January 13, 2003. Copyright Graphic News, All rights reserved.

Rendezvous with a comet 

January 13 -- Graphic News, London: Scientists are anxiously awaiting the launch of a pioneering spacecraft designed to look back 4.6 billion years to an epoch before planets existed, when only a vast swarm of asteroids and comets surrounded the sun.

But the European Space AgencyÕs 700 million euro (US$740 million, £460 million) Rosetta mission to orbit and land on a comet now faces a delay of weeks or even months following the failure of EuropeÕs new super rocket, the Ariane 5, which exploded over the Atlantic on its maiden flight in December. 

Rosetta is due to blast off from Kourou in French Guiana on a voyage past the planets to a rendezvous with Comet 46P Wirtanen. Rosetta, which will be the first spacecraft ever to go into orbit around a comet nucleus, will launch a small lander onto its icy surface. It will also be the first to observe the changes that occur in comet activity as the nucleus travels towards the inner solar system.

Named after the Rosetta Stone, the key which enabled linguists to discover the secrets of ancient Egypt by deciphering hieroglyphics for the first time, it is expected that the modern Rosetta will prove instrumental in unravelling an even older mystery, the birth of the solar system. 

Because these primordial leftovers were never subject to the kinds of thermal and chemical change that happened during the formation of the planets, comets are, in a sense, time capsules that can be used to find out more about conditions in the early solar system.

These Òdirty snowballsÓ may also be able to answer specific questions about the early history of the earth. For example, some scientists suggest that much of the water in the earthÕs oceans, and the original gases in its atmosphere before life got to work on them, were ÒplasteredÓ on to the planet by collisions with comets, rather than emerging from its interior through volcanic vents.

If the water found on comets contains an earth-like ratio of deuterium (a form of hydrogen that has a heavy nucleus) to ordinary, light hydrogen, that would favour the plastering theory. 

A more intriguing area that this mission is meant to cast light on is the origin of life -- or, rather, the chemical precursors of life. Comets contain amino acids, which are the building-blocks of proteins. If comets ever did contribute to the earthÕs oceans, they may have brought part of the complex carbon-based chemistry of life with them. RosettaÕs Cometary Sampling and Composition experiment, which will analyse dust for carbon-based molecules, may indicate that life on earth is indeed a legacy of the stars. 

The spacecraft consists of two main components, a box-like orbiter carrying communications, solar arrays, propulsion systems and payload of 11 scientific instruments, plus a nifty 100kg lander called RoLand.

Rosetta will weigh a chunky 2.9 tonnes at launch, with over 50% of the mass being onboard propellant to keep the spacecraft moving throughout its 11 year mission, particularly during the delicate and complex manoeuvres in orbit around the comet nucleus.

As a result, ESA will use EuropeÕs most powerful launcher, the Ariane 5, to boost Rosetta towards its destination. Even then the flight will also require a gravity-assist flyby of Mars in 2005 and two similar flybys of Earth, one in 2005 and another in 2007, to put Rosetta on track for rendezvous with Wirtanen in November 2011. En route, Rosetta will wake from its hybernation to study two asteroids at close range. The first is a small 20km-diameter body called 4979 Otawara in July 2006, followed by a flyby of the much larger, 110km-diameter 120 Siwa in July 2008. 

On reaching the comet, Rosetta will first match velocities, and then drift along carrying out observations until orbital insertion around the nucleus in the late summer of 2012. Once in orbit, mission control will refine the course of the spacecraft using images sent back by the probe to move Rosetta into a tighter orbit, as close as 2km above the nucleus, to provide the most detailed views yet obtained of a comet. 

Once detailed mapping has been completed and a landing site selected, Rosetta will release the lander which will descend onto the surface and secure itself in position against the cometÕs feeble gravity using a harpoon fired into the surface. 

The lander will operate for at least a month, transmitting its data back to Earth via the orbiter which will continue to observe the comet until the expected end of the mission in July 2013.
/ENDS

Source: European Space Agency