Sunday January 18th 2009

Herschel is what is termed a “consumables-limited mission”. What that means is that when the helium that cools the instruments runs out, the mission is over. In the infrared you need to be cold to observe and the further into the infrared, the colder you need to be, which is why the inside of the cryostat will be the coldest place in the Universe (that’s a wonderful phrase). How long the helium lasts though depends on a lot of factors.

Effectively, shortly before launch, the helium dewar on helium will be topped-up for the last time and the fairing closed. From then on, helium will start to boil off. Heat from outside will gradually seep into the tank, warming it slowly. In space, we have a sunshield to keep the telescope and cryostat in shadow: only a tiny amount of heat will be conducted through it. On the launch pad though, we will be a lot warmer than in space. The air conditioning inside the fairing will be run at maximum to keep it cool before launch but, in the end, the best way to conserve precious helium is to launch as fast as possible after the tank is filled and to get into the cold of space. A great deal of thought and effort has gone in to shortening as much as possible the interval between closing the fairing and thus shutting of all access to the telescope and the moment of launch; even so, it will be around two days.

Once closed and ready for launch, we have a short launch window (typically it ranges from about 40 minutes to an hour and a half for most launches from Kourou, with Herschel’s being at the shorter end of the range). If we cannot launch, we then try again at the same time next day, provided that the temperature inside the cryostat is still low enough. However, by next day we have already lost a significant amount of helium and we may lose weeks, or even a few months off the mission duration. If we are unable to launch at the second attempt, the whole rocket assembly has to be rolled-back to the assembly building, the fairing is opened, and the helium tank will be topped-up. Once done, the Ariane will then be wheeled back to the launch pad. This operation will take 10 days. As the launch window opens on April 10^th and lasts around 30 days, we effectively have two goes at launching in April before the window closes and we have to wait for a new opportunity in May.

So, what everyone wants is a successful launch as early as possible on April 16^th, avoiding going into the reserve day. Some cool weather around that time would also be a bonus.

The opiniones expressed here are the personal opinions of the author and should not be identified with those of the European Space Agency in any way, shape or form.