Slamming into the Martian atmosphere at 13,000 mph and enduring temperatures of up to 3,800 degrees Fahrenheit, a peak deceleration of up to 15 Gs, and the jerk of a supersonic braking parachute--that's just the opening act.
For NASA's Mars Science Laboratory, the real fun will start 50 seconds before touchdown when the one-ton nuclear-powered rover falls free of its parachute for a nail-biting rocket-powered final descent to the surface. (For the main story in this package, see "On Mars, satisfaction awaits Curiosity.")
Unlike past Mars missions, the Curiosity rover will not set down atop a legged lander or bounce to the surface surrounded by shock-absorbing airbags. Instead, it will be lowered to the ground and set on its wheels by a slowly descending "sky crane" designed to unreel the lander like a lure on a fishing line.
The sky crane will carry an inertial measurement unit and a radar altimeter that will feed velocity and altitude data to a sophisticated flight computer inside the rover. The computer, in turn, will command canted hydrazine rocket thrusters on each corner of the sky crane to put the craft in a controlled vertical descent at a sedate 1.7 mph.
Finally, at an altitude of around 65 feet, a gravity-fed harness system will lower the MSL rover away from the sky crane, which will continue its descent until the rover's wheels touch down. Sensing the change in its load, the computer will send commands to release the harness and cables and the sky crane, its job complete, will fly off to a crash landing a safe distance away.
The rover, now safely on the ground, will be ready to begin two Earth years of exploration to look for carbon compounds and evidence of past or present habitability.
At first blush, the sky crane seems more Rube Goldberg than NASA, prompting an immediate "you're kidding" response from the uninitiated. But as it turns out, the sky crane provides a clear advantage when it comes to getting a rover away from its lander and safely onto the surface.
The more traditional rocket-powered legged lander, or even an airbag system like the ones used for NASA's Mars Pathfinder and the later Spirit and Opportunity rovers, require an added level of complexity to ensure that a wheeled rover can safely roll off its landing platform and onto the surface.
It also requires a bit of luck. If a lander sets down on a slope or large rocks, a rover could have major problems getting down to the surface.
Airbags were never an option for MSL, which is the size of a car and weighs 1,950 pounds, almost five times as much as Spirit and Opportunity (also known as the Mars Exploration Rovers).
Airbags cannot be scaled up enough to support MSL and still fit inside the launcher. And previous airbag-equipped landers made unsteered, ballistic entries, resulting in a relatively large landing zone "footprint," or error ellipse, limiting the number of possible landing sites.
Complicating landing site selection, previous airbag-equipped landers were solar-powered and limited to targets within 10 to 15 degrees of the Martian equator.
Enter MSL and the sky crane.
With an ability to steer the entry vehicle during hypersonic flight using thrusters and spring-ejected weights to control the spacecraft's lift, the MSL lander can fly a precisely guided entry to a much smaller landing footprint, or ellipse, than previously possible.
With a nuclear generator producing heat and electricity, MSL can safely land at higher altitudes 45 degrees to either side of the Martian equator. And with the sky crane bridle/umbilical system, the rover can be lowered directly to the surface, ready to roll, without the additional risk of getting off a lander.
Wringing out the bugs
That's the good news. The bad news is the system cannot be fully tested on Earth. Designed to cope with the weaker gravity of Mars, an end-to-end test in Earth's gravitational field is not possible. Instead, engineers are forced to rely on component testing and countless computer simulations.
"It's a very complicated spacecraft. It's got a lot of bells and whistles," Adam Steltzner, manager of the MSL entry, descent, and landing project, told CNET. "And that means the act of putting it together right and testing it to make sure it's doing what it's supposed to is a huge effort. And it certainly feels daunting at times."
Still, Steltzner expresses no doubt the landing is going to be successful. "I think we have an extremely thorough program in place," he said, "to wring out the bugs and make sure that the vehicle's ready to go and will perform when we get there."
Doug McCuistion, Mars exploration manager at NASA headquarters, said he believes the MSL landing is only slightly more risky than the parachute and airbags used for the Mars Exploration Rovers and the rocket-powered Phoenix polar lander, a more traditional legged spacecraft that landed on Mars in May 2008. In his view, guided entry is what puts MSL in a class by itself.
