The question is straightforward: how to get a car-size rover safely to the surface of Mars? And not just anywhere, but to a very precisely defined bull-s-eye on the floor of a broad crater, within roving distance of a 3-mile-high mountain.
In earlier ventures to Mars, spacecraft have either bounced to the surface cocooned in giant airbags or made the trip atop a rocket-powered descent stage. But neither approach was an option for NASA's Curiosity rover, the centerpiece of the.
Tipping the scales at one ton, the nuclear-powered Curiosity, a rolling laboratory equipped with a suite of state-of-the-art cameras and instruments, is too massive to use airbags like the ones that cushioned the landings of NASA's much smaller Pathfinder and the hugely successful Spirit and Opportunity rovers.
While a legged lander could do the trick, powerful braking rockets would be needed to get Curiosity to the surface. The sheer size of the descent stage would result in a daunting engineering challenge: get a bulky rover safely down to the surface from a perch many feet above the ground atop its lander.
"This rover is 900 kilograms, it is a beast, it is the size of a car," said Steve Sell, an entry, descent and landing engineer at the Jet Propulsion Laboratory. "So you're trying to land something very heavy, so that means you need sizable engines."
More powerful rocket engines could kick up billowing clouds of dusty debris as the lander approached touchdown "so you tend to want to keep the engines farther away from the surface," Sell said.
"That makes you want to have longer legs, and having longer legs means your center of gravity is higher and then it's much easier to tip over," he said. "Or you need to be very wide. It tends to drive you [to a] larger and larger [lander] in order to do that and be stable."
Then there is the little matter of getting the rover down to the surface after landing.
"Let's say you solved all that and were able to land and you now had to drive a one-ton rover off the top of a platform, either down ramps or some other kind of mechanism," Sell said. "If you were oriented in a way that maybe wasn't favorable to your wheels, like you were tilted to your side, you could slide sideways off the ramps. Maybe there are rocks where the bottoms of the ramps would normally touch and you can't deploy the ramp in the first place."
Faced with those and other sobering scenarios, the MSL engineers came up with a novel alternative. Instead of rolling the costly rover off an elevated, possibly tilted lander, why not do away with the landing legs altogether? Why not attach the rover to the bottom of the rocket-powered descent stage and then lower it directly to the surface on the end of a long cable?
It was a seemingly crazy idea, but it would solve a host of technical challenges, leaving the rover ready to begin its mission without any risky post-landing maneuvers to deploy and roll down ramps of some sort to reach the surface.
But the so-called "sky crane" concept brought its own set of challenges. Unlike the kind of crane one might find on Earth, this one has to fly, using eight rocket engines on each corner. There is no cab, and the nearest operator is 154 million miles away, with 14 minutes between every action its confirmation.
The rover and its jetpack would have to handle the challenging descent autonomously and, during the last few hundred feet, carry out the sky crane maneuver, a never-before-attempted two-body balancing act.
"First, you think it's crazy. Then, you know, you're like OK, all right, maybe it'll work, but you're still crazy," laughed Sell.
"Eventually people come around to saying OK, this makes sense. When you break it down, it is really the result of very careful, reasoned thought and what we all believed were the best choices to land something this big in the most efficient way possible."
Entry, descent and landing -- EDL -- will begin 10 minutes before atmospheric entry when Curiosity and its descent stage, tucked away inside a protective heat shield and backshell, separate from the drum-shaped cruise stage that provided power, communications and propulsion during the long flight to Mars.
One minute later, thrusters will fire to cancel out the spacecraft's 2-rpm rotation and the vehicle will reorient itself, heat shield forward. Two 165-pound tungsten weights then will be ejected, changing the spacecraft's center of mass to provide aerodynamic lift.
Controlling that lift during the high-speed portion of entry is what will enable Curiosity's flight computer to precisely target the landing zone in Gale Crater, steering as required to compensate for differences in atmospheric density, wind speed and other variables.
MSL is the first robotic mission to attempt a guided entry.
"Although we're using the Apollo algorithms and had help from a lot of people who've worked on that stuff in the past, this thing doesn't land successfully if guided entry doesn't work because we've got to kill the energy," said project manager Pete Theisinger. "So that part of it's got to work well."
