Nasa unveils mission to map frozen craters on Mercury

Space scientists are to build a probe to map Mercury, the most mysterious and least explored planet of the inner solar system.

Space scientists are to build a probe to map Mercury, the most mysterious and least explored planet of the inner solar system.

One objective of the $286m (£204m) mission is to find out whether deep craters at the poles of Mercury are filled with ice despite surface temperatures elsewhere on the planet soaring to 450C (840F).

The American National Aeronautics and Space Administration (Nasa) said that construction of the Messenger space probe would begin immediately, to meet a scheduled launch date of March 2004. Five years will be needed for the probe to reach its final destination via a circuitous journey that will take it twice past Venus and Mercury.

The only other spacecraft to have visited Mercury was Mariner 10, which flew past the planet three times in 1974 and 1975 but could take snapshots of only half the surface area of the Sun's nearest planet.

This time, the Messenger probe will stay in orbit for at least a year to make detailed maps of the planet's strange topography as well as measuring its magnetic field and other geophysical features. A protective shield will save Messenger from the heat of the Sun.

Scientists are particularly interested in the planet's polar craters. The deepest parts are in continual shadow and temperatures there never rise above minus 200C. Radar experiments in 1991 indicated the craters could contain frozen water deposited by passing comets over billions of years.

Jay Bergstralh, chief scientist at Nasa's Solar System Exploration Division in Washington DC, said the Messenger spacecraft was the seventh in Nasa's Discovery Programme aimed at exploring the solar system using small and relatively cheap probes.

" Messenger is our most complex and challenging Discovery-class mission. Our goal is to do something never before attempted, conducting a year-long mission to orbit a planet only 36 million miles from the Sun for relatively low cost," Dr Bergstralh said.

The Earth is almost three times further from the Sun than is Mercury, the "winged messenger" of Roman mythology, and is visible only just after sunset or before sunrise. It is so close to the Sun that even the Hubble space telescope cannot take pictures of it.

Mercury is one of the four rocky or "terrestrial" planets – the others being Venus, Mars and Earth.