The two spacecraft will land at different sites and take complementary approaches to their explorations.

"Beagle will remain in one place and use its mole to burrow under the surface for samples: it will also study the Martian atmosphere. The rover will move about and analyse samples from different positions on the surface," says Colin Pillinger, Beagle Principal Investigator.

One of Beagle's main aims will be to look for the chemical signatures of past and present life by studying organic matter and isotopic composition. The rover will act as a "robotic field geologist". "It has everything a human field geologist has, and then much more," says Steven Squyres, Principal Investigator of the main scientific package to be carried by the rover.

Both spacecraft will carry grinders to remove the weathered surface from rocks and expose the pristine interiors for analysis. Both will also look for evidence of water and asses how hospitable the environment is, or has been, to life.

NASA's rover will be based on technology used successfully for Mars Pathfinder's rover, Sojourner, in 1997. Like Beagle 2, it will descend to the Martian surface cushioned by airbags.

Last week's announcement was made after months of re-assessment of NASA's Mars programme after the failure of the Mars Climate Orbiter and Mars Polar Lander missions last year. NASA is still considering whether to send another rover at the same time and is due to take a decision shortly.