Mercury is a strange world of rock. It is the closest world to the Sun and is so hot that zinc and lead would melt on its surface.

It is a neglected planet. Only distant Pluto at the outer reaches of the solar system has been visited less. Just one spacecraft has been there, Mariner 10 which passed Mercury three times in 1974-5.

Its only visitor: Mariner 10

There are only pictures of just one side of Mercury, a world that looks very much like our moon. But there are some differences.

Mercury is heavier and it appears to have shrunk. All over its surface are signs of compression like the skin of a shrivled apple.

The new mission is still in its planning stage. It will be carried out jointly with Nasa.

So hot lead would melt

"We want to go to Mercury because it's the least known and least understood of the inner planets," said Hirobumi Saito, project manager for the Mercury probe, speaking at the 21st International Space Technology and Science Symposium being held in Japan.

The probe will be launched in August 2005 and will complete a triple swing-by of Venus to adjust its course before heading for Mercury.

Preliminary calculations suggest it will get to the planet in January 2008 before entering orbit on September 30 2009.

The probe will also be powered by a new type of thruster called an ion engine.