Crunch time approaches for a decision on how to free Spirit from a sand trap.
After being stuck in soft soil on Mars for six months, Spirit, one of two NASA rovers on the red planet, is about to attempt an escape.
"It's likely that this process will take months and we don't even know if we'll be successful," says John Callas, project manager for Spirit and its twin rover Opportunity at NASA's Jet Propulsion Laboratory in Pasadena, California. Spirit landed on Mars in January 2004 for what was originally planned as only a three-month mission.
The attempted drive out of the soft, floury soil that Spirit drove into accidentally in May this year comes after a seven-member review panel took a close look at the recovery effort on 28 October. The panel recommended that the rover project team should try to extract Spirit as soon as possible. Time is of the essence — as winter approaches, power from Spirit's solar panels wanes. "The coming winter could pose a risk to the rover," says Callas.
Back in May, as Spirit drove across a sloping area near the planet's equator called Home Plate, the solid crust gave way to reveal soft ground beneath, and Spirit's five (out of six) functional wheels couldn't get enough grip to drive out. Instead, the rover sank deeper like a car spinning its wheels in deep snow. The sixth wheel hasn't worked since March 2006.
Before getting stuck, Spirit had experienced other technical problems: its flash memory — used to store data overnight while the rover is powered down — occasionally failed to record any data. This also happened earlier this year, on the rover's 1800th Martian day, or sol, in January, and again on the 1874th and 1876th sols in April. On sol 2065, on 24 October, the problem returned. Callas explains that a putative cause for the latest flash-memory problem has been identified and instructions have been sent to Spirit to reformat the flash drive, which they hope will sort it out. If not, Spirit will need to be kept awake for longer to wait for the daily data-relay pass made by an orbiting satellite, instead of taking a nap beforehand.
The problem is, going in that direction is going to a point of no return, deeper into a treacherous place. John Callas , NASA Jet Propulsion Laboratory
Apart from recommending a rescue as soon as possible, the review panel also told the project team to reconsider their options for the rover's initial drive out. The panel suggested that the team should reassess the possibility of a turn slightly left and downhill, which would allow gravity to assist the rover's movement.
"The problem is, going in that direction is going to a point of no return, deeper into a treacherous place," says Callas. Going forwards would mean that the wheels become embedded deeper in their tracks, he says. The project team prefers the option of driving back out the way they went in.
Whichever route they choose, the manoeuvre is likely to begin as soon as 11 November. But safe ground is up to 2 metres away, and the rover will be going slowly — maybe only 1–2 centimetres a day. Each tiny movement will be scrutinized to check that the rover is not getting itself into deeper trouble. The process could take months.
The choice on which way to drive will not be made lightly. "It's a very, very tough decision," says Anders Elfving, manager for the European Space Agency's ExoMars rover, based at ESTEC, the agency's research and technology centre in Noordwijk, the Netherlands. The ExoMars rover is set to visit Mars in 2018. Spirit might also have a rock beneath its belly, Elfving says, which could leave it almost hanging in the air, making any attempted manoeuvres risky.
If the rescue attempt fails, Callas is hopeful of at least another year of science exploration by Spirit. "This is a scientifically interesting location," he says. The ground at Home Plate is the most sulphate-rich soil seen by either rover. This could imply that the soil contains minerals churned up by a volcano, says Callas.
Mark Bullock, an expert on Martian and other planetary atmospheres at the Southwest Research Institute in Boulder, Colorado, agrees. "Can Spirit produce useful science if it stays stuck? The answer is a resounding 'yes' because it can be our ground meteorological station for as long as it lasts," he says. Spirit's panoramic camera and thermal-emission spectrometer can point at the sky to monitor the levels of dust and cloud over time, and to reveal how atmospheric temperatures vary, Bullock says.
If the rover does remain stationary, there's also the possibility of using Spirit's arm to analyse the soil more closely and to measure the area's seismology. Ultimately, the rover might be destined to be a weather station. Bullock says that this future for Spirit is not so bad: "Retiring its journeys to become an active meteorological station is not a bad fate for an ageing rover. It should definitely not be turned off."
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Sanderson, K. Mars rover plans its escape. Nature (2009). https://doi.org/10.1038/news.2009.1066