The first Mars Expedition Rover is scheduled to depart aboard a Boeing-made Delta II launcher at 2:05 pm (1805 GMT) Sunday from an Air Force base adjacent to the Kennedy Space Center in Cape Canaveral, Florida, weather permitting.

If all goes according to plan MER-A will traverse 500 million kilometers (300 million miles), then drop into the Gusev crater, 15 degrees south of the Martian equator, on January 4, 2004.

"This will continue NASA's long goal of finding the water. On Earth, wherever we find water, we find life. There was water on Mars billions of years ago and maybe just a few hours ago," said Ed Weiler, associate administrator for space science for NASA.

The US space agency, which has chalked up two costly failures in its quest to explore Mars in recent years, warned that the task would not be easy.

"It's not a trip to the beach on a Sunday afternoon," Weiler said Friday.

"We can have a bad day on Mars. Wind. Bad weather. We are trying to predict the weather on Mars for January 2004," he explained, noting that of 30 attempted Mars missions over the past 40 years, just 12 have succeeded.

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

The US MERs will be competing with a European rival to shed light on Mars's mysteries. Europe's Mars Express left on June 2 and is scheduled to finish its trip a few days before the first MER arrives on January 4 next year.

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

The National Aeronautics and Space Administration is spending 800 million dollars to get this mission right.

It aims to overcome the disappointment of the Orbiter, which took off in December 1998 and disappeared on arrival, as well as the Polar Lander rocket, which crashed into the planet in 1999 when its landing system broke down.

A US Pathfinder rocket was the first vehicle 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 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 Martian surface before coming to a standstill.

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

Powered by solar energy, the robots will be able to move 40 metersfeet) 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 color 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 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 points before venturing out 500 meters each over the course of the mission.

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

SPACE.WIRE