The agency's associate administrator for space science Ed Weiler warned 48 hours before the rocket carrying Spirit took off in June: "It's not a trip to the beach on a Sunday afternoon."

The six wheeled Mars Exploration Rovers are 1.6 metres (5.2 feet) long and has a 1.5 metres (4.9 feet) high camera mast that gives it the look of a golf buggy adapted for Star Wars.

Each is packed with sensitive equipment to search for signs that their may have been water on Mars in the past that could have sustained life.

The equipment includes a Miniature Thermal Emission Spectrometer from Arizona State University, a Mossbauer Spectrometer from Johannes Gutenberg University in Germany, an Alpha Particle X-ray Spectrometer from the Max Planck Institute for Chemistry in Germany and a Microscopic Imager from NASA's Jet Propulsion Laboratory in California, which is managing the whole 800-million-dollar project.

A rock abrasion tool, called a RAT, grinds on the rocks that have been found on the surface so they can be examined and data sent back to Earth. The spectrometers identify different minerals.

The panoramic 360 degree camera on the mast sent back the most spectacular colour and infra-red images of the surface ever seen.

The whole thing has been moved around by the six wheel drive buggy, which has a 'rocker bogie' suspension system which bends at its joints rather than using springs. This enables the rover to cross rocks bigger than the wheel diameter of 26 centimetres (10 inches).

The solar robotic explorers can trek up to 40 meters (about 44 yards) each Martian day.

Scientists were to command the vehicles to head for specific targets chosen from the images and date received each day.

But this has halted since NASA lost contact Wednesday with Spirit, the first of the two robotic probes, which landed on Mars on January 3.

The second, Opportunity, is due to land on another part of the planet this weekend. Each was intended to work for at least 90 martian days, which take the mission until late April.

Before the takeoff from Earth, NASA had delayed the launch because of a problem with its cables that could cause the robots to short-circuit.

Problems with the airbags that cushioned the January 3 landing held up the start of the exploration on the surface.