Rover rescue rehearsed

Posted: Wednesday, July 08, 2009 7:02 PM by Alan Boyle

NASA / JPL-Caltech

After commanding a test rover to drive forward through a sandbox filled with fake Martian dirt at NASA's Jet Propulsion Laboratory, rover driver Paolo Belluta measures how much the rover slipped sideways during the maneuver.

An earthbound rover is finally spinning its wheels in fake Martian dirt - marking one small step in NASA's efforts to get the real thing out of a Red Planet sand trap.

The first wheel-spinning test took place on Monday at NASA's Jet Propulsion Laboratory, where engineers have filled a slanted sandbox with fake Martian dirt. A test copy of the Mars Exploration Rover was buried in the stuff up to near the top of its wheels, with a rock sitting underneath.

The Spirit rover is in a similar fix on Mars, in a patch of slippery soil known as Troy. Spirit has roamed over almost five miles of Martian terrain in the five and a half years since it bounced to its landing - but it's had to tarry at Troy for more than two months.

Engineers at JPL are carefully considering the best method for freeing up the six-wheeled scientific star. One of those six wheels is no longer working - so to simulate the situation more accurately, the engineers are using only five of the test rover's wheels. NASA says they're also making accommodations for the fact that the rover on Earth is three times as heavy as the machine on Mars.

In this week's first test, the engineers tried out the simplest scenario for getting Spirit out of the sand: driving forward with all five wheels. The wheels made enough revolutions to go tens of yards if they could have found traction, but all the rover did was move slightly forward as well as sideways down the sloping sand.

Engineers' dilemma, scientists' delight
Over the weeks to come, engineers will be trying out a variety of other scenarios, an exercise that will eventually help them choose the best path for setting Spirit free. In the meantime, scientists are intrigued by the sandy layers they're seeing at Troy - so intrigued that they may be sorry to see Spirit go.

The evidence suggests that Martian winds have sorted the sand grains into those layers, and that they've been cemented into place by thin films of water. It was just the engineers' bad luck, or the scientists' good luck, that Spirit broke through the crusty top layer and got caught up in the disturbed soil beneath.

Speaking of good luck, Martian winds have swept off Spirit's solar arrays so well that the rover has more electrical energy at its disposal that it has had in years. To keep the batteries healthy (and keep the rover from overheating), Spirit has been burning off some of that energy at twilight and at night.

Universe Today highlights one of the astronomical images that Spirit captured, showing the star Canopus in the night sky. You'll also find plenty of views of a Martian sunset among the raw images being sent down by Spirit. All these observations are designed to gather more data about the Red Planet's dust-filled atmosphere.

Speaking of sunsets, one of the top atmospheric scientists on the Mars rover team, Mark Lemmon, explained why Martian sunsets can look blue in an explainer we published 11 years ago, based on Mars Pathfinder data. A similar phenomenon is behind the blue moon that is currently visible from dust-swept Tehran. You'll find further details and imagery at SpaceWeather.com.

What about Oppy?
Spirit's mechanical twin, the Opportunity rover, has also captured blue sunsets during its five-plus years of operation. Right now Opportunity is making its way toward 14-mile-wide Endeavour Crater, the biggest crater ever targeted by the rover. But it's had to make its way somewhat more slowly than expected, JPL spokesman Guy Webster told me.

Engineers are starting to see electrical problems relating to the rover's right front wheel - the very same wheel that went out on Spirit. As a precautionary measure, Webster said the rover drivers are giving Opportunity more rest periods and also mixing up its travel modes. The rover might go on a series of short drives rather than one long haul, for example, or turn itself around and drive in reverse. Which all goes to show that you can teach an old rover new tricks after all.

Update for 1:35 a.m. July 9: You've gotta see this! Venus and Earth, as seen from Spirit's vantage point on Mars. The good folks at UnmannedSpaceflight.com have put together the rover's sky imagery to create a movie of our tiny, pale gray dot moving relative to brighter Venus. (Venus is closer to Mars than Earth is right now, and that's why our planet looks dimmer, as explained in this blog posting.)

If you scroll down the UnmannedSpaceflight forum page a little farther, you'll see a dandy little movie of a Martian sunset as well.

Mark Lemmon, the expert on Martian skies who is now based at Texas A&M, basically pointed me toward these gems when he sent this e-mail late Wednesday:

"...We have continued with the night imaging as described earlier. Earth shows up passing by Venus during the observations we made. Earth is faint, but distinguishable, and Venus is brighter. The UnmannedSpaceflight.com forum has an animation made from uncalibrated images. The are a few other star fields we've imaged, but last I checked they were still in the queue to downlink.

"We are going to slow down the night campaign since we have to juggle many resources, not just energy. In this case, the plan complexity has increased too much, and we need to balance using energy efficiently at night with the need to come up with safe plans every day. That won't stop night ops, it just means we cannot do too many complex things in any given plan."

Thanks to Mark for the pointer, and thanks to UnmannedSpaceflight.com for their good works.

Update for 9 p.m. ET July 9: The rover testing team has worked its way through four of the nine scenarios for getting Spirit out of the sand trap, according to Sharon Laubach, integrated sequencing team chief for the Mars Exploration Rover project at JPL. Among those strategies are driving forward, straight out; driving backward; making a crablike motion forward and uphill (to push away from a potential obstacle); and turning downhill in place.

Has the test rover popped out of its sandbox using any of those strategies? "I wish!" Laubach told me.

"While we do see progress, it is very small progress with high slippage," she said. "We are seeing motion, and it is promising. But from what we see on Earth, we still believe it is going to take a while once we start the extraction process on Mars."

As for Spirit's twin on Mars, Laubach said that Opportunity was due to hit the road on Friday after two weeks of rest. Actually, that's two weeks of rest for the wheels - but the rover team has been keeping Oppy's robotic arm and scientific instruments quite busy in the interim. "We stop in scientifically interesting areas so we can make the best use of Opportunity while she has to rest her wheel," Laubach said.

That reference to "her" is not a typo, by the way. For years, the rover team has thought of Oppy as female ("Little Miss Perfect," in the words of principal investigator Steve Squyres).

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