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Endless Summer?

August 09, 2004

This black and white image features thin, whispy clouds rolling over the stadium-sized 'Endurance Crater' on the rover Opportunity's 153rd sol on Mars.
Clouds Roll Over "Endurance Crater"
Using its left navigation camera, the Mars Exploration Rover Opportunity sought to capture some clouds on its 153rd sol on Mars (June 28, 2004).
Image credit: NASA/JPL
Link to Full Res
Martian Winter Moving In

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 25 degrees. If you add 14 degrees to 25 degrees, then the angle of the sun from straight overhead is 39 degrees down, in the worst part of winter," said Dr. Larry Soderblom, a rover science-team member from the U.S. 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."

This black and white image based on real mission data shows rough terrain rising to hills in the distance.
3-D Mesh Map of Spirit's Location on sol 192
This mesh map represents the topography of Spirit's location on sol 192. Creating this map is the first step in locating "hot spots."
Image credit: NASA/ARC
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Steering Clear of the Shade

Since each rover's mission began nearly eight months ago, rover planners have been using topographical maps generated from 3-D images. The team at JPL's Multimission Image Processing Lab (MIPL) reviews images taken from both of the rover's "eyes" at the same rover position and identifies common features in both view angles. They can then calculate triangles and figure out the distance of common objects from the rover. [Learn more about the art of engineering.]

This is a black and white image layered with colors that represent the places where the rover's arm can and cannot reach.
MIPL Reachability Map
Reachability map created by the MIPL team to determine which points are reachable by the robotic arm. Colors represent what areas are reachable by the various science instruments.
Image credit: NASA/JPL
Link to Full Res
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 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."

MIPL has been generating similar maps from the beginning of the mission that are used for precise placement of the rovers' arms. Now, instead of using a rock abrasion tool (RAT)-sized patch, they are using a rover-sized patch (created by rover driver Chris Leger).

This image features a cartoon mock-up rover placed on hilly terrain. The image is awash in mostly green and blue, representing good and medium solar exposure.
Solar Energy Map with Virtual Rover
A 3-D mesh map flooded with color allows rover drivers to hunt for "hot spots." Blue areas represent optimal sun exposure, while red patches indicate dangerously low solar exposure. The colors in between represent a moderate amount.
Image credit: NASA/ARC
Link to Full Res
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."

This is an image of Bob Deen, chief software developer for MIPL sitting at his computer creating a terrain map.
Colorizing Martian Terrain
Bob Deen, chief software developer for MIPL, creates a 3-D map on his computer.
Image credit: NASA/JPL
Link to Full Res
Rovers Leaping to Get the Blues

Colors have illuminated shadows and are helping rover planners define the terrain with which they are dealing. Red represents dangerous, low-sun areas and blue identifies the solar-rich sections. The colors in between represent marginal exposure. 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 sits in a rover operations room, viewing computer models.
Virtually on Mars
Michael Sims, a research scientist from NASA's Ames Research Center and co-investigator on the Mars Exploration Rover mission, uses his models to look for "hot spots."
Image credit: NASA/JPL
Link to Full Res
Michael Sims, a research scientist from NASA's Ames Research Center and co-investigator on the Mars Exploration Rover mission, used the MIPL 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."

Dr. Larry Soderblom poses for a quick picture.

Leapin' Lily Pads!
Dr. Larry Soderblom, a science-team member from the U.S. Geological Survey, brought the idea of making solar energy maps to MIPL.
Image credit: NASA/JPL
Link to Full Res
The Final TKO

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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