It was only supposed to survive three months on the Martian surface; it lasted 6 years. What it discovered with its array of instruments and intrepid camera has literally changed the way we look at the planet Mars, while adding vastly to the storehouse of human knowledge.

The rover Spirit, along with its indomitable companion rover Opportunity, was a mission done on the cheap by NASA standards. The dual rovers cost the taxpayer around $800 million - about 1/3 less than most interplanetary missions. Now Spirit - silent for more than a year - has officially been retired as NASA will no longer listen for a sign of life.

New York Times:

Spirit was a spectacular success. A three-month mission, beginning in 2004, turned into six years of exploration. Even the accidents were profitable. When Spirit bogged down, permanently, at a location called Troy, efforts to free it revealed unexpected subsurface sulfates, which scientists believe are part of the Mars water cycle.

This is not like calling off the search for a missing human explorer. And yet it feels similar, even though Spirit is a six-wheeled robotic vehicle, not even remotely human in appearance, even by Wall-E standards. Still, it is strangely easy to personify Spirit. Over the years, it has seemed intrepid, valiant, determined. It has no consciousness, but there has been something self-knowing in the photographs it has taken of itself, with Mars in the background. In its plight - stuck on the edge of a small crater tens of millions of miles from Earth - we feel a celestial solitude, as if we were marooned there ourselves.

What made Spirit all these things, of course, were the engineers and scientists who built and operated it, who reveled in the data it returned and who did their best to keep it running, year after year. The most human thing about Spirit, after all, was the impulse that sent it to Mars in the first place, a planet we now know in a way that would have seemed unimaginable only a decade ago.

The spectacular panoramic pictures sent back by the rover along with its ingenious array of geological instruments (it had the ability to drill into a rock and "smell" what it was made of) made Spirit one of the most successful probes in NASA's history. Meanwhile, Opportunity continues its explorations with little sign that it is slowing down.

One day, man will walk on the surface of Mars. When we do, we will owe a debt to the scientists and engineers who designed and operated the Mars Exploration Rovers for 7 years now and counting.