Editorial: Indefatigable Spirit
NASA’s twin Mars rovers have outlasted their planned three-month missions for so long that they seem indestructible. Nearly six years on, their presence on the Red Planet is taken for granted, as if they are immutable parts of the Martian landscape.
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But we may soon have to confront a new reality. Spirit, which has always suffered more hardships than Opportunity, is facing its toughest challenge yet. When Â鶹´«Ă˝ went to press, the rover was set to begin a risky push to free itself from a sand trap it has been mired in for six months. Mission engineers say it may not survive the attempt. “She’s in a very precarious situation, and we don’t know for sure if we’re going to get her out,” says rover driver Scott Maxwell of NASA’s Jet Propulsion Laboratory (JPL) in Pasadena, California.
Spirit encountered its latest difficulty in April, as it made its way south along the edge of Home Plate, a plateau of hardened ash that has been its main haunt for more than three-and-a-half years. Its wheels started to slip in the soft, sandy soil filling an 8-metre-wide crater. By the time the rover team decided to stop driving, Spirit was essentially just spinning its wheels.
Both rovers have managed to escape sand traps before, but this time it’s different. Spirit is tilted by 12 degrees, with its three left wheels buried almost entirely in the sand (see illustration opposite). Unfortunately its right-front wheel, which is the least buried, is useless – it seized up in 2006 and has been dragged or pushed by the remaining five wheels ever since.
To make matters worse, a pointed rock appears to be touching the rover’s belly. So if Spirit’s wheels sink further as it moves, the rock could end up bearing some of the rover’s weight, further reducing traction. “I look at it as on steroids,” says Spirit scientist of Washington University in St Louis, Missouri.
As a result, NASA has been extremely cautious about moving the rover, spending months testing escape manoeuvres on two prototype rovers at JPL.
But now, with the Martian winter approaching, a review committee has recommended that NASA start moving the mired rover as soon as possible. Engineers will attempt to drive it out the way it came in, a plan likely to involve a combination of driving straight and angling the rover’s wheels uphill to aim for firmer ground.
Progress will be slow, as rotating the wheels enough to produce what would ordinarily be 5 metres of motion might only cause the rover to move a millimetre or two, says , rover project manager at JPL. “It will be kind of like watching your grass grow,” he says. Arvidson adds that there are no guarantees the rover will escape. “This vehicle is not invincible,” he says. “We may not get out.”
At this critical crossroads, Â鶹´«Ă˝ is taking the opportunity to look back at the rovers’ highs and lows.
Slow start
NASA’s twin rovers, Spirit and Opportunity, landed on opposite sides of the Red Planet in January 2004.
Almost immediately after touching down, Opportunity found evidence that its landing site at Meridiani Planum had once hosted ancient acidic lakes.
Spirit was not so lucky. Orbital images had suggested that Gusev Crater, where it landed, was once a lake. But for its first six months on the planet, the rover turned up little besides basaltic lava flows – which may have paved over any ancient lake sediments.
High ground
Spirit enjoyed an upturn in its fortunes when it reached the higher terrain of the Columbia hills in June 2004. The hills, named in honour of the astronauts who died in the shuttle disaster of 2003, are even older than the 3-billion-year-old volcanic plains on the floor of Gusev Crater. Here, Spirit found intriguing sulphur-rich rocks that appear to have formed in the presence of water. After cresting the Columbia hills in March 2006, Spirit’s right-front wheel seized up. That forced engineers to drive the rover backwards, dragging the wheel behind it. Fortunately, the rover managed to limp to a sunlit slope to sit out the winter.
Watershed discovery
In May 2007, scientists reported that Spirit had made a major discovery: it found evidence of rocks made almost entirely of silica, a telltale sign they were formed in water. Ironically, its immobilised wheel – which dragged behind it like a dead weight – gave rise to the find when it uncovered a patch of bright, silica-rich soil.
The soil likely formed when large amounts of water interacted with hot volcanic material. This area, dubbed “silica valley”, lies near a raised plateau called Home Plate that may have formed when layers of volcanic material filled in a crater whose surroundings then eroded away.
Opportunity’s trials
Opportunity has always been the “golden” rover, travelling about 19 kilometres – 2.4 times as far as Spirit – since it landed. But it too has suffered setbacks. In April 2005, Opportunity got stuck for about five weeks on a ripple of soil nicknamed Purgatory Dune.
When it got stuck, its priority had been speed. It was cruising along at 200 metres a day and most of its software safeguards – which could have detected its wheels slipping – were turned off. That caused the rover to dig itself in more deeply than it would have otherwise. Engineers learned from the mishap, freeing the rover by backing it out as it had come in and instituting periodic “slip checks” for its wheels. In June 2006, those changes helped the rover quickly escape from a sandy spot nicknamed Jammerbugt – Danish for “Bay of Lamentation”.
Editorial: Indefatigable Spirit
Still working
Spirit has been immobile for the past six months, but it has not been idle.
The rover’s own observations have revealed the cause of its plight – a small crater filled with yielding, yellow-brown sand. The sand had been hidden beneath a dust-covered crust of weakly cemented sand particles.
Spirit’s wheels punched through this centimetres-thick crust, exposing soil with the highest concentration of sulphate minerals ever found by either Spirit or its twin, Opportunity.
Sulphates form in the presence of water, so the find further reinforces the idea that, billions of years ago, the area surrounding Spirit was rich in the liquid.
Researchers suspect Spirit’s stomping ground was once a site of intense hydrothermal activity. Pools of hot water and steam vents may have dotted the area, making it a good place for future missions to look for evidence of ancient Martian life, says Spirit scientist of Washington University in St Louis, Missouri. “If it were Earth with that kind of environment, it would be teeming with microbes,” he says.
Nonetheless, the rover team is keen to move on. “It’s an interesting area to be in over the summer,” says Arvidson, “but we’re ready to leave.”
The local winter will descend in the first half of 2010, and NASA is anxious to move the rover before light levels drop drastically, cutting power to its solar panels. The rover might be able to survive the winter in its current orientation, but “just barely”, says rover project manager John Callas of NASA’s Jet Propulsion Laboratory in Pasadena, California. “Spirit will be one dust storm – or change in dust accumulation – away from not surviving.”
Where will Spirit go if it does make it through? Two long-dead volcanoes 170 metres to the south – one with a wealth of exposed bedrock – would be compelling destinations.
