NASA's Spirit rover (search) is in stable condition after sending three short streams of data, as engineers attempt to determine the source of its problems while also preparing for the arrival of another spacecraft on Mars.

For two days, the six-wheeled vehicle transmitted only gibberish or sporadic beeps to acknowledge commands from Earth. Coherent data about Spirit's status finally arrived Friday in three bursts lasting 10 minutes, 20 minutes and 15 minutes.

"We believe, based on everything we know now, we can sustain the current state of the spacecraft from a health standpoint for an indefinite amount of time," said Peter Theisinger, rover project manager at NASA's Jet Propulsion Laboratory, Pasadena, Calif., giving the team time to diagnose and perhaps fix the issue. 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.

Engineers have determined that Spirit's flight software has rebooted the rover's computer more than 60 times in the past three days. A motor that moves a mirror for the rover's infrared spectrometer was partway through an operation when the problem arose, suggesting a hardware problem as the root issue.

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 $820 million double mission to learn if Mars was once a wetter planet 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.

Opportunity should signal Earth within minutes of its landing, just as Spirit did.

Opportunity's target is an area called Meridiani Planum (search), 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 (search), which typically forms in marine or volcanic environments rich in water. The landing site is a 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, the Mars Orbiter Camera on NASA's Mars Global Surveyor orbiter snapped an image of Spirit's landing region that shows the spacecraft's lander platform on the ground. The jettisoned parachute, backshell and heat shield are also visible, noted Dr. Michael Malin of Malin Space Science Systems, San Diego, lead investigator for the orbiter's camera and a member of the rover science team.

The Associated Press contributed to this report.