Curiosity will slam into the discernible atmosphere of Mars at an altitude of about 81 miles and a velocity of 13,200 mph. At that point, it will be about 390 miles -- seven minutes -- from touchdown in Gale Crater
One minute and 15 seconds after entry, the spacecraft's heat shield will experience peak temperatures of up to 3,800 degrees Fahrenheit as atmospheric friction converts velocity into heat, accounting for 90 percent of the spacecraft's deceleration.
Ten seconds after peak heating, that deceleration will max out at 15 times the force of Earth's gravity at sea level.
Plummeting toward Mars, the rover's flight computer will continue to steer the spacecraft as required, firing thrusters to make subtle changes in the flight path to compensate for actual atmospheric conditions.
The guided entry phase of flight will come to an end about four minutes after entry. Six 55-pound weights will be ejected to move the center of mass back to the central axis of the spacecraft to help ensure stability when its parachute deploys.
At that point, at an altitude of about seven miles and a velocity of some 900 miles per hour, the huge chute will unfurl and inflate to a diameter of 51 feet, delivering a 65,000-pound jolt to the still-supersonic spacecraft.
"We have done a series of tests, but none of them have been at Mars-like conditions," Sell said. "This is a supersonic chute, we're deploying it at about Mach 1.7, which we've done in the past on Mars, we're not deploying faster than we have in the past. But what we are doing is we're deploying the largest chute we've ever flown. This thing's 21-and-a-half meters in diameter, it's a very large chute, and it barely fit in the wind tunnel at Ames Research Center."
The inflation load is "the amount of force the chute will experience when it first inflates and catches that first blast of supersonic air. We're never able to test it at supersonic speeds on Earth, so there's always that chance that there's something missed there, there's a percentage chance or so that the chute won't inflate the way we expect it to."
Assuming it does, in fact, inflate as expected, the heat shield will be jettisoned 24 seconds later, at an altitude of about five miles and a descent rate of 280 mph, exposing the rover's undercarriage to view.
A sophisticated radar altimeter will begin measuring altitude and velocity, feeding the data to the rover's flight computer, and a high-definition camera will begin recording video of the remaining few minutes of the descent.
"It does eight frames per second, high-definition-quality video from the backshell coming off [all the way] to the ground," Theisinger said. "So it's a lot of data, it'll take a long time to get it back. But it should be a tremendous movie when it does."
Six minutes after entry, plunging toward Mars at nearly 200 mph just a mile or so from the surface, the most critical moments of the descent will begin as the rover and its rocket pack are cut away from the parachute and backshell, falling like a rock through the thin martian atmosphere.
An instant later, eight hydrazine-burning rocket engines, two at each corner of the descent stage, will ignite to stabilize and quickly slow the craft's vertical velocity to less than 2 mph.
Then comes the moment of truth for the sky crane concept.
Just before the maneuver begins, four of the eight rocket engines, one on each corner, will shut down. Then, about 15 seconds before touchdown, at an altitude of just under 70 feet, Curiosity will be lowered on the end of a 25-foot-long tether. As the support and data cables unreel, the rover's six motorized wheels will snap into position for touchdown.
Finally, seven minutes after the entry began and descending at a gentle 1.7 mph, Curiosity's wheels will touch the surface of Mars. Radio confirmation of landing is expected at 1:31 a.m. ET on August 6, but atmospheric conditions and other factors could result in a touchdown 40 seconds or so to either side of that target.
Whatever the actual timing, the flight computer, sensing "weight on wheels," will send commands to fire small explosive devices that will sever the cables connecting Curiosity with the still-firing propulsion system.
Its work done, the descent stage will fly to a crash landing at least 500 feet away and possibly twice that far. Curiosity, meanwhile, will continue sending telemetry to NASA's orbiting spacecraft for relay back to Earth where engineers will be standing by for post-landing health checks.
"Of anything I could possibly be disappointed with in this mission, there's not somebody standing there [on Mars] with a camera filming the whole thing from the surface," Sell said.