The Voyagers are, perhaps, the best known example. Launched in the 1970s to explore the outer planets, the iconic spacecraft have zoomed far beyond their original targets to the edge of interstellar space itself, 9 billion miles from Earth and still making discoveries. Pioneer 10 and 11, Ulysses, Stardust-NEXT, Deep Impact, and others have similar track records. It has become almost routine for superbly-engineered probes to wrap up their prime missions, then travel a few million (or billion) bonus miles for extra science.

Against this backdrop of sweeping overachievement, we pause to salute a robot that might never move another inch.

Well done, Spirit.

"For the past 7 years, Spirit has been on a journey as extraordinary as any mission in NASA history," says Mars Exploration Rover project manager John Callas of JPL. "But now it may be time to say thanks and farewell."

NASA hasn't heard from Spirit in more than a year, and on May 25th, 2011, the agency sent a final transmission in its series of attempts to regain contact.

The trouble began in April 2009 when the rover trundled into a sandtrap in a place called "Troy," breaking through an apparently safe crust into soft sand below. Stuck in place, Spirit couldn't turn its solar panels squarely toward the sun; at the same time, dust accumulated on the panels, reducing sunlight even more. These impediments curtailed power just when Spirit needed power most, during the deep freeze of an approaching Martian winter.

"Where Spirit is, winter temperatures drop as low as -130 C, far colder than any place on Earth," says Callas. "Without sufficient electricity to power internal heaters and warm critical systems, Spirit went into hibernation."

NASA has used Deep Space Network antennas and two of the agency's Mars orbiters to try to reestablish contact--but no luck. Whether the rover is damaged or merely "sleeping," no one can say, but most engineers believe the possibility of contact is now extremely remote accomplishing its primary science goals, the rover went on to work for almost six more years.

In all, Spirit has traveled almost 8 km, explored several large craters, scrutinized thousands of rocks, scraped off topsoil to reveal hidden minerals, photographed Martian dust devils and sunsets, observed the moons of Mars, and took the first picture of Earth in the night sky of another planet. Bonus-time, indeed.

Asked to name Spirit's top scientific discoveries, Callas lists three:

(1) Evidence of ancient hot springs. "This came about because of the failure of one of Spirit's wheels," he recalls. "Two years into the mission, the right front wheel stopped working--we're still not sure why. Spirit had to drag it along, cutting a furrow in the ground. This revealed deposits of amorphous silica widely thought to have formed in hydro-thermal systems. Apparently, Mars once had water and the energy to warm it. We might never have found this if not for the serendipity of the broken wheel."

(2) Evidence of a thick atmosphere and "sweet" water. Today the atmosphere of Mars is so thin, most life as we know it couldn't survive there. Spirit's discovery of carbonates at the Comanche Outcrop is compelling evidence that it wasn't always so.

Callas explains: "The carbonates Spirit found formed in surface water that could only exist with a thick atmosphere sitting on top of it to prevent rapid evaporation. Moreover, the chemistry of the carbonates tells us that the water wasn't acidic like other ancient water on Mars." Life would have liked this place, billions of years ago.

(3) Evidence of an active water cycle. The first thing Spirit did when it got stuck at Troy was to try to break free. Spirit's spinning wheels churned up the soil, uncovering sulfates. "These minerals appear to have come in contact with water perhaps as recently as a million years ago," says Callas. In geological terms, that's very recent, suggesting an active water cycle on the Red Planet.

Alone, any one of these discoveries would have been considered a resounding success by the mission's original planners in the 1990s. All three, plus others not listed, place Spirit squarely atop the pantheon of NASA's great overachievers.

Meanwhile, Spirit's twin rover Opportunity is halfway across the red planet, still going strong.

"Opportunity is in good health," says Callas. "The rover is about to log 30 km of distance since landing in 2004. We never dreamed of such a trek when the mission began. And years of additional service appear possible."

He cautions that Mars is a dangerous place, and the rover itself is in its senior years. At any moment, Opportunity could be engulfed in a storm, fall in a sandtrap, or simply break down due to old age.

When one of these things inevitably happens, Opportunity will join Spirit as a silent monument to grit, luck, intelligence ... and oh so many extra miles.