Herschel’s orbit around L2 was much larger than that of the Moon around Earth: the average distance of Herschel from L2 was about
Because of this large orbit, Herschel’s distance to Earth varied between 1.2 and 1.8 million km. In addition, orbits around L2 are slightly unstable and subtle disturbances cause the satellite to drift such that Herschel had to perform small orbit correction manoeuvres every month.
Herschel operations were arranged in 24-hour cycles. The spacecraft communicated with the ground station for three hours every day. This short period dedicated to data download and command upload left ample time for the observatory to conduct scientific observations undisturbed.
The remaining 21 hours were dedicated to science observations. The data collected during observations were stored on board. In fact, it was also possible for Herschel to use the three-hour communication period to continue some scientific observations.
Herschel was operated by the Mission Control Team at the Mission Operations Centre (MOC) located at ESA’s European Space Operations Centre (ESOC) in Darmstadt, Germany, which was responsible for the health and safety of the satellite. The team also maintained all necessary contact with the spacecraft via ESA’s New Norcia (Australia) and Cebreros (Spain) deep space stations. From its orbit around L2, it took about 10 seconds for Herschel to communicate with Earth (two-way).
During the three-hour communication slot, the Mission Control Team topped up the mission operations timeline for the next but one day. In this way, 48 hours of the timeline was stored on board at the end of each communication slot; and a minimum of 24 hours at any given time.
Herschel executed this mission operations timeline autonomously. In parallel, all scientific and housekeeping data, i.e. telemetry related to the service module, stored on board, were downloaded during each communication slot.
The mission operations timeline was based on a schedule of observations produced by the Herschel science operations team in the Herschel Science Centre (HSC) located at ESA’s European Space Astronomy Centre (ESAC) in Villanueva de la Cañada, Spain. The HSC is the science office responsible for Herschel and is the centre for all interaction with the worldwide astronomical community that uses the observatory.
The spacecraft’s housekeeping and scientific data were downloaded from the spacecraft and routed from the receiving station to the MOC in ESOC, and from there to the HSC at ESAC. There the data were processed, archived in the Herschel Science Archive, and made available to observers. Relevant data were distributed to the instrument control centres, where they were used to monitor and optimise instrument performance.
The Herschel instrument control centres were, for PACS: the Max Planck Institute for Extraterrestrial Physics, Garching, Germany; for SPIRE: the Rutherford Appleton Laboratory, UK; for HIFI: SRON Netherlands Institute for Space Research, the Netherlands. An additional centre is the NASA Herschel Science Center located at the California Institute of Technology Infrared Processing and Analysis Center, Pasadena, California, USA.