The billion-euro Herschel space telescope has been switched off. On Monday its controllers emptied the satellite’s fuel tanks and commanded the observatory to sever all communications.
The spacecraft is now in a slow drift around the Sun, around 2.14 million km from Earth.
Mission controllers sent the final command to the Herschel satellite on Monday at 12:25 GMT (14:25 CEST), marking the end of operations for ESA’s hugely successful space observatory. Herschel’s scientific mission had already ended in April after the exhaustion of the crucial liquid helium that cooled the observatory’s instruments close to absolute zero. However, the spacecraft had to be kept active for a few more weeks, during which the final manoeuvres and ‘passivation’ activities were to be performed.
Almost immediately after helium exhaustion, engineers at ESA’s European Space Operations Centre (ESOC) in Darmstadt also seized the rare opportunity to conduct a series of technological tests on the satellite, which remained fully functional although no longer capable of scientific observation.
“Normally, our top goal is to maximise scientific return, and we never do anything that might interrupt observations or put the satellite at risk,” says Micha Schmidt, Herschel’s Spacecraft Operations Manager at ESOC. “But the end of science meant we had a sophisticated spacecraft at our disposal on which we could conduct technical testing and validate techniques, software and the functionality of systems that are going to be reused on future spacecraft. This was a major bonus for us.”
Micha says that requests for in-orbit validation and analysis of hardware and software came from the mission control teams at ESOC, from the European industry teams that built the satellite and its components and from science instrument teams.
“For example, the ExoMars team asked us to do some validation using Herschel’s Visual Monitoring Camera, a similar model will fly on their mission. And the Euclid team asked us for some reaction wheel tests.”
The final command issued on Monday was the last step in a complex series of flight control activities and thruster manoeuvres designed to take Herschel into a safe disposal orbit around the Sun and passivate its systems.
The most spectacular event came on 13–14 May , when Herschel depleted most of its fuel in a record 7-hour, 45-minute thruster burn.
This was the main manoeuvre in a series that ensured the satellite was boosted away from its operational orbit around the L2 Sun–Earth Lagrange Point and into a heliocentric orbit, further out and slower than Earth’s.