached the end of its life. It’s in great shape, but it has run out of cold gas for the micro-thrusters used by its drag-compensation system to keep the masses in a state of controlled free fall. It’s no longer able to acquire science data, so the time has come to passivate it.”
Unlike most spacecraft in orbit, the Microscope satellite doesn’t have chemical thrusters to deliver enough energy to de-orbit it. After the passivation sequence, which consists in making the satellite as inactive as possible so that no residual sources of chemical, pneumatic, chemical or electrical energy are remaining, Microscope will become an item of orbital debris in Earth orbit at an altitude of 710 km. Due to its small size and mass (only 330 kg), it has less surface area for the rare atmospheric particles capable of slowing its speed at this altitude, meaning that it would take 73 years before burning up in Earth’s atmosphere. Vigilance with respect to space debris is a key concern for CNES, especially since France is responsible since 2008 for applying the French Space Operations Act (FSOA), which requires de-orbiting and orbital debris risk-reduction strategies to be in place.
That’s why Microscope is carrying IDEAS (Innovative DEorbiting Aerobrake System), consisting of two 4.5-metre inflatable booms that each act like a flexible ‘wing’. “The system will increase Microscope’s surface area by 9 square metres, which will generate much more atmospheric drag to slow the satellite and modify its orbit over time. Broadly speaking, we expect it to burn up in the atmosphere after 27 years,” says Guidotti. “Passivation operations will begin on 15 October and the final sequence will be activated on the 16th. That will last about 9 hours, which will be a nail-biting time for us because once we’ve clicked and sent the command, there’s no going back. The satellite will complete its passivation on its own, step by step, and shut down all communications. It’s during this sequence that IDEAS will be deployed, so we’ve only got one chance to get it right!”
The IDEAS system being used for the first time in orbit will therefore be monitored from the ground. Using powerful ground radar, the first step will be to determine if the two booms have deployed correctly: in the stowed position, the system is only 25 cm long and its booms are folded like origami. “We don’t know if we’ll be able to see the results right away, because for the radar to see both booms the Microscope satellite has to be in a certain position, and as it will no longer be controlled at this time in the mission, there’s no way of knowing where it will be pointing. What’s more, when IDEAS deploys it’s possible that when the booms inflate they’ll cause the satellite to spin on itself,” says Petitbon. “We’ll know in the months ahead when we detect if the satellite is effectively being braked by the system.” But the teams have done their homework and IDEAS has even been tested in zero-gravity conditions (in 2007) and two patents have been developed for CNES and Airbus D&S, which built the system with Air Liquide and Erems. For now, the system is still only a technology demonstrator and other satellites with small thrusters could also be equipped with it, while several manufacturers are showing interest in the innovative inflatable concept. So between the end of its life and future scientific publications, Microscope is sure to come under close scrutiny yet.