Nine months ago, the Mars Science Laboratory rover Curiosity left the east coast of Florida on a one-way trip to a crater on Mars. Launch was a violet event, full of light, smoke, and noise on the way uphill and eastward out of Earth's gravity well. Since Florida is relatively easy to get to from where I live, and I know people who work on the rover for the living, I was able to see the launch from a few miles away and blogged about it here.
Curiosity's landing site, Gale Crater, is a bit more inaccessible than Cape Canaveral Air Force Station. Not only has no one seen or inspected the rover for nine months and a hundred million miles, Mars is so remote from the Jet Propulsion Lab's control room in Pasadena that it now takes 14 minutes for telemetry radioed back from the rover to reach mission controllers. This means that if anything happens quickly, this complex automated system built by a team of imperfect people is completely on its own for decision-making and execution of those decisions. Nothing much exciting happens during an interplanetary cruise, so that hasn't been worrysome up until tonight. Once Curiosity hits the top of Mars's atmosphere in a few hours, though, things will start happening very quickly.
The following things need to happen, without fail, for the pride of NASA and JPL to reach the red dust on Mars's surface intact and ready to do science:
The rover, now an aircraft rather than a spacecraft, needs to decelerate from around 30 times the speed of sound to twice the speed of sound by converting the kinetic energy of spaceflight into heat energy surging into an incandescent wall of plasma on the leading edge of the heat shield. The rover's electronics cannot overheat while this happens. That would ruin everything.
The rover needs to explosively launch a parachute out the back end while still flying faster than rifle bullet. There will be shock waves snarling the parachute's shroud lines and coalescing into sheets of high-pressure air that will slam into the canopy like blasts from a grenade. On Earth, supersonic parachutes are considered something to avoid, but the air on Mars is simply to thin to wait any longer to deploy the chute. It needs to be out while the rover is still in a screaming dive toward the surface.
As it turns out, the parachute still isn't enough to slow Curiosity down to a tolerable landing speed. Rockets will fire, nulling the rover's horizontal speed and cutting its vertical speed to a gentle, constant descent. Meanwhile Curiosity's onboard computer will be snapping pictures of the terrain beneath her, comparing those pictures to borrowed snapshots taken by the fleet of Mars orbiters, and will attempt to guide the rover to a pre-programmed ideal landing point.
Once the rover has closed to within fifty or so vertical feet of the surface, the rover itself will be slung under the rocket platform on cables. The platform (actually a skycrane. Like, for real, that's what its called) will descend until rover wheels touch down, then cut the cables and fly away to a (hopefully) distant impact point.
Then the rover can power up the instruments with the heat of an onboard plutonium source and go do science. In case you're not properly terrified of how horribly this can all end, here's a video which adds some visual detail to the steps outlined above:
The engineers at JPL, Lockheed Martin, and Boeing, who designed, built, and simulated the rover are smart people. They're experienced, they've accomplished difficult things before, in short, they know what they're doing. That said, this is a much, much more dangerous event in the life of Curiosity than was launch. More things have to happen in faster succession on a machine that's been on a lonely journey since November. Before launch there were people ready to inspect and repair the rover up until the final hours before engine ignition. That luxury is unique to Earth.
NASA has sunk more than two billion dollars into this spacecraft, and the future of our Mars exploration program for a generation may be hanging in the balance of what a simple-minded robot can do in seven minutes in the atmosphere of another planet. Tonight is a big night, and I'll be breathing easier after 10:30 my time.