A year ago last Monday, an ungainly little robot spacecraft named Spirit boinged down onto the rocky surface of Mars. Three weeks later, Spirit was joined by Opportunity, and the pair began separate exploratory sojourns designed to last about 90 days. Twelve months later, amazingly, they are still going.

Last year was not a good one for Earth. Looking back on it now with the perspective afforded by a week or so of life in 2005, it appears like one long diorama of unmitigated disappointment and disaster, from the ballooning misadventure in Iraq to the horror of last month’s killer tsunami.

It’s just as well that NASA’s two little rovers with the hopeful nicknames were far, far away on Mars, getting on with life oblivious to the bleakness on their home planet. They reminded us that good things do happen — and that sometimes they even work out better than they were supposed to.

People often question the social worth of scientific ventures, especially such costly and quixotic ones as space exploration programs. In the case of the Mars rovers, however, the answer is obvious: In a year that seemed custom-designed for pessimists, Spirit and Opportunity made the case for optimism. Reading of the futile search for Osama bin Laden, or the futile quest to end the murderousness in Sudan, or the futile push for a just peace in the Middle East, or even the futile efforts to restart the space-shuttle program, one is consoled by recalling how these golf-cart-size ambassadors have achieved goal after goal in their silent haven.

Never mind that the goals weren’t all that grand. Last week, NASA reported that Opportunity had climbed out of a crater and was poised to investigate its own discarded airbags. A Martian day has been counted a success if a rover scrubbed an inch of rock with its Rock Abrasion Tool Brush or laboriously dislodged “a potato-sized rock” from a wheel, as Spirit had to do last month. Grand goals or humble, the point is that the rovers reached them. Then they would wake up next morning, assimilate their assigned mini-goals for that day and go out and reach them, too.

Spirit recently climbed out of a crater and, according to NASA, “examined some of its own tracks that it had laid down prior to entering the crater.” (This is not as trivial as it may sound: Didn’t the philosopher George Santayana advise humanity 100 years ago to do the same with its own tracks lest it end up traveling them again?)

Their handlers at the Jet Propulsion Laboratory in California, of course, do not see the rovers’ goals as either trivial or philosophical — or as random as their daily log makes them sound. In fact, every minor trek and task undertaken by Spirit and Opportunity has been designed with major practical questions in mind.

In the foreground: Was there ever water on Mars? (That question has been persuasively answered in the affirmative by the rovers’ discoveries.) And in the background, lurking behind each tiny bit of evidence of a watery Martian past: Was there ever life on Mars, and did it arise independently?

Spirit and Opportunity were actually not equipped to answer those bigger questions. Once their scrutiny of rocks and minerals had confirmed the past presence of water, their mission was basically over. Questions about life on the red planet will be on hold until a more sophisticated explorer arrives in 2010. And yet the pioneering pair laid the groundwork.

No wonder we find the Mars mission updates so cheering. They remind us of the usefulness of taking one day at a time, focusing on the task at hand — no matter how nonsensically small — with faith that over time the mosaic-like bits will resolve into a picture.

Spirit and Opportunity have so far taken more than 50,000 photographs of the Martian terrain, from extreme close-ups to scenic panoramas. Individually, they don’t amount to anything very startling, at least to an untrained observer. Taken together, they create a view of another world that even the most uninformed layperson can see is stunningly complete. Somehow, the idea helps us carry on with our own messy lives and seemingly inconclusive projects.

Even more encouraging, though, is the rovers’ unexpected longevity. Everything they have learned since April, when their 90 days was up, has been pure bonus, what Horace Walpole called serendipity — the happy discovery, “by accidents and sagacity,” of things one is not in quest of.

As Earth steps gingerly into a new year, soured and shaken by the setbacks of the last one, we should think of that. Spirit and Opportunity are twin proofs that even though life seems very bleak just now, bonuses are still part of the mix.

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