Mars Global Surveyor, the hardy ship that has orbited Mars since 1997, has not been heard from in two weeks, and as Michael Meyer, NASA's lead Mars scientist, put it, "We may have lost a dear old friend and teacher."
The robot probe, launched from Florida in November 1996, took more than 240,000 images of the Martian surface during its mission and helped establish that in the distant past, there had probably been flowing water on Mars.
But in early November a solar panel began to have trouble pointing properly at the sun to provide electricity, and the ship automatically turned to compensate -- possibly cutting off communications.
Over the weekend, NASA ordered a newer probe, Mars Reconnaissance Orbiter, to try to take a long-distance picture of Global Surveyor. But engineers, who stayed up into the early hours this morning looking at the results, say the pictures revealed nothing.
They will try one more trick -- to see if Global Surveyor relays a signal through Opportunity, one of the two rovers now driving around the Martian surface. But the odds of success, they say, are slim.
"While we have not exhausted everything, we believe the prospect of recovery is not very good at all," said Fuk Li, Mars program manager at NASA's Jet Propulsion Laboratory in California.
"We are still holding out hope, but we are prepared for the mission to be over," he said.
At the time of its arrival on Mars, Global Surveyor was overshadowed by the flashier Mars Pathfinder -- the first ship to land on Mars since 1976, and only the third to do so. Pathfinder deployed a small rover, called Sojourner, that puttered around the landing sites, sniffed at rocks, and won the hearts of millions of earthlings.
Meanwhile, Mars Global Surveyor was scanning the planet from one pole to the other. It spotted thousands of gullies as it passed overhead; geologists said they had clearly been formed -- perhaps just a few thousand years ago -- by water spurting from underground aquifers.
NASA's mantra at the time was "follow the water" -- that would be a sign of whether Mars could ever have been a good environment for life. Mars Global Surveyor went looking for water, and successfully got its feet wet.
Did the mission pay? NASA managers say the original budget to build the spacecraft was $156 million -- a hefty hunk of change, for sure, but a bargain by space-program standards. Assembly actually came in slightly under that.
Add money for the Delta rocket to launch it, and funding to keep it running during its mission, and the total comes to $377 million.
By its later years, NASA was running the Global Surveyor project for about $6 million annually; about 20 scientists and engineers worked on it part-time. In addition to images and other measurements of Mars, the ship scouted out landing sites for later probes and relayed their signals once they arrived.
The original plan called for Mars Global Surveyor to last until around 1999, about two Earth years orbiting Mars. Instead, it worked for a decade.