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A simulated Mars Exploration Rover roams a simulated planet. In January it all becomes real. (NASA/JPL)

Next Stop Gusev Crater

If planetary scientists could do whatever they wished, they'd probably send a spacecraft to land on the floor of Valles Marineris.

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While having more information made picking the MER landing sites easier, it didn’t always seem that way to the scientists. The two spacecraft, named Spirit and Opportunity, are the most advanced robots ever sent to Mars. Not only are their instruments better—the cameras sharper, the spectrometers for determining rock chemistry and mineralogy more discriminating—but their aiming will be more precise thanks to upgraded navigation software and other improvements since 1997. Pathfinder needed a landing zone so big—about 60 by 150 miles—that there were only a few places flat and smooth enough to touch down. But the MER landers can home in on a target area about a quarter that size.

When Parker sat down at his high-powered computer to study digital maps of the surface, he kept finding more places where he could fit a stretched-out ellipse outlining the minimum runway MER needs to land.

He managed to identify about 155 candidate ellipses in all sorts of terrain. A couple of the potential sites—Melas Chasma and Eos Chasma—were within Valles Marineris, the largest canyon in the solar system. Another fell near Apollinaris Patera, a volcano that shows signs of having once sent water gushing to the surface. Still another covered what looks like a vast system of river channels called Athabasca Vallis, just downstream from where a major flood appears to have raged.

With 155 possibilities, it became much tougher to decide where to land, especially when scientists could study each one in more detail than ever before. Public meetings started two years before launch so that scientists could argue for their favorite sites; this added another layer of outside review and countered the criticism that NASA liked to make closed decisions.

“Not only did we have more data than any other landing site selection process, but we also had more people, more eyes, more minds thinking about this than ever before,” says John Grant, a geologist at the Smithsonian’s National Air and Space Museum who co-chaired the landing site committee. “It meant there were no surprises.”

It also was no surprise that some of the favorites reflected individuals’ own research interests. Some scientists pushed for sites that would help them understand the magnetic orientation of Mars. Others wanted to know about ice near the poles. Most, though, agreed that the search for life should drive the mission. They ranked the top sites based on whether they might turn up evidence of water—a basic ingredient of life—and whether any signs of past biological activity might remain.

Superimposed on this scientific priority list were other concerns, mostly having to do with safety—how many hazardous rocks littered the landing zone, what was the likelihood of dust clouding the rover’s cameras or blanketing the solar arrays. Even the local scenery was considered—disguised, in unemotional science-speak, as “public engagement” and “site aesthetics” that would appeal to taxpayers. Once these factors were added, the 155 candidates dwindled to fewer than 10.

On occasion, amiable tug-of-wars broke out, with engineers preferring to avoid risk and scientists pulling for a site full of intriguing mesas and ravines. “Sometimes you could tell from [the engineers’] body language,” recalls project scientist Joy Crisp. “It said ‘No, I don’t want to go there.’ That made the rest of us nervous.”

More often, though, “it was the scientists saying ‘We want to go there’ and the engineers saying ‘Let’s see if we can,’ ” recalls JPL’s Mark Adler, the mission’s lead engineer. Meridiani Planum, for example, captivated scientists because it holds a big outcropping of hematite, a mineral that typically forms in the presence of water. But its elevation was a bit too high. Engineers solved that by upgrading the MER navigation system so the spacecraft could better measure the distance to the ground, and adding rockets to compensate for unpredictable winds. That provided a large enough safety margin to make Meridiani a go.

To better simulate landing conditions at the top candidate sites, mission planners hired a landscape architect to lay out a Martian scene, created with rocks selected from the California desert. Trucks hauled the rocks to Sandusky, Ohio, where crews bolted them to a platform inside the world’s largest vacuum chamber, at NASA’s Glenn Research Center. With air sucked out of the chamber to simulate the thin Martian atmosphere, the crews dropped a dummy spacecraft onto the platform to see how it handled different slopes and rock configurations. A bungee cord attached to the model yanked it down at the speed the 1,200-pound landers will hit the surface of Mars.

About Michael Milstein

Michael Milstein is a freelance writer who specializes in science. He lives in Portland, Oregon.

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