New probe for Mars

An artist's impression of the new Mars rover

The American space agency, Nasa, has announced plans to send at least one new unmanned probe to Mars.

A Nasa spokesman, Dr Edward Weiler, said a robot vehicle capable of travelling 100 metres (325 ft) a day on the Martian surface would be launched in 2003 for a landing a year later.

This mission will give us the first ever robot field geologist on Mars

Scott Hubbard, Nasa Mars programme

The Mars rover will be a larger and more sophisticated version of the tiny Pathfinder Sojourner vehicle, which successfully landed on Mars in 1997 and sent back thousands of photographs.

The project announcement comes a year after Nasa lost two Mars probes - the Mars Polar Lander and an orbiting vehicle - when they disappeared on their final approach towards the planet.

The loss of these probes was a great embarrassment to Nasa - particularly as it emerged that the orbiter crashed because the mission engineers had confused metric with English measurements.

Search for water

The new landing probe will send colour and infra-red images back to Earth, and is equipped with tools and instruments to search for evidence of liquid water which scientists believe once existed on Mars.

It was chosen after consideration of a competing proposal - a Mars orbiter that was to take high-resolution pictures of the planet.

An artist's impression of the lost Mars Polar Lander

"This mission will give us the first ever robot field geologist on Mars," Scott Hubbard, Nasa's Mars programme director, said.

"It not only has the potential for breakthrough scientific discoveries, but also gives us necessary experience in full-scale surface science operations which will benefit all future missions."

Nasa also said it was considering sending a second landing probe at the same time to explore a different area of Mars. This move would add about $150m on to the $350m price tag of a single mission, excluding launch and operations.

Already in place is the Mars Global Surveyor, which has been circling the planet for several years.

Recent pictures from it suggest that water might exist just below the surface and could still emerge on to the Martian surface as springs.

BBC correspondents say that if liquid water does still exist on Mars then it raises the tantalising prospect that primitive life might also be found on or just below the planet's surface.

A plentiful supply of water would, they say, also make it far easier for humans to travel to the Red Planet.

Bouncy landing

Nasa will have to face the difficult question of how to get the wheeled robot down safely - unlike Mars Polar Lander last year.

Nasa officials said a landing site for the probe had still to be selected, but it is expected to be a dry lakebed or water channel.

Mars is thought to have once been rich with water and its surface bears features that apparently were carved by water.

There is evidence that water once flowed freely on Mars

The 136-kilo (300-pound) lander will be launched on its seven-and-a-half-month journey to Mars on top of a Delta II rocket.

If all goes to plan, it will enter the Martian atmosphere on 20 January, 2004.

After a parachute descent, air bags will inflate to cushion the probe's landing.

Engineers say the craft will bounce about a dozen times after touching down and then roll, perhaps as far as a kilometre (just over half a mile).

This method of landing was pioneered with the Pathfinder Sojourner and proved such a success that the Nasa engineers now feel confident to attempt it with this larger, more expensive craft.

When the air bags deflate, the rover will roll off its landing platform to start exploration.

Surface operations will last for at least 90 days, but could continue longer.