As the sky-crane descent-stage hovered over the landing site, lowering the rover Curiosity to the surface of Mars on cables, its thrusters blasted deep enough to give investigators an unexpected chance to look all the way down to bedrock. But the excavation zone is also giving mission planners pause as they consider where Curiosity should start its exploration.
It remains to be seen where the rover will drive first. “We probably aren't going to know for a week or two, until we have completed CAP1B [commissioning phase], and we don't have to make that decision at this point,” says John Grotzinger, mission chief scientist.
The choice is boiling down to three main options, the first of which is to study the excavation zone created by the descent thrusters' blast. Grotzinger says the(JPL) team is “concerned about the level of contamination—not just the hydrazine [rocket fuel] and heating, but also the fractured rock. The team has thought about it and we think the first sample ought to be Mars, rather than something we did to alter it.”
A second option is an area of interesting morphology some 500 meters (1,640 ft.) to the east of Curiosity, where images indicate the convergence of three separate forms of terrain. This could present a target-rich environment where the fluvial features indicative of an ancient watery past give way to “everything else downstream” says Grotzinger. “We are pretty sure that mantling material is obscuring that, and there's lots of interest in answering that question.”
The third option is simply to characterize the landing site and then “get going,” he adds. Curiosity's overall transit speed between areas of interest is going to be slow at first, while checks continue and operators become familiar with the rover. However, Grotzinger says that although the rover incorporates better autonomous systems, these are not “quantum” improvements over the Mars Exploration Rovers. “I honestly don't think we will move much faster than Spirit or Opportunity,” he says.
The JPL team expects Curiosity to take about one year to navigate through dark-colored active sand dunes and around obstacles to make it to a target area “a few hundred meters up in elevation” on the slopes of the 5.5-km-tall (18,000-ft.) Mount Sharp, according to MSL Deputy Project Scientist Ashwin Vasavada. The rover is expected to drive “something like a football field a day” at top speed, and will “have to find a gap in the dune field to give us safe passage.” There will also be “some challenges that will slow us down,” Vasavada adds.
And the team scientists will want to stop along the way to explore interesting terrain. “Anytime we characterize something, it's probably a few weeks of activity, because it might mean acquiring a sample, for example,” Vasavada says. “[It] takes a few weeks to find a place to get the sample, deploy the arm, acquire the sample and process it in our lab.”
Scientists started mapping the terrain inside Gale Crater even before Curiosity landed, using high-resolution data from cameras in orbit around Mars. Dawn Sumner, a science team member from the University of California at Davis, says she and her colleagues divided the target landing and roving area into 1-mi. squares and are inventorying the features in each using the overhead data.
“We'll use this map to find a path from where we landed to the main target at the base of Mount Sharp, which is south of where we landed,” she says. “So we'll drive on the northwest side of the dunes and go through a break in the dunes, but on the way we're going to have a lot of interesting geology to look at.”
In deciding what to do next, team scientists will try to balance the attraction of a particular feature on the crater floor with the ultimate target, the sedimentary rock on the mountain that will allow Curiosity to sample its way through the planet's history.
The rover's Martian day is divided into battery-powered science and nighttime “sleep” while the radioisotope thermoelectric generator (RTG) recharges the batteries with electricity generated from the heat of decaying plutonium 238. During that time, a group of about 100 scientists and engineers decides what commands to upload to Curiosity for the next day's activity, and then writes, checks and sends as many as 1,000 commands through one of the Mars orbiters or directly to the high-gain antenna on the rover.
“It's a challenging mission to actually do the operations, because we can't joystick the rover because of the time delay and . . . a highly resource-constrained vehicle,” says Andy Mishkin, the JPL engineer who heads the operation. “The amount of power that we're getting from the RTG is basically a little more than you might need to power a 100-watt light bulb.”
The team's commands must meet “hundreds” of flight rules designed to keep one activity from hampering another or damaging hardware, and to make sure the data load that is generated will fit into the available communications links. The exacting, deadline-driven work is complicated by the need to live on Martian time, where a sol is 40 min. longer than a day on Earth—a requirement some team members describe as permanent jet-lag.