Rosetta space probe lined up for Mars swing-by

BERLIN, Feb 22 (Reuters) Europe's pioneering Rosetta space probe will swing around the back of Mars early on Sunday in a critical phase of its ten-year mission to meet a distant comet.

The so-called ''swing-by'' of the Red Planet is being coordinated by the European Space Operations Centre (ESOC) in Darmstadt, Germany, and is the second of four such manoeuvres that the probe must make on its ambitious journey.

The mission's crowning moment will be in late 2014 when it releases a landing vessel in the first attempt at a controlled landing on a comet but first it must gain enough speed to catch it.

''Rosetta is a very long mission,'' said Paolo Ferri, head of solar and planetary missions at ESOC. ''It is not necessarily the distance but the fact that we want to reach the comet and stay in the vicinity for one or two years.'' ''In order to do this we have to achieve the same orbit and the same velocity as the object which is travelling extremely fast,'' he said.

The three-tonne Rosetta comet chaser will orbit Mars just 250 km above the planet's surface but Ferri said the risks of anything going wrong were minimal.

''You can never exclude this but making an error of 250 km and crashing into the planet is basically impossible,'' he said.

Rosetta, launched in March 2004 on an Ariane five rocket from French Guiana, will catch up with and monitor the comet 67P/Churyumov-Gerasimenko, in one of the most ambitious missions made by the European space project.

GRAVITATIONAL PULL To gather momentum while conserving as much rocket fuel as possible, controllers are using the gravitational pull of Earth and Mars to catapult the probe closer to its target.

One such ''gravity assist'' manoeuvre -- commonly known as a swing-by -- was performed around Earth two years ago. The Mars manoeuvre is the second and most complicated of the mission, while two more Earth manoeuvres are due in 2007 and 2009.

''I don't know if this is closest swing-by in history but it is one of the most daring,'' Ferri said. The process will take around half a day with the critical manoeuvres taking place in the early hours of Sunday morning.

While measurements show the probe is on the right trajectory, Ferri remains concerned about the solar-powered Rosetta passing through the Martian shadow.

For 24 minutes, it will lose the source of power for its major instruments, leaving it reliant on a brace of tiny batteries which were not designed for the task.

In the worst case, the probe may fail to reestablish contact with Earth when it emerges on the other side of the Red Planet.

''This is a big worry because we hate in space flight to do things for the first time,'' Ferri said.

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