Monday, July 12, 1999 Published at 15:35 GMT 16:35 UK
Harpoon probe for comet
Comets - flying mountains of rock, ice and frozen gases
By BBC News Online Science Editor Dr David Whitehouse
The American space agency Nasa has announced two spectacular low-cost missions. They are a return to the neglected planet Mercury and a space probe that will fire a harpoon into a comet.
The so-called Deep Impact comet mission could be one of the most spectacular carried out.
Launched in January 2004, the spaceprobe will spend 18 months traversing the solar system en route to a rendezvous with comet Temple 1.
Manoeuvring alongside the 5km-wide nucleus of the comet, composed of rock, ice and frozen gases, it will use its cameras to survey the surface.
The surface of the comet consists of a loose collection of dust and fluffy frozen particles. In places, there are geysers where dust and gas erupt from below the surface creating jets and streamers that billow out into space.
Scientists back on Earth will select a place on the comet to fire a copper-tipped projectile into the nucleus.
Sensors on the spaceprobe will monitor the explosion. Back on Earth, scientists will watch a plume of gas and dust erupt into space carrying material from the heart of the comet.
The material blasted into space is expected to be unchanged since the formation of the Solar System.
Return to Mercury
The other mission is to be called Messenger and it will go to the neglected planet Mercury.
Mercury has only been visited by one spaceprobe, Mariner 10 in 1974. It is a world that is superficially similar to our Moon but there are significant differences, especially in its internal structure.
To get a spaceprobe into orbit around Mercury will require a series of flybys of Venus as well as two Mercury flybys before entering orbit in September 2009.
One of Messenger's goals will be to discover whether Mercury has water ice in its polar craters like the Moon. Radar observations of Mercury suggest that it has.
"These low-cost missions are both fantastic examples of the creativity of the space science community," said Dr Edward Weiler of Nasa's space science division.