Herschel Status Report - April 2013
06 May 2013 10:04Report for period 6 to 29 April 2013
Herschel's long and successful lifetime of scientific observations has come to an end. On 29 April 2013 the spacecraft ran out of liquid helium coolant required to maintain the operational temperatures for the instruments' detectors. Mission operations of the Herschel space observatory continued nominally during the reporting period, with the spacecraft and subsystems all performing as expected.
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 has been steadily used by the spacecraft's cooling system to cool the payload and now, almost four years after launch, Herschel's onboard supply of the liquid helium coolant has been exhausted.
On 29 April 2013, about two hours prior to the start of the daily communication pass, the temperatures in the Herschel cryostat started rising. This fortunate timing meant that, during the daily window of just three hours in which the spacecraft is in contact with the ground segment, the rise of the temperatures as measured by several key sensors could be followed in real time. At 15:20:01 UT the temperatures from two key sensors had passed set limits and "end of helium" was officially declared, thereby establishing the end of Herschel's in-flight science mission.
Operations for all three instruments, PACS, SPIRE and HIFI, have been nominal during the reporting period. The only exception remained the one channel in the PACS photometer for which observations continued with a degraded signal to noise ratio (see previous status report).
Just as had been predicted by the cryogenic specialists, the Herschel payload operations proceeded smoothly until their final operational day and the exhaustion of the liquid helium coolant. As yet it remains to be established exactly how many of the science observations from the last operational day were executed nominally.
Ground segment operations have been nominal and 100% of the data continues to be recovered. As of 26 April 2013, the approximate completion status of the different programme parts was:
For more details of the different programme parts, see the "overview of Herschel observing" linked from the right-hand menu.
Following the end of helium, for a period extending into early June, post-helium engineering testing and wrap-up activities will next be conducted with the spacecraft and the instrument electronics.
Now that Herschel has completed its operational lifetime for scientific observations, the spacecraft will leave its current orbit around L2. 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 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.