ESA's ailing Herschel telescope finally retires

ESA's Herschel satellite - the largest infrared telescope ever launched - has finally been switched off, ending more than three years of pioneering observations of the cosmos.
Mission controllers sent the final command to ESA's stalwart satellite yesterday, marking the end of operations for the agency's hugely successful space observatory.
With the exhaustion of its helium coolant, Herschel's science mission ended on April 29 this year.
However, the satellite continued providing value to the end, serving as an orbiting testbed for control techniques that can't normally be tested in flight.
Herschel's science mission had already ended in April upon exhaustion of the crucial liquid helium that cooled the observatory's instruments close to absolute zero.

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 technology 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," said 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," said Schmidt.

The final command issued 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 May 13-14 , when Herschel depleted most of its fuel in a record 7-hour, 45-minute thruster burn.
Herschel has made over 35,000 scientific observations, amassing more than 25,000 hours worth of science data from about 600 observing programmes.