BBC News Online: Sci/Tech

Nasa to crash probe into comet

The $279m (£197m) Deep Impact mission will be launched in January 2004 and take 18 months to approach the Tempel 1 comet.

Once at the comet in July 2005, a flyby spacecraft will launch a smaller 770 lb (350 kg) impactor, which will slam into Tempel 1 at a speed of 22,300 miles per hour (36,000 kilometres per hour).

Nasa says the mission is designed to learn about the comet's composition, structure, and how its interior differs from its surface.

Preserved in space, comets are thought to contain pristine examples of the primitive material that formed the Solar System 4.5 billion years ago.

Huge crater

The collision - to be broadcast live back to Earth - will create a crater the size of a football field some seven storeys deep.

Tempel 1 was discovered in 1867 and orbits around the Sun every 5.5 years.

Nasa says the fact that the comet has made many passages through the Solar System means it is a good candidate for studying evolutionary change in its mantle, or upper crust.

A Nasa spokesman said: "Scientists are eager to learn whether comets exhaust their supply of gas and ice to space or seal it into their interiors.

"They would also like to learn how a comet's interior is different from its surface. The controlled cratering experiment of this mission could provide those answers."