Meanwhile, like a swarm of paparazzi, the craft now orbiting Mars photographed each of the frontrunner zones, so the top few sites are probably now the most studied places on the planet. Researchers looked at infrared images to determine how quickly regions cooled after the Martian sunset. Since big rocks retain heat, the images gave hints about the number of boulders lying about. The scientists had the orbiters fire radar waves at the surface to make sure it was solid enough to support the rovers. Such reconnaissance showed that Athabasca Vallis contained a jumble of ragged rocks, rather like Devil’s Golf Course in Death Valley. Even the maneuverable rovers could get stuck there, so it was out. Steepness ruled out Gale Crater—the landing ellipse was too big to avoid the walls.
By January of last year the choice had come down to four, two of which would be relegated to backup status. One of the finalists was a broad Martian channel called Isidis Planitia, where scientists hoped to find a variety of rocks washed down from the highlands. Isidis lost some of its appeal when a closer look suggested the rocks might be buried instead of exposed. Researchers likewise lost interest in Elysium Planitia, which was plenty safe but wasn’t especially intriguing geologically. When scientists at the final committee meeting last January broke into groups to consider the pros and cons of each site, the two most popular spots lured about 50 people each. Isidis drew two and Elysium a lonely one. (“The only reason it had one person was because we assigned him to it,” says Grant. “And he didn’t want to be there.”) In April, based on the scientists’ recommendation, NASA gave the final nod to Gusev and Meridiani.
That doesn’t mean all doubts have been quelled. The biggest unknown still facing MER is the Martian wind. It was wind, in fact, that ruled out landings in the canyons of Valles Marineris, which may act as a wind tunnel, whipping up gusts of 80 mph. Even though JPL’s supercomputers have spent months trying to forecast a few days’ worth of weather, the Martian winds remain largely unpredictable.
So when the landers arrive at their destination in January, JPL engineers and the scientists who rely on them will be sweating every last foot of the descent. All the careful targeting, all the workshops and agonizing over where to touch down, could be thrown off by a gust blasting the landers sideways. But if they land safely, it will take just one small discovery—perhaps a photo of water ripples in Gusev’s dried-up lakebed, or confirmation of hematite and other water-formed minerals at Meridiani—to make scientists glad they took the trouble.