By Jonathan Amos

Science reporter, BBC News

PLANCK SPACE TELESCOPE Planck will need three correction manoeuvres in total Planck will survey the famous Cosmic Microwave Background This ancient light's origins date to 380,000 years after the Big Bang It informs scientists about the age, shape and evolution of the cosmos Planck's measurements will be finer than any previous satellite

Planck prepares to go super-cold It must feel a little like the attendant outside a hotel who is given the keys to a supercar and is asked to go and park it. The excitement is almost overwhelming but so too is the fear of scratching the gleaming mega-motor. The mission controllers who will oversee the delivery of Europe's Herschel and Planck telescopes to their science observation positions in space are very conscious of the responsibility that rests with them. The combined value of these two European Space Agency (Esa) programmes is 1.9bn euros (£1.7bn; $2.5bn). More than that, the effort put into the missions by some scientists and engineers stretches back over 20 years. Satellites do not have wing mirrors, but if they did well, you get the picture. "It's not a bad analogy; these are very complex and very expensive pieces of equipment," says Chris Watson, the spacecraft operations manager (SOM) whose team is responsible for Planck. "You're very aware that there are a lot of scientists out there who are waiting on what we do," concurs Micha Schmidt, the Herschel SOM. The telescopes will be hurled into space atop an Ariane 5 launcher from the Kourou spaceport in French Guiana on Thursday. The powered portion of the flight lasts about 26 minutes, before the observatories are ejected from the upper-stage of the rocket. Herschel is first out; Planck follows shortly after. It will be about 10 minutes before they are picked up by the ground station and contact is established. It is at that moment that the "keys" are passed to the European Space Operations Centre (ESOC) in Darmstadt, Germany, and the big parking job begins. Herschel and Planck take many weeks to get to their observation stations Gravity conditions at L2 allow for cheap (in fuel terms) orbital corrections Environmental conditions (heat & radiation) are more stable than at Earth L2 takes its name from its discoverer, Joseph-Louis Lagrange (1736-1813)

"The spacecraft become active at separation and it's a pre-programmed list of commands that is run through," explains Mr Schmidt, who says Herschel should already have begun to orientate itself and started to activate sub-systems by the time the ground link is made. "The first thing we send to the spacecraft is a test command. It's a connection test; it doesn't do anything apart from verify that we can command it." The early days of the mission, known as LEOP (Launch and Early Operations Phase), are spent in a dedicated room at ESOC. It is there that controllers will tick off a long list of health-checks as the satellites cruise away from Earth. The destination for both telescopes is a remarkable position in space known as the second Lagrangian point (L2). It is one of five gravitational "sweet-spots" around the Sun-Earth system where satellites can maintain station by making relatively few orbital corrections. HERSCHEL SPACE TELESCOPE The Herschel concept has been in development for over 20 years The observatory is tuned to see the Universe in the far-infrared Its 3.5m diameter mirror will be the largest ever flown in space Herschel can probe clouds of gas and dust to see stars being born It will investigate how galaxies have evolved through time

Telescope seeks cold cosmos L2 is some 1.5 million km from Earth on its "night side". The observatories will circle this point, keeping the Earth and our star in a near-straight line. Currently, only one spacecraft inhabits L2: the US space agency's WMAP probe (which is making observations on which Planck hopes to build). In the years ahead, however, many more astronomical spacecraft will choose to sit at this location because of the very stable conditions it enjoys. It does not experience, for example, the big swings in temperature and light endured by space telescopes positioned much closer to Earth. That is very important for Herschel and Planck which will both be investigating aspects of the "cold Universe". "You can maintain a good geometry - the Earth and Sun are always in the same direction," says Martin Hechler who has helped to design the mission path out to L2. "This is a very nice way of shielding. In previous missions in Earth orbit, you lost a lot of the mission time because you could not observe when the Earth itself was shining in the telescope. "At L2, you look out [into space], and as you move around the Sun, you see 360 degrees in the galactic frame; so you can look in all directions in one year." Dr Hechler and his mission analysis group are breaking new ground for Esa. The agency has never sent a spacecraft to L2 before and the knowledge gained will be invaluable for future missions. But it is not simply a case of pointing a rocket and sitting back. As good as Ariane is, a series of orbital corrections will be required to nudge Herschel and Planck into precisely the right paths to take up their parking positions. Both spacecraft will fire their thrusters on Day 2 of the mission. Herschel will have another correction, probably around day four. Planck will need three manoeuvres in total to put it in a much tighter "hover" about L2. "The first correction for our transfer is a very important milestone in our mission," says Chris Watson. "The longer it takes you to make it, the more fuel it costs. So if you can correct any errors very early, it is relatively cheap in terms of fuel." Herschel sits on top of Planck for the ride into orbit, and is first out Planck needs to spin for its mission; Ariane will start up that motion The telescopes need to make path corrections to get out to L2 The graphic shows their pattern on the sky at L2 as viewed from Earth Planck's design requires an Earth aspect of less than 15 degrees

Good economy will pay dividends later in the mission when fuel is needed to make the regular small tweaks in the telescopes' parking slots. "L2 is unstable and we'll need to use our thrusters once a month," says Mr Watson. "It's like trying to balance a pen or a stick on your hand - if you can do it precisely enough, you spend very little fuel because you are making tiny corrections." Schmidt (near) and Watson (far) work through yet another simulation Please turn on JavaScript. Media requires JavaScript to play. Of course, this all assumes a trouble-free passage. Problems could arise at any moment; and the teams have had to practise for all kinds of eventualities. In one simulation exercise last month, the Herschel team had to cope with a spacecraft that had a computer failure immediately after being released from the Ariane. The controllers worked through the issue and recovered the situation only to discover that Herschel was then being bombarded by meteoroids. Again, Micha Schmidt and his team had to move swiftly to regain full command of the satellite. "You said it was like someone saying: 'Here're the keys to a Ferrari, don't scratch it'. Well in the simulation, that is precisely what we do. In real life, we hope these things won't happen; but if they do, we are very well prepared for them." It will take a little over six weeks to get both telescopes into their final science orbits. The super-satellites can then be handed over to the world's researchers with the wing mirrors still attached. Jonathan.Amos-INTERNET@bbc.co.uk



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