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Stranded rover will become a static science base, if it can last through the coming Martian winter

2 Дек 2014

rover21-fullNASA is in a race against time to prepare its stranded Mars rover Spirit for survival through the upcoming Martian winter after abandoning further attempts to free the bogged-down vehicle.
The decision, which effectively marks the end of Spirit’s six-year sojourn across part of Mars’s southern hemisphere, follows nine months of foiled attempts to dislodge the rover from a sand trap dubbed Troy.
However, if Spirit can be maneuvered into a better position to collect solar energy–either on level ground or facing northward on a slant – NASA wants to maintain the rover as a stationary science platform. “It’s driving days are likely over, but its contribution will continue,” says Doug McCuistion, director of the Mars Exploration Program.
Operators at NASA’s Jet Propulsion Laboratory in Pasadena, Calif., were forced into the difficult decision following the failure of a second wheel on the six-wheeled rover during the most recent phase of extraction attempts. The key driver, says NASA, was the increasingly urgent need to prepare the vehicle for the lower temperatures of winter, which begins in March and reaches its depth in May.
“It’s a much more difficult and dangerous regime for Spirit,” says NASA Rover Project Manager John Callas.
The problem with the right-hand side rear wheel emerged during operations to try to free Spirit from soft soil where it has been trapped since late April 2009, when it broke through an unseen crust covering an old impact crater. The hope now is that operators at NASA’s Jet Propulsion Laboratory can maneuver Spirit on its four remaining wheels to receive sufficient solar energy through the winter.
“We’re counting on the fact we’re sitting on a hill, and if we can back up that hill, we can point the solar panels toward the Sun,” says Ashley Stroupe, one of the rover’s drivers. “We have maybe three weeks of driving activity before we have insufficient energy to do that anymore,” adds Callas.
In normal operations, the rover requires a sustained 160 watt-hours to maintain contact with Earth. As energy from the Sun wanes with the lower incidence angles in winter at Spirit’s high-latitude location at 16 deg. S., the rover will use more of its battery power. When the batteries drop below a minimum charge, Spirit will enter a hibernation mode called a low-power fault in which the vehicle automatically powers down all systems apart from its internal clock. It is programmed to briefly wake itself up to conduct daily internal checks, and will re-establish contact with NASA operators if it detects that its batteries are being recharged by the solar arrays.
“If we can get the right tilt, we can avoid tripping this low-power fault,” says Callas, who cautions that surviving the winter in hibernation mode holds no guarantees of a successful revival. “The concern is that temperatures will get very cold for Spirit, … down to -40 or even -46C,” he adds. However, if these fall to -55C, they could damage the vehicle’s electronics. “Right now our estimate is the rover will stay within design limits, but these were for a new rover fresh out of the box, not one that’s been on the surface for six years and endured grueling temperature cycles.”
If Spirit enters and successfully survives hibernation, JPL hopes it could be awakened in six months to begin a new life as a stationary science platform. While not ruling out movement using its four remaining wheels, the main focus is now shifting to conducting a new series of geophysical science experiments using the rover’s systems from a single location.
The prime new mission will involve tracking the rover’s radio signal to produce a three-dimensional picture of Spirit’s motion through space. The data will give scientists an unprecedented accurate track of a precise location on the Martian surface, enabling them to characterize the motion of the planet’s orbit and its inherent wobble. From this observation, scientists hope to answer once and for all whether the inner core of Mars consists of a solid structure, as now widely believed, or a liquid or semi-liquid core.
“There is a very strong belief Mars has an iron core because when you look from orbit you see the remnants of an ancient magnetic field,” says Steve Squyres, rovers principal investigator from Cornell University. Traces of the field are “frozen into the rock,” which provides “compelling evidence that Mars had a pretty powerful traditionally generated magnetic field. But now it does not have one, so something has changed. The question is, has the core frozen, or partially solidified?” he says.
Other studies will focus on local atmospheric-surface interactions and analysis of the highly unusual sulfate-rich soil in which the rover is bogged down.
Spirit landed on Mars on Jan. 3, 2004, and its twin, Opportunity, arrived later the same month. Originally expected to last for three months, both have continued exploration for six years, or 3.2 Mars years. Spirit discovered evidence of a “steamy and violent environment” on ancient Mars, ironically thanks largely to the failure of a front wheel in 2006 that forced JPL to drive the rover backward. “By dragging the wheel behind us we were digging a trench which revealed silica and sulfur that had to be put there by hot water,” says Stroupe. Among its other achievements, Spirit is also credited with taking the first photograph of Earth from the surface of another planet.
Opportunity continues to operate in the warmer equatorial region and is currently en route to a large crater called Endeavour. It began the 12-mi. drive to the crater in mid-2008, covering 3.3 mi. in 2009, its fastest travel rate since arriving on Mars. Overall, Spirit traveled almost 12 mi. during its roving days, while Opportunity has notched up a similar travel distance and has so far returned more than 133,000 images.

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