NASA's best effort to find a missing Mars space probe have failed, scientists said Tuesday as they began to lose hope for the 10-year-old planet-mapping workhorse.

After more than two weeks of silence from the Mars Global Surveyor, NASA will make other tries to locate it, but scientists were pessimistic.

"We may have lost a dear old friend and teacher," Michael Meyer, the lead scientist for NASA's Mars Exploration Program said in a news conference.

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The $154 million surveyor, which was supposed to last only two years, but continued sending data for almost a decade, is the oldest of six different active space probes on or circling the red planet.

Among its accomplishments are more than 240,000 pictures of Mars, offering the best big-picture view of the planet.

Meyer credited the probe with proving that Mars once had water.

"Every good thing comes to an end at some point," said Arizona State University scientist Phil Christensen. "It certainly in my mind greatly exceeded our wildest expectations of what to hope for. It revolutionized what we were thinking about Mars."

On Monday night, NASA had hoped to catch a glimpse of the surveyor from the camera on the new Mars Reconnaissance Orbiter. But the orbiter failed to spot it.

Now NASA will try an even less likely search effort.

Engineers will send a signal to the silent spacecraft, asking it to turn on a beacon on one of the two Mars rovers below.

If the rover beacon turns on, NASA could figure out where the lost Mars Surveyor is, said project manager Tom Thorpe.

"While we have not exhausted everything we can do ... we believe the prospect for recovery of MGS is not looking very good at all," said Fuk Li, Mars program manager at NASA's Jet Propulsion Laboratory in Pasadena, Calif., which controls the probe. "We're still holding out some hope."

NASA will keep trying small-scale communications efforts. Experts believe the surveyor, which lost contact Nov. 2, probably developed a problem with a device that moves solar panels, causing it to lose communication.

The entire Mars Global Surveyor program cost $247 million, including launch expenses and a decade of in-flight operations. NASA had just approved a two-year mission extension for $6 million a year.

Launched on Nov. 7, 1996, the probe gave scientists the best topographic map of any planet in the solar system, said Cornell University astronomer Steve Squyres, who didn't have an instrument on board the probe but was part of NASA's scientific review team.

The probe gave Earth its first detailed views of massive Martian dust storms and gullies. It also revealed a new mystery about Mars: It once had a magnetic field.

The low-cost probe rose "from the ashes" of a dramatic Mars failure, Squyres said.

In 1993, the $813 million Mars Observer disappeared just before getting to the planet. Most of that probe's instruments were built again and included on the Mars Global Surveyor.

Christensen called the global surveyor "a workhorse" because of its numerous and diverse scientific instruments.

"It really has opened up new vistas of Mars that we hadn't the foggiest notion of," said Arizona State University geologist Ron Greeley.