A European Space Agency probe will pass within 250 kilometres of Mars to correct its course en route to a distant comet.

The Rosetta space probe's flyby of Mars is a critical phase of an ambitious and costly 10-year project to land a probe on Comet Churyumov-Gerasimenko.

Artist's impression shows the Rosetta probe as it approaches Mars en route to comet Churyumov-Gerasimenko.
(C. Carreau/European Space Agency)

The billion-euro project began with the launch of the probe on March 2, 2004. It is scheduled to reach the comet in 2014.

The probe is scheduled to make its closest approach to Mars at 8:57 p.m. ET on Saturday, using the gravity of the planet as a braking mechanism to reduce speed and adjust its course back to Earth for a flyby of our planet.

To correctly navigate a path to the comet, Rosetta will have passed Earth three times and Mars once.

The Mars flyby provides an opportunity for the probe to test its instruments, perform scientific observations and take pictures of the Red Planet.

But the manoeuvre is not without risks. A case of bad timing caused by a launch delay means the probe's solar panels will be shadowed from the sun by the planet. Ground controllers will also lose contact with the probe for 15 minutes as it passes behind Mars.

The ultimate goal of the probe is to land on Churyumov-Gerasimenko to analyze its surface.

Like the planets, comets are thought to have spun out of the sun during the formation of the solar system 4½ billion years ago. But unlike planets, comets have remained largely unchanged in composition, making them a unique time capsule of the process and elements at work during the formation of the planets.

NASA has crashed a probe into the comet Tempel 1 to study the dust and ice released during the collision. The Stardust spacecraft obtained dust samples from the Wild 2 comet by flying close by. But no spacecraft has ever attempted to land on a comet.