Herschel Status Report - March 2013
The spacecraft continues to be in good health and is operating nominally.
At launch, in 2009, Herschel's cryostat was filled with over 2300 litres of superfluid liquid helium, weighing about 335 kg. The helium is steadily used by the spacecraft's cooling system to cool the payload, so the amount of remaining coolant directly determines Herschel's lifetime for scientific observations.
Best estimates for the date of the exhaustion of the liquid helium (LHe) fell in the second-half of March this year. However, at the end of the period covered in this report, on 5 April, Herschel continues to operate with the LHe lifetime moving well into the upper range of the estimates. The in-flight science operations are expected to finish any day now.
A period of about six weeks following the eventual LHe exhaustion will be assigned for post-helium engineering testing and wrap-up activities with the spacecraft and the instrument electronics. The spacecraft will then leave its operational orbit around L2 and will be injected into a no-return heliocentric orbit.
Operations for all three instruments, PACS, SPIRE and HIFI, have been largely nominal during the reporting period, with the exception of the PACS photometer.
For one of the PACS photometer's two red arrays the bias voltage can no longer be controlled. This channel had experienced an anomaly earlier in February (see previous status report) and could not be fully restored. Observations in this channel continue, but with a degraded signal to noise ratio. The other PACS photometer channels, as well as the PACS spectrometer, are unaffected and continue to operate nominally.
One single event upset for PACS resulted in about one day of science observation time lost for the instrument.
Ground segment operations have been nominal and 100% of the data continues to be recovered. As of 5 April 2013, the approximate completion status of the different programme parts was:
|KPGT||Key Programme Guaranteed Time||:||100%|
|KPOT||Key Programme Open Time||:||100%|
|GT1||First in-flight Guaranteed Time||:||100%|
|OT1p1||First in-flight Open Time, priority 1||:||99.4%|
|GT2||Second in-flight Guaranteed Time||:||100%|
|OT2p1||Second in-flight Open Time, priority 1||:||100%|
|OT1p2 + OT2p2||First and Second in-flight Open Time, priority 2, top||:||98%|
|First and Second in-flight Open Time, priority 2, middle||:||20%|
|First and Second in-flight Open Time, priority 2, bottom||:||1.7%|
For more details of the different programme parts, see the "overview of Herschel observing" linked from the right-hand menu.
Mission operations were conducted with the support of ESA's New Norcia ground station. Throughout the reporting period, observational data stored on-board Herschel was received on ground during daily communication passes, each lasting approximately three hours.
In preparation already for the end of scientific operations after LHe exhaustion, an orbit control manoeuvre was successfully performed on 15 March 2013. This "departure manoeuvre" (with a delta-V of 10.51 m/s in the "escape direction") ensured that, in the event that the spacecraft might be disabled during the post LHe exhaustion technology tests, an escape away from the Earth-Moon system would be assured; i.e. the spacecraft would still leave its operational orbit around L2.
The manoeuvre, however, was not the mission's last planned manoeuvre. A series of final departure manoeuvres will take place in May and June, which will inject Herschel into its selected final no-return heliocentric orbit. In this context "no-return" means the spacecraft not returning to the potential well of the Earth-Moon system for at least 300 years. The window for the manoeuvre required to put the spacecraft in its final heliocentric orbit opens on 5 May 2013.
The ground segment is operating nominally. Data products are generated routinely and ingested into the Herschel Science Archive (HSA).
- April 2013: Expected end of operational lifetime for scientific observations (exhaustion of the liquid helium for the spacecraft's active cooling system)
- Post-helium engineering testing and wrap-up activities with the spacecraft and the instrument electronics
- Injection into a no-return heliocentric orbit
- 1 July 2013: Start of Post-Operations Phase