The two asteroids to be visited by the Rosetta mission have just been named.
The big rocks will be visited on the way to the comet
Europe's space scientists are to direct their recently launched spacecraft to flyby rocks called Steins and Lutetia.
Steins is only a few km across and will be studied in September 2008; while Lutetia, a 100km-wide asteroid, will be inspected from close up in July 2010.
The £600m Rosetta mission was blasted into space on 2 March with the ultimate goal of orbiting and landing on Comet 67P/Churyumov-Gerasimenko in 2014.
Its outward 10-year journey around the Solar System will include two passes into the asteroid belt, the band of rocky debris between the orbits of Mars and Jupiter left over from the formation of the planets.
Only a few asteroids have so far been observed from nearby. They have been shown to be very different in shape and size, ranging from a few km to over 100km across, and in their composition.
Following Rosetta's successful launch from Kourou in French Guiana, scientists have been able to work out the probe's precise trajectory in space and identify the asteroids it could conceivably visit without using up too much fuel.
The pair of asteroids now selected are said to be of high scientific interest and well within the fuel budget of Rosetta.
The Steins rendezvous will take place on 5 September 2008 at a distance of just over 1,700km. This encounter will be conducted at a relatively low speed of about 9km/s during Rosetta's first excursion into the asteroid belt.
The Lutetia pass will occur at a distance of about 3,000km on 10 July 2010 at a speed of 15km/s. This will be during Rosetta's second passage through the asteroid belt.
Rosetta will take pictures of the rocks, and its instruments will endeavour to get data on mass, density, and temperature. This should tell scientists more about the asteroids' make-up.
But the mission's main goal is to meet up with Comet Churyumov-Gerasimenko in early 2014.
At this time, the comet will be cold and inactive, and Rosetta will be able to release a washing-machine-sized lander, called Philae, on to the object's surface.
The orbiter and lander will then record changes in the comet as it hurtles in towards the Sun at speeds up to 135,000km/h.
Rosetta was launched into space by an Ariane rocket
"Comets and asteroids are the building blocks of our Earth and the other planets in the Solar System," said Professor David Southwood, director of the European Space Agency's science programme.
"Rosetta will conduct the most thorough analysis so far of three of these objects.
"Rosetta will face lots of challenges during its 12-year journey, but the scientific insights that we will gain into the origin of the Solar System and, possibly, of life are more than rewarding."