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The clock is ticking for stalwart Spirit rover

Posted: December 16, 2009

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The Spirit rover is facing an approaching deadline to drive out of a Martian sand pit, as engineers on Earth race to analyze the health of two wheels and a potentially significant electrical glitch.

Teams have been trying for a month to extricate the stuck rover from a field of thick sand. Spirit unintentionally drove into the sand trap in April, and the rover has not moved since then.

Spirit is now in an unfavorable orientation to collect sunlight for power production during the approaching Martian winter. Officials think the rover's chances for survival are slim at its current location.

"Now is the time to do something because we have the energy to do something. If we wait too long, we will have fewer and fewer options," said John Callas, project manager for the Spirit and Opportunity rovers at the Jet Propulsion Laboratory in Pasadena, Calif.

Callas said Spirit's survivability will become an issue after March, when the sun's elevation in the sky could be too low for the rover's solar panels to produce enough electricity. The timing is uncertain because of changing dust accumulations on the solar panels.

The winter solstice in the southern hemisphere of Mars is on May 13.

Despite repeated attempts to free the rover, Spirit has barely budged. More than 30 feet of wheel motion has moved Spirit just a fraction of an inch.

Drivers haven't commanded the rover to move since late November. Spirit's right-rear wheel unexpectedly stalled, and engineers have spent the last two weeks conducting diagnostic tests to determine the cause of the unusual behavior, according to Callas.

Officials initially feared Spirit would be left with four operating wheels. The rover's right-front wheel has not worked since 2006, and engineers attributed its failure to a bad actuator.

Since the 2006 wheel failure, controllers have driven Spirit backwards, dragging the bum wheel behind the rover.

An image of Spirit's right-front wheel. Credit: NASA/JPL
But during tests last weekend, the right-front wheel showed signs of working. The indication came during a rotor resistance test in which controllers apply low voltages to wheel motors to verify electrical continuity, Callas said.

The right-rear wheel continued to exhibit high resistance and showed no movement, but the right-front wheel motor made one revolution, moving the wheel by one-quarter of a degree. Previous test results indicated an open circuit in the right-front wheel in 2006.

"The resistance measurements for the right-front wheel showed it to be normal," Callas said.

Callas cautioned the results were preliminary and there could still be a problem with a motor or actuator in the right-front wheel.

Engineers are currently analyzing an unusual voltage in the mobility system that could be the cause of the wheel troubles on Spirit.

"There's a voltage we measure between what's called the spacecraft chassis and the spacecraft electrical ground, and that voltage is normally zero and it has been zero for most of the mission," Callas said. "We're now seeing a voltage show up on that that seems to be correlated with the drives."

Spirit's other four wheels are showing no signs of trouble, according to project officials.

"It's suggesting there might be an electrical problem with the mobility system, specifically those wheels in question," Callas said.

Engineers are in the early stages of diagnosing the electrical anomaly, and Callas wants to be sure it is not indicative of a larger problem affecting other parts of the rover.

"Our concern, now that we've observed this electrical problem, is we don't want to brown out the rover," Callas said. "We don't want to cause a short somewhere that could potentially be devastating. We have to proceed cautiously, so we're trying to investigate this as quickly as we can. The difficult thing is we may not come up with any kind of clear path forward on how to deal with this electrical problem."

The rover team spent six months developing intricate plans to drive Spirit out of the sand pit, but engineers now have less time for analysis and must act quickly because of the upcoming winter.

"We have to start doing something with the vehicle because we're running out of time," Callas said.