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.
- By Michael Milstein
- Air & Space magazine, January 2004
(Page 2 of 3)
Despite these constraints, MER had a huge advantage over past Mars landers. Two spacecraft currently in Martian orbit—the 1996 Mars Global Surveyor and 2001 Mars Odyssey—had photographed potential landing zones in unprecedented detail. Mission designers had millions of close-up snapshots to work with, compared to fewer than 8,000 less-detailed pictures taken by NASA’s Mariner orbiters prior to Viking, the first successful Mars landings in 1976. Looking back on that mission, Tim Parker, a JPL planetary geologist who took a leading role in the MER site selection, says, “There was so much we didn’t know, there were probably some definite luck factors” in Viking’s success.
Even with Pathfinder, “the ignorance we had was probably rather blissful,” Parker says. Since then, closer looks at the planet have revealed that the landing zone has potentially dangerous gullies and hillocks that mission planners didn’t know about at the time.
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.