Illustration of NASA's Mars Expedition Rover robots, the first of which is due to launch on Monday, which are the most sophisticated robots ever sent to another planet. The rovers will travel over the surface of Mars and search for signs of water, past or present. [Reuters]

The US space probe Spirit was launched toward Mars for a mission to study the geological structure of the Red Planet and try to find out if life could ever have existed there.

The Boeing-built Delta II rocket carrying the Spirit -- with the Mars Expedition Rover A (MER-A) aboard, the first of two US robots that will closely study the red planet -- launched Tuesday at 1:58 pm (1758 GMT).

"We see a clean separation," a mission control spokesman said. "MER-A, have a safe journey and a successful mission, and MER-B, we'll see you soon."

A camera mounted on the rocket showed images as it sped on its flight path above the Florida coast then the upper strata of the atmosphere at more than 15,000 kilometers (9,300 miles) per hour.

The spacecraft is to traverse some 500 million kilometers (300 million miles) over seven months, then drop into the Gusev crater, 15 degrees south of the Martian equator, in early January 2004.

The second, MER-B, dubbed Opportunity, is scheduled to launch on June 25 and to land on January 25, 2004 in an area of Mars known as the Meridiani Planum.

The aim of the mission is to look for signs of water which would indicate that there could have been life or that life was possible.

"This will continue NASA's long goal of finding the water. On Earth, wherever we find water, we find life," Ed Weiler, associate administrator for space science for NASA said earlier this week.

"There was water on Mars billions of years ago and maybe just a few hours ago."

Of 30 attempted Mars missions over the past 40 years, just 12 have succeeded, Weiler noted.

Europe's Mars Express left on June 2 and is scheduled to finish its 400 million kilometer (250 million mile) trip a few days before the first US MER arrives on January 4 next year.

The two rivals launched so close together because Mars is now at its closest position to the Earth, which only happens every 26 months.

After an investment of some 800 million dollars the mission will aim to overcome the disappointment of the failure of the Orbiter mission which took off in December 1998 and disappeared on arrival and the Polar Lander rocket which crashed into the planet in 1999 when its landing system broke down.

A US Pathfinder rocket was the first to land on Mars in 1997. A small robot was put on the Red Planet to gather information.

This time, NASA has decided to revert back to its Pathfinder landing system which uses parachutes and air bags to slow down the spacecraft and cushion the impact.

Each vessel will bounce about 10 times on the frozen surface of Mars before coming to a standstill.

The two six wheeled robots will be put out on the opposite poles of Mars and go walkabout for three months collecting geological samples.

Powered by solar energy, the robots will be able to move 40 metres (125 feet) each Martian day, as much as during the whole Pathfinder mission in 1997.

The MER robot has a telescopic arm including a camera which will be able to take 360 degree colour images. It also has equipment to scratch and dig into the surface.

A squad of 150 scientists at NASA's Jet Propulsion Laboratory in Pasadena, California, will gather data transmitted by the robots directly to Earth via two US satellites in orbit around Mars: the Global Surveyor and Odyssey probes.

The data will help scientists on Earth decide the robots route on Mars for the following day.

The robots first will explore the area around their drop point before venturing out 500 meters (1,640 feet) each over the course of the mission.

Joy Crisp, project scientist for the Mars Exploration Rover project at the laboratory said in an interview in May: "We cannot exactly predict what we will find" on Mars, though the project's "overall objective is to determine if there was water on Mars."

"We picked our landing sites by using data and images that are obtained from orbit and we tried to pick sites that are reasonably smooth," she told AFP.

"But until you go down and look up close, you don't know what exact rocks configuration of rocks you will discover at the scale that matters to your rover."

Water in its liquid form is not a feature of the Martian surface but topography which seems in part to have been criscrossed by running water has made many researchers believe it may have been there in the past.

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