Wednesday March 18^th 2009. Launch - ?? days.

Today we have had a couple of pieces of news that make it obvious that the prospective date that we had been given for the launch – a date not agreed by Arianeespace and thus totally unofficial – has become as firm as it can be without an official announcement. We know that Mission Control has calculated an orbit for Herschel and Planck based on a supposed launch date and time and is about to distribute them and that the instruments will be requested to make their plans for testing based on this orbit file. The orbit file, which specifies Herschel’s trajectory based on a launch at a given date and time, is a key piece of information because the exact position of the spacecraft on a particular date affects what objects can be seen in the sky: for safety reasons we can only see two areas of the sky from 60-120 degrees from the Sun on either side of it around the ecliptic and the entire polar region of the sky.

Planets and asteroids, which we use to calibrate the instruments though, keep to the ecliptic and we have only the restricted band of sky around the ecliptic amounting to one third of its circumference, available any particular day. So, on any particular day we can only see certain objects. This band of visibility moves by about a degree per day as the Earth moves around its orbit and, as the spacecraft moves in its orbit we see nearby objects such as asteroids with a constantly moving perspective, even when they are in the area of sky accessible to us so that an object that is visible today may not be visible tomorrow. We also see these planets and asteroids changing position from minute to minute and hour to hour as they move, so where we would have to point the spacecraft to observe the asteroid Vesta right now, is not the same place that we would have to move in one hour’s time; this in turn affects how far we have to shift the telescope to get to our next target afterwards and, in turn, how long it takes to make that move, which affects how much time we need to observe it… and so on!

Given that changing the orbit file and getting everyone to use a new orbit implies a fairly major job of re-planning, I would assume that we are gearing up seriously. However, after launch we will still have to fine-tune the orbit with the exact launch date and time.

Then again, the exact launch time has an even more profound effect. We fill the on-board helium tank and seal it inside the fairing about 50 hours before launch. From then on, warmth from outside is slowly penetrating into the satellite, warming the tank and the instruments – the limit is -263ºC… if we reach that temperature, we cannot launch and we would prefer not to go anywhere near that high because the higher the temperature goes, the more precious helium we boil off, shortening out mission.

Once in space we do two carefully controlled manoeuvres. First we warm the telescope itself, allowing it to reach a high enough temperature that any water that has frozen on it will evaporate (this will be about -150ºC, so it is not exactly hot! However, it is sufficient in the vacuum of space), before letting it cool down in the shadow of the sunshade to around -200ºC. At the same time we are giving the instruments inside the cryostat time to cool back down to a stable temperature. However, depending on whether we launch right at the start of the launch window on the first day, or on the reserve day, it might take the instruments an extra 10 days to become cool enough to use. So, the whole issue of what day we launch and even the time that we launch within our launch window has profound effects further down the line.