A European comet-chasing spacecraft is set for a nail-biting close encounter this weekend with Mars. The billion-euro (1.3-billion-dollar) probe Rosetta will come within 250 kilometers (156 miles) of the Red Planet's surface, using Martian gravity to correct its course in one of the longest and costliest treks in the history of unmanned space exploration.
The European Space Agency (ESA) probe, launched in March 2004, is designed to rendezvous with Comet Churyumov-Gerasimenko in 2014 after a voyage of 7.1 billion kilometers (4.4 billion miles).
It will send a refrigerator-sized lab, called Philae, to the comet's surface to investigate the rock's chemistry.
To make the meeting, it is using four gravitational assists from Earth and Mars.
From 0215 GMT on Sunday, Rosetta will start to swing behind Mars, cutting itself off from radio contact but also from the sunlight that powers its two giant solar panels.
During this 13-minute "eclipse," all of the spacecraft's non-vital equipment will be closed down to conserve power, leaving it to run on batteries alone.
Normally, planetary swingbys are used to build up speed but Sunday's operation in fact is designed as a brake.
"We have to put Rosetta in the best position for the next gravitational assist, which will be with Earth towards the end of the year," said Gerhard Schwehm, head of ESA's Solar System Science Operations Division.
Rosetta's first Earth flyby was in 2005. Its encounter with Earth later this year will be to gain speed.
