The moment of best flying is called 'glass-off' - when the thermal plumes rise most visibly and this hopping strategy pays off for the pilot. For other times of the day, a distant cloud overhead is another good sign to fly towards as a likely sign of flying ease ahead.

It turns out that similar strategies for tracking the sun's warmth can be found hundreds of millions of miles away, on the surface of Mars.

Both Mars Exploration Rovers are fighters, beating the count a handful of times on Earth and on Mars. Having flown in the face of "old age" and impending demise, Spirit and Opportunity might also be able to resist the reduction of direct sunlight and Mother Nature's minions of martian winter clouds.

With the help of software and imagination, the rovers will not be scrambling aimlessly for their last tan; they'll be strategically driven to "hot spots" that hold the promise of endless summer.

"Spirit is headed for winter, being about 14 degrees south of the equator. Now the tilt of the axis is about 27 degrees. If you add 14 degrees to 27 degrees, then the angle of the sun from straight overhead is about 41 degrees down, in the worst part of winter," said Dr. Larry Soderblom, a rover science-team member from the US Geological Survey in Flagstaff, Arizona.

"That is a very severe penalty to the rover's solar panels and the ability to collect solar energy and convert that to electricity and store it in the batteries."

What the team refers to as "surface normal" tells what the ground is like: it points up on flat ground, a little to the side on a hill, or horizontal on a cliff. It is made from the 3-D maps.

"If you know where the sun is, you can compare the direction of the sun with the surface normal, or the tilt of the ground," explained chief software developer for Multimission Image Processing Lab (MIPL), Bob Deen.

"If you gather those two things and get the angle between those points and the position of sun, that tells you how squarely the sun is hitting the ground and, thereby, the solar panels."

When chasing the sun, however, it's important to know not just where hazards are, but where their shadows fall.

These voids are the latest, and perhaps the most threatening, hazard to the rovers as martian seasons begin to change. Colorized 3-D maps allow rover drivers on Earth to "feel the martian heat."

"Now, if we just drive the rover on flat surfaces, the amount of power we get out is about 350 watt-hours. So you can run a 100-watt light bulb for three and a half hours. It's not a lot of energy," Soderblom noted.

"However, if we can drive the rover to find little hills and depressions in the topography so that we can always keep our panels tilted toward the north, then we can boost that solar energy by a significant amount."

"Currently, instead of getting 350-watt hours on Spirit, we're getting about 410 by keeping the wheels on the north side of the spacecraft down. So we're driving in little pockets and so forth."

Likening the rovers to frogs, Soderblom whimsically refers to the optimal solar areas as "lily pads." The rover team needs to search strategically for these locations so their twin rovers can charge and recharge.

"If the rovers ended up facing the wrong way it would be bad," said MIPL's Bob Deen.

"If they ended up on a really, really bad slope they might not be able to wake up until [martian] summer. You could, essentially, lose your power for the day and you'd be in a world of hurt. Even a few degrees can make a huge difference."

Michael Sims, a research scientist from NASA's Ames Research Center and co-investigator on the Mars Exploration Rover mission, used the rover images to build virtual-reality models.

"Because we've been using these tools, we've been getting more power and that's given us a lot more options," said Sims. "If we continually, judiciously choose our paths, we might not even have to hibernate for martian winter."

"We'll probably have to slow down, but as long as we can maintain situations that are power-positive, we can have a power-intensive day that includes using the rover arm and its instruments."

So, when answering the frequently asked question, "when will the rovers stop working?", engineers and scientists can truly respond that it's up to the sun and the rovers.

Spirit and Opportunity will likely never reach their "golden years," but a little creative "leapfrog" is helping them stay in the ring and stave off retirement once again.

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