High-stakes Mars mission relies on untried ‘sky crane’
CNET News- 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 .5 billion Mars Science Laboratory mission.
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.”