On Thursday, the European Space Agency announced that the Rosetta Science Working Team had selected two asteroids for the Rosetta comet rendezvous mission  to observe at close quarters during its journey to Comet 67P/Churyumov-Gerasimenko.

Rosetta's scientific goals always included the possibility of studying one or more asteroids from close range. However, only after Rosetta's launch on March 2 and its insertion into interplanetary orbit could the ESA mission managers assess how much fuel was actually available for fly-bys. Information from the European Space Operations Centre (ESOC) in Germany enabled Rosetta's Science Working Team to select a pair of asteroids of high scientific interest, well within the fuel budget.

The selection of these two excellent targets was made possible by the high accuracy with which the Ariane 5 delivered the spacecraft into its orbit. This of course leaves sufficient fuel for the core part of the mission, orbiting Comet 67P/Churyumov-Gerasimenko for 17 months when Rosetta reaches its target in 2014.

The targets selected for Rosetta, Steins and Lutetia, have rather different properties. Steins is relatively small, with a diameter of less than few kilometres, and will be visited by Rosetta on 5 September 2008 at a distance of just over 1060 miles (1700 km). This encounter will take place at a relatively low speed of about 5 miles (9 km) per second during Rosetta's first excursion into the asteroid belt.

Lutetia is a much bigger object, about 330 miles (100 km) in diameter. Rosetta will pass within about 1860 miles (3000 km) on July 10, 2010 at a speed of 9 miles (15 km) per second. This will be during Rosetta's second passage through the asteroid belt.

Rosetta will obtain spectacular images as it flies by these primordial rocks. Its onboard instruments will provide information on the mass and density of the asteroids, thus telling us more about their composition, and will also measure their subsurface temperature and look for gas and dust around them.

"Comets and asteroids are the building blocks of our Earth and the other planets in the Solar System. Rosetta will conduct the most thorough analysis so far of three of these objects," said Prof. David Southwood, Director of ESA's Science Programme. "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."

Image Credit: ESA/AOES Medialab