NASA?s decision this week to give up trying to move stuck Mars rover Spirit marks a major turning point in the plucky robot?s storied six-year history on the red planet, scientists say.

On Tuesday, NASA said that Spirit will stay permanently trapped in the deep Martian sand that snared it in the red planet's southern hemisphere in May 2009.

?It?s kind of a poignant moment for us, you know,? said Steve Squyres of Cornell University in Ithaca, N.Y., principal investigator of Spirit and its twin rover Opportunity, during the announcement. ?We built these vehicles with the intention of driving around the surface and Spirit has done that magnificently for the better part of six years.?

Though it's sad to see the rover stop roving, scientists say, Spirit can still do important work while standing still.

"There's months, if not years, of good-quality leading-edge landed science we can do," said John Callas of NASA's Jet Propulsion Laboratory in Pasadena, Calif., who is project manager of the rovers.

Engineers are now trying to work the Spirit rover into a pristine, permanent parking spot on Mars ? one that will position the vehicle in the best possible angle toward the sun to feed its top-mounted solar arrays to generate power. Meanwhile Opportunity is still mobile, and is currently making its way to a large crater named Endeavour.

Spirit's transition from rover to stationary outpost on Mars is emotional for the scientists who have worked with the vehicle since it landed on Mars in January 2004.

"We're scientists and engineers, but we're human too," Squyres told SPACE.com. "Over the course of a mission like this you get very attached to these things. You attach so much hopes, dreams, frustration, pride. When they're doing well, you're proud of them and when they do badly you get angry and if they're about to stop doing what they do best, that's sad."

But despite that sadness, mission managers are still thrilled that Spirit kept moving this long.

"Every morning I come into work thinking, 'Boy, are they still alive?'" Squyres said. "These things were designed to last three months and it's been six years."

Indeed, Spirit and Opportunity were both originally commissioned for 90-day missions, and were designed to travel about 0.6 miles (1 km). Spirit has trekked across 4.7 miles (7.5 km), and Opportunity has traveled more than twice that far.

"We voided the warranty so long ago," Squyres said. "Anybody who tells you that they expected Spirit to last this long, I think, is lying."

Given that the rovers are now in uncharted, unexpected territory, it's tough to predict how much longer they'll last. Spirit especially will be vulnerable when Martian winter sets in a few months from now, because the sun will dip so low in the sky that the rover may have trouble gathering enough solar energy to run itself and insulate its hardware. Opportunity at least can adjust its position to maximize its sun exposure.

Even if the end is nigh, the scientists said the rover missions have been a resounding success.

"We're so far beyond predictions," Callas said. "We're amazed that we got this much."

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