Herschel mission timeline
Commissioning: Days 1 – 60 (mid-May – mid-July 2009)
The first two months of Herschel’s life in space are being used to check out thoroughly all aspects of the spacecraft and the instruments to make sure everything has survived the launch and works properly in space. For most of this time Herschel will be moving steadily away from the Earth, on the way to its final orbit about the L2 point.
A key moment during Commissioning will be the opening of the cryostat lid. The instruments are contained in vacuum inside the cryostat, and for testing on the ground, the vacuum was maintained by an airtight lid on the top. Once in space, where there is a vacuum, the lid can be removed so that the light collected by the telescope can get in to the instruments. Removing the lid is a critical operation – if it is not removed successfully, Herschel will not be able to make any observations because the instruments will not be able to see the sky. This will be done around Day 35 (June 17). The reason it was not done straight after launch is that it takes a few weeks for the spacecraft to “out-gas” traces of water in its various materials (inevitable in anything manufactured on Earth). The lid is kept closed while this happens to make sure that none of the evaporating water can get into the cryostat and condense as ice inside the instruments.
The image on the left shows Herschel set-up for testing on the ground before launch. The lid is on, keeping the instruments under vacuum. The instruments can’t actually see the telescope – this will happen for the first time in space when the cover is removed. The image on the right shows Herschel in space after the lid is removed. The light collected by the telescope can now get to the instruments.
This movie shows a test of the lid release mechanism. Once released, it springs back and after a few oscillations it settles in a position that does not block the light collected by the telescope from entering the cryostat and being detected by the instruments. This is scheduled to happen about a month after launch. [Credit: European Space Agency]
Performance verification: Days 60 – 150 (mid-July to mid-October 2009)
After everything has been checked out, the three Herschel instruments will be put through their paces by making every possible kind of observation and setting up each observing mode to give the best results. The data processing software will also be tested and improved if necessary to make sure that the highest quality scientific results can be produced. All that will take another three months to complete. Then Herschel will be fully ready for science observations.
Science demonstration: Days 150 – 190 (mid-October – end November 2009)
To show the world what Herschel can do, and to generate some early scientific results, the next six weeks (Days 150-190) will be used to make scientific observations covering the whole range of Herschel’s capabilities – a little bit of everything that Herschel can do covering the solar system out to the most distant reaches of the Universe. The results will be analysed quickly and presented at a European Space Agency workshop in December.
Herschel will then settle down to at least three years of operation. Roughly half of that time has already been allocated to “Key Projects” – big observational programmes designed for in-depth investigation of some of the main questions that Herschel has been designed to answer. The second half of the mission will be available for smaller programmes and for following up some of the work done during the first part.
Extended lifetime?When the helium in Herschel’s cryostat has all evaporated, the instruments will warm up and will no longer be able to operate – the mission will then be at an end. The minimum lifetime was designed to be 3.5 years, but the Herschel cryostat has been very well designed, and everything went very will with the launch, so it is possible that it might last even longer – an extra six months or a year would be very welcome to the queue of astronomers who want to use it.