"This is not 'Hit the atmosphere and sooner or later you're ballistic and then you're going to pop a chute and land,'" he told CNET. "We're going to steer that thing through the atmosphere through use of ballast and thrusters and guide that entry to reduce that landing-error ellipse.
"The payoff is massive for science, for future systems that are robotic, as well as future human systems. The payoff is enormous, and worth a little added risk. But I don't think it's dramatically riskier. In fact, really sky crane is not dramatically different than Phoenix, just with the engines on top.
Another major difference, of course, is the bridle/harness system that will lower MSL to the surface, providing physical support along with power and data.
"Well, that's not too bad," McCuistion laughed. "I do a lot of fishing, you know, and we use a reel and line all the time!"
MSL will make the voyage to Mars inside an entry "aeroshell" attached to a solar-powered interplanetary cruise stage. Ten minutes before atmospheric entry in August 2012, the lander will separate from the cruise stage, stop its 2-rpm spin and orient itself, heat shield forward, at the proper angle of attack.
Entry interface, the somewhat arbitrary point that defines where the spacecraft enters the discernible atmosphere, will occur at an altitude of around 78 miles. The spacecraft will be subjected to peak heating within the first 81 seconds, rapidly bleeding off speed through atmospheric friction.
With the vehicle descending at a supersonic velocity, the flight computer will eject ballast as required, steering the craft by adjusting its lift, to reach the desired target.
Slowing to just over 1,000 mph about four minutes after entry, the craft's main braking parachute will deploy at an altitude of about 6.2 miles. About 28 seconds later, at an altitude of 4.3 miles and a velocity of 358 mph, the heat shield will be jettisoned, exposing the rover's wheels and undercarriage.
MSL's landing radar will begin pinging the ground and feeding altitude and velocity data to the flight computer aboard the rover as the parachute descent continues. The sky crane's eight rocket engines will be primed for ignition moments later, five minutes and 35 seconds after entry, starting at an initial 2 percent throttle setting.
Now down to an altitude of 1.1 miles and just 47 seconds or so from touchdown, the sky crane and rover will drop away from the parachute and back shell for the rocket-powered descent to the surface.
It will be the moment of truth for the sky crane concept.
"We are at a low throttle setting for about a second after we come out of the back shell to allow ourselves to fall toward Mars and, more importantly, to allow the back shell and the parachute to accelerate viciously away from us," said Steltzner.
With most of the weight now gone, the parachute and back shell will suddenly decelerate at about 10 Gs.
"The first thing we do is divert out to get away from it," Steltzner said. "And in that divert maneuver, we're also canceling out horizontal velocity and bringing our vertical velocity to 32 meters per second [71 mph]. And then we come to a waypoint, as it were, nominally 200 meters [656 feet] over the surface."
As the lander descends vertically, the flight computer will begin decelerating the sky crane, reducing its downward velocity to about 1.7 mph.
"At this point, we've been running on eight engines, we throttle four of those engines down to the 2 percent [level]," Steltzner said. "There's a little transient, we give ourselves a second or so to let that transient die out, and then we separate the rover."
At an altitude of around 65 feet--about six minutes and 20 seconds into the descent--the rover will be lowered on a 25-foot-long bridle/harness directly below the sky crane.
"The descent stage has got the IMU [inertial measurement unit] in it, it's got the radar on it, it is still following a commanded vertical profile of three-quarters of a meter per second, zero horizontal," Steltzner said. "It keeps going down."
When the rover's wheels touch the Martian surface six-and-a-half minutes after entry, the sky crane will briefly continue its descent, adjusting its rocket engine throttles to maintain the same descent rate. When the throttle setting is half of what was needed when the weight of the lander was present, the computer will cut the bridle and the sky crane will fly off to a crash landing about 500 feet away.
"We're going to hope Mars has taken up the weight of the rover, but either way, our job here is done," Steltzner said. "By the way, the rover is doing all this thinking. The computer is actually located on the rover, but the sensors are on the descent stage. The rover says OK, and issues commands to cut the descent stage, first the bridle, then the umbilical...And then we're down."