Herschel Status Report - June 2013
This is the final entry in the series of Herschel status reports, which covered the activities of the mission during in-flight operations. The archive of all Herschel status reports, running from October 2009 to June 2013, is available here.
The series of post-helium technology tests, which began on the spacecraft and instruments soon after the exhaustion of the cooling system's liquid helium coolant on 29 April, were completed on 13 June. Herschel was no longer capable of performing science observations, but the otherwise fully functional in-orbit spacecraft provided a unique opportunity to test specific spacecraft and subsystem operations, before Herschel would be finally switched off.
During the post-helium technology tests the performance of various sub-systems was explored over a parameter space, or in modalities, not routinely available during normal science operations. For example the star-trackers (one prime and one redundant), the reaction wheels, the Visual Monitoring Camera (VMC), and some elements of the redundant chains of the science instruments, were all exercised. (See also "Herschel ends operations as orbiting testbed").
Two of the Herschel Space Observatory's three instruments, PACS and HIFI, were already switched off in the previous reporting period. The third instrument, SPIRE, was switched off on 11 June, but it was later reactivated purely to generate enough heat to stop the hydrazine propellant freezing.
Ground segment operations have been nominal during the reporting period.
Mission operations were conducted with the support of ESA's New Norcia ground station. Up to the end of Herschel operations on 17 June 2013, data stored on-board the spacecraft was received on ground during regular communication passes, each lasting approximately three hours. In the last days, some of these daily passes were omitted as they were not needed.
On 17 June the mission's final delta-v manoeuvre, the "Draining Manoeuvre", was performed. The manoeuvre entailed the last thruster burn to deplete the little remaining fuel as part of the spacecraft passivation. It was the last in a series of final departure manoeuvres, performed in May and June, that were designed to boost Herschel from its operational orbit around L2 of the Sun-Earth system into its final no-return heliocentric orbit. In this context "no-return" means not returning to the potential well of the Earth-Moon system for at least 300 years. The main manoeuvre in this series, the "Disposal Manoeuvre", was successfully executed already on 13-14 May (see the previous Herschel status report, for May 2013).
The final Draining Manoeuvre was started as planned in the morning of 17 June 2013. For the manoeuvre operations the Malargüe station was used in addition to the New Norcia station. To allow a stable telemetry link during the manoeuvre the spacecraft's low-gain antenna was used. The manoeuvre's progress was monitored and when the low fuel level caused first instabilities in the spacecraft's attitude the thruster burn was manually terminated. The spacecraft was stabilised again using the reaction wheels.
Shortly after, at around 12:25 UT, Herschel mission operations were concluded by manually sending the last telecommand to the spacecraft. This final command triggered a previously loaded sequence on-board that disabled the spacecraft transponders (for downlink) and effectively switched off the spacecraft.
Herschel is now trailing the Earth in a wider orbit around the Sun.
The Herschel Science Ground Segment, comprised of the Herschel Science Centre (HSC) at ESAC, the three Instrument Control Centres (ICCs) and the NASA Herschel Science Center (NHSC), is now effectively in the Post-operations Phase.
The sixth bulk reprocessing of the data in the Herschel Science Archive (HSA) began in late June, upgrading the data products using the latest reduction algorithms and calibration. This is the first reprocessing to run over the data of the entire mission. Several more bulk reprocessing cycles of the archive data are foreseen in the coming years, as the Herschel data processing software will continue to be updated.
The Herschel Science Centre, based at ESAC, Madrid, is steadily assimilating refined programme-specific data products from the Herschel Key Programmes, into the HSA.
Over 2585 scientists are now registered as HSA users, about 25 new registrations occurring per month on average, but 110 in the past month. This large number of new registrants is attributed to a recent data processing training workshop and to publicity surrounding the end of the operational mission.