The European Space Agency (ESA) is preparing for the reactivation of its spacecraft Rosetta next month, en route to a history-making landing on a comet in November.
"Nobody has ever done this before," said Paolo Ferri, head of mission operations at ESA's European Space Operations Centre in Darmstadt, Germany.
"We expect to learn revolutionary things about our solar system." Rosetta was launched in March 2004 on an Ariane 5 rocket from Europe's spaceport in Kourou, French Guiana, and has since travelled around the sun five times and made several "gravity-assist" fly-bys of Earth and Mars to put it on course towards comet 67P/Churyumov-Gerasimenko.
To conserve power and fuel, the three-ton spacecraft was put into deep-space "hibernation" in June 2011.
Rosetta's automatic wake-up is set for January 20, when it is to reestablish communication with Earth. It is expected to reach the comet, estimated to be about four kilometres in diameter, in August.
Equipped with 11 sophisticated measuring instruments, the Rosetta orbiter is to remain within a few tens of kilometres of the comet's icy nucleus, map the surface and analyse the gas and dust grains that are ejected - forming a tail - as it heats up heading back towards the sun.
Three months after its rendezvous with 67P/Churyumov-Gerasimenko - named after the two Soviet astronomers who discovered it in 1969 - Rosetta is to release its box-shaped lander Philae onto the comet's surface.
Upon touchdown a harpoon will anchor Philae to keep it from bouncing back into space, unrestrained by the comet's infinitesimal gravity.
Philae's 10 instruments are to analyse the composition and structure of the comet's surface and subsurface material, and cameras will take panoramic pictures.
Rosetta's main objective, ESA said, is to help understand the origin and evolution of the solar system. It notes that comets preserve the earliest record of material from the nebula out of which our sun and planets formed more than 4.6 billion years ago.
Comet impacts are also thought to have delivered much of the water in Earth's oceans, and perhaps the complex organic molecules crucial to the evolution of life.
The ESA spacecraft was named after the famous Rosetta Stone, the key to deciphering ancient Egyptian hieroglyphs. Philae was formerly an island in the Nile River where an obelisk that contributed to the decipherment was found.
The nominal end of the mission, costing about 1 billion euros ($A1.55 billion), is in December 2015, four months after the comet's closest approach to the sun. By then, Rosetta will have travelled 7.1 billion kilometres. It is hoped that Philae will be able to work for at least six months.
