End Of Operations For Herschel Space Telescope
April Flowers for redOrbit.com – Your Universe Online
The European Space Agency (ESA) announced on Monday, June 17, that the Herschel Space Telescope had been shut down, marking the end of operations for the hugely successful space observatory.
BBC News reports Herschel was the most powerful observatory of its kind ever put in space. In four years of operation, Herschel used its 3.5m mirror and three state-of-the-art instruments to gather pictures and other data at far-infrared wavelengths that have transformed our understanding of star formation and galaxy evolution.
The satellite exhausted its supply of liquid helium coolant that cooled the observatory´s instruments close to absolute zero on April 29, effectively putting an end to observations, but the satellite was kept operational as a technology test bed for control techniques that can´t normally be tested in flight.
“Normally, our top goal is to maximize 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 ESA´s European Space Operations Center (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.”
Requests for in-orbit validation and analysis of hardware and software, according to Schmidt, 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.”
As the last step in a complex series of flight control activities and thruster maneuvers, the final command was issued this week that will take Herschel into a safe disposal orbit around the Sun and passivate its systems.
Perhaps the most spectacular event came on May 13 and 14, when Herschel depleted most of its fuel in a record 7-hour, 45-minute thruster burn. The burn was a final move in a series of events 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. The ESOC team executed one final thruster burn on Monday to ensure that all fuel is depleted. Moving Herschel in this way ensures that it will not impede other spacecraft that want to use L2´s very stable temperature and light conditions.
As Herschel drifts, probably in a slow tumble, it will continue to charge its batteries and provide power to the onboard computer.
“In normal circumstances, there is an automatic recovery function whereby Herschel would try to switch on the transponder, but we have overridden this,” Schmidt told BBC News.
“It will never contact Earth again. We could re-command it. This mode is hardwired and we can’t overcome this. But we have no intention of doing that.”
The final command was sent via ESA´s 35 m-diameter deep-space antenna at New Norcia, Australia. The immense distance between Herschel and Earth meant it took six seconds each way for the satellite to receive the command to shut down, and for ground stations on Earth to confirm the loss of signal.
“Herschel has not only been an immensely successful scientific mission, it has also served as a valuable flight operations test platform in its final weeks of flight. This will help us increase the robustness and flexibility of future missions operations,” noted Paolo Ferri, ESA´s Head of Mission Operations.
“Europe really received excellent value from this magnificent satellite.”