Vol. 33, No. 20

5-18-2004
By: Pam Frost Gorder

Engineer reflects on Mars mission

In a scene that any golfer could appreciate, NASA’s Opportunity rover landed on Mars by bouncing a bunch of times and dropping right in a hole.

When the rover poked up its robotic head and transmitted the first pictures of its new surroundings on Jan. 24, Ron Li was still working full time at the Jet Propulsion Laboratory (JPL) in Pasadena, Calif. Now back at Ohio State, the professor of civil and environmental engineering and geodetic science described his role on the mission science team during an April 27 seminar.

“We knew we were in a hole, but we didn’t know where. The panorama just showed a crater wall,” Li remembered.

Though NASA had a rough idea of where the rover would touch down — Meridiani Planum, a dusty plateau on the opposite side of the planet from where the Spirit rover landed — Opportunity’s exact location was anyone’s guess. It could have been sitting in any of half a dozen craters.

NASA used Ohio State software to build a detailed 3D image of the hole’s edge and compare it to photographs from the descent. This method, along with others, enabled scientists to pinpoint the rover’s location: Eagle Crater, just a short distance from other important target sites on the surface.
But the lucky crater trumped all the other sites so far, when its exposed bedrock yielded evidence of ancient water on the red planet.

“Eagle Crater was fascinating,” Li said. “It had vertical wall stratigraphy like the Grand Canyon.”

After sampling the bedrock, the rover climbed out of the crater and continued its mission, again using data and maps provided by Li’s team.

If Eagle Crater was truly a hole on a golf course, most of Meridiani Planum would be a sand trap. As Opportunity rolls over sand dunes, its wheels occasionally slip, which creates a problem for navigation. That’s because NASA measures wheel rotation among other factors to calculate how far the rovers have traveled.

So every day that the wheels spin in the Martian sand, the measurements of the rovers’ locations become a little less accurate.
Li and his colleagues determined that Opportunity’s measurements are off by 20 meters — almost 12 percent of the total distance it’s traveled since January. Spirit, which is traveling in a less sandy location, is off by only 2 percent.

Opportunity is currently circling the much larger Endurance Crater while scientists try to decide whether a trip to the bottom would be worthwhile. The cliff-lined crater could contain further evidence of water. The space agency will weigh its options over the next few weeks.
And on Mars, as everywhere, time is money.

Li explained that mission scientists hotly debate where the rovers should travel, and which observations should be made with which instruments each day. “We could be looking at a very interesting rock, and someone will say ‘This rock just cost us too much time and energy,’” Li said.

NASA is keenly aware of such pressures, and its scientists log on to a secure area of the Ohio State Web site every day to download related mission information and get detailed maps for planning the rovers’ future routes.

Since NASA has extended the mission, Li will be working at JPL part time until autumn quarter, when he plans to give another public talk.