Landing a rover like Curiosity on Mars is not easy. From a fiery launch to scintillating entry and the nerve-wracking landing, there's a lot that could go wrong. But, it's the final obstacle - the martian surface itself - that could prove the most frustrating: an inauspiciously-placed boulder, an unexpected sand dune, or a too-steep slope could all end a Mars mission before it even begins.
John Grant and Matt Golombek, who served as the Mars Science Laboratory's landing site steering committee co-chairs, had the task of making sure these potential disasters didn't come to pass.
Golombek has been involved with landing site selection since the late '90s, when he scoured Ares Vallis for a safe place for Mars Pathfinder to set down. Back then, landing site characterization was as much an art as a science: when a satellite image's single pixel covers several square meters, you're left reading the tea leaves to determine if there are lurking dangers. There were ways to get creative - by looking at dark areas of a single location at different Sun angles to back out relief and, possibly, rockiness, for example - but it wasn't an exercise in certainty. Golombek operated in the language of probability and hope-for-the-best optimism.
Fortunately, the landing-site-selection team's job has become easier over the last decade as the arsenal of Mars orbiters has provided increasingly detailed images of the surface. The watershed moment came in 2006 when the HiRISE camera aboard the Mars Reconnaissance Orbiter went live, able to resolve rocks the size of a basketball on the surface. "The images and data available to study the landing sites are so good and detailed that there are very few surprises in what we have seen at the surface in terms of safety," says Golombek, leaving ample room for scientific discovery.
But, treacherous rocks are just part of the story; other requirements for a suitable landing site include, among other criteria, an equatorial latitude (to maintain thermal stability and preserve mechanical components), a low elevation (more atmosphere to slow the spacecraft during entry), a sufficiently large ellipse (a big enough target for the navigation team), low slopes (a toppled rover has diminished future discovery potential), and minimal deposits of thick dust (a stuck rover quickly becomes a lander).
Now that Curiosity has landed safely, the science team is beginning to piece together the geologic setting and put the rover's immediate surroundings into a larger context. Grant, for one, wasn't too surprised by the initial photographs: "We are able to see a variety of surface with different characteristics from the ground that map well into what we had seen from orbit before landing," he says. "Once we start to drive and cross various surfaces we will be able to better evaluate how they formed."
Curiosity effectively avoided the rocks during landing, but now it's ready to hunt them down and learn the stories they have to tell about Mars' intriguing past.