WASHINGTON — NASA's best efforts to find a missing Mars space probe have failed, scientists said this week 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.

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. 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.