NASA's Spirit rover lay in critical condition Friday on Mars, coughing up only a few gasps of data, as engineers struggled to diagnose the ailment and also deal with the impending arrival of its twin spacecraft on the Red Planet.

Controllers heard three times from Spirit after two days in which the six-wheeled vehicle transmitted only gibberish or sporadic beeps to acknowledge commands from Earth. The last batch of data, relayed to Earth by the 2001 Mars Odyssey spacecraft, indicated Spirit's power system was OK, said Ed Weiler, NASA associate administrator for space science. More data was expected early Saturday.

Engineers believe some sort of underlying hardware problem triggered the crisis. That has wreaked havoc with Spirit's software and forced the rover to reboot its computer more than 60 times, project manager Pete Theisinger said. Spirit's prognosis was uncertain.

"The chances that it will be perfect again are not good and the chances that it will not work again are also low," Theisinger said at NASA's Jet Propulsion Laboratory.

Asked how he would rank its condition if it were a patient, Theisinger said "critical." Even under the best of circumstances, Spirit would not be back to normal for many days or even a couple of weeks, he added.

At the same time engineers dealt with the crisis, Spirit's twin, Opportunity, neared Mars for its own landing late Saturday.

Opportunity is the second half of an US$820 million double mission to learn if Mars was once a wetter world capable of supporting life.

Three hundred scientists and engineers, divided into two teams, are working on the double mission. Theisinger has encouraged engineers to stay focused on Opportunity and not dwell exclusively on Spirit and its problems.

Nevertheless, the problems with Spirit will probably force a delay in the sequence of activities that Opportunity will go through after its landing, scheduled for 9:05 p.m. PST on Saturday. It took Spirit nearly two weeks after its own Jan. 3 landing to unfold and roll onto the martian soil.

Spirit began malfunctioning early Wednesday, its 19th day on Mars, transmitting little more than random strings of zeroes and ones, after NASA commanded it to run a motor inside its 5-foot mast. The rover never completed the sequence of tasks.

Spirit's software has not worked properly since, confounding the "onion-peeling" efforts of engineers trying to uncover what is wrong, Theisinger said. Attempts to communicate with Spirit, either directly from Earth or through two NASA satellites in Mars orbit, have been spotty.

Spirit can stay in its current condition for some time while scientists work on the problem, Theisinger said. But the rover has been staying up through the night when it should be asleep. That can draw down its rechargeable batteries and trigger further problems.

The rover had taken thousands of pictures and had begun to carry out its first analysis of a martian rock when what had been a steady flow of science data came to an abrupt halt.

As for the other rover, Opportunity should signal Earth within minutes of its landing, just as Spirit did.

Opportunity's target is an area called Meridiani Planum, whose terrain scientists believe will be dramatically different from all the previous rosy-tinged pictures from Mars. Meridiani is expected to be dark gray or black and relatively dust-free.

The region is believed to be rich in a mineral called gray hematite, which typically forms in marine or volcanic environments rich in water. The landing site is 45-mile-long ellipse that is one of the smoothest and flattest places on Mars.

"Expect the unexpected," said Jim Garvin, lead scientist for NASA's Mars exploration program.

To the west of the landing site, the landscape appears corrugated. Scientists are not sure what formed the features. They do know they don't want Opportunity anywhere near them.

"If we landed there, it would be like plowing your way out of a labyrinth," said John Grotzinger of the Massachusetts Institute of Technology, a member of the science team.

Not since the 1976 landing of the twin Viking landers has NASA had two working spacecraft on the surface of Mars. Sending two rovers was seen as a way to increase the chance of success: Only one in three international efforts to land on Mars has succeeded; some of the other spacecraft blew up, crashed or disappeared.

The casualties may include the British lander Beagle 2, which has not been heard from since attempting to set down in December.

"We are not yet good at this," said Rob Manning, manager of the entry, descent and landing phase of the NASA project. "We're not at the point where you get in your car, turn the key, and get to your destination."

Meanwhile, European scientists reported Friday that the Mars Express, a European Space Agency spacecraft in orbit around the planet, has found the most direct evidence yet of water in the form of ice on Mars. The spacecraft detected molecules vaporizing from the Red Planet's south pole.

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