Spirit's Quiet Anniversary

Space diehards took a moment yesterday to observe an exploration milestone. It was seven years ago, January 3, 2004, that a rolling robot named Spirit landed on Mars.

Together with its twin, Opportunity, which arrived 22 days later, these Mars Exploration Rovers easily exceeded their basic 90-day mission plans and have since surpassed even the most optimistic expectations.

But 2010 was not an easy year for Spirit. Having become entrenched — literally and unfortunately — in a patch of soft sand in May 2009, the "elder" rover struggled to free itself despite having lost the use of two of its six wheels. It was beginning to make some hard-won progress when, a year ago, Martian winter set in. The craft fell silent in March and hasn't been heard from since.

Engineers at the Jet Propulsion Laboratory know that Spirit put itself into electronic hibernation to conserve energy until enough sunlight could fall on its solar-cell panels to recharge its batteries. But the depths of winter have come and gone at its current location, and the rover should have awakened by now.

(For the record, Spirit is mired in a soft spot called "Troy", on the lip of the tiny crater "Scamander", which is next to the small plateau "Home Plate", located inside much larger Gusev crater.)

The team now believes the rover got so cold (down to at least –148°F (–100°C) that the internal clock lost power and the spacecraft has lost track of time. This is the first time winter's cold has forced either rover to hibernate, so engineers are in untried territory. But according to the MER playbook, Spirit should now be waiting for a command from Earth to wake up and report in. It's just that the rover is only listening for 20 minutes each day — and it's impossible to know when that window is occurring.

Although Spirit's handlers remain optimistic that it will revive, the mission's periodic status reports have been discouragingly headlined "Spirit Remains Silent at Troy" since August.

A dust storm swept over the area in October, and the winds might have either partially cleaned the power panels of accumulated grit (a plus) or added more to what's already there (a definite minus). The sunlight's strength should peak in about two months, and the wake-up calls from Earth will continue until then and beyond.

Meanwhile, Opportunity is faring somewhat better. It's more than midway through a long-distance drive to Endeavour, a largish crater about 14 miles (22 km) across. Recently Opportunity has been rolling along backward, an attempt to reduce wear on its wheels' aging gear trains.

Mission scientists have interrupted Opportunity's trek a couple of times to take in the local scenery. A few months ago it chanced upon a meteorite, which scientists dubbed Oileán Ruaidh (the Gaelic name of an island in northwestern Ireland). Right now the rover is checking out Santa Maria, a relatively fresh crater about the size of a football field.

Opportunity is still about 4 miles (6½ km) from Endeavour, but the spacecraft's cameras can already see its uplifted rim. Once the scientific study of Santa Maria ends, the rover will make a beeline for what promises to be the mission's grand prize: a geologic candy store of outcrops along the rim that could reveal much about the role of water in the planet's past.

For a much more thorough review of the rovers' recent trials and triumphs, be sure to read A. J. S. Rayl's excellent update for the Planetary Society. You can also get status reports and recent images at the mission's website.