Helicopter inspires NASA’s Mars lander

By Brian Berger

Staff Writer

updated 2:06 p.m. ET Nov. 29, 2006

If NASA’s 2009 Mars Science Laboratory reaches the red planet’s surface in one piece, the agency will owe a debt of gratitude to the Sikorsky S-64 Skycrane heavy-lift helicopter.

Like its namesake, NASA’s Sky Crane carrier platform will hover above its drop site — albeit with retrorockets rather than rotor blades — and lower its payload, the compact car-sized MSL rover, to the surface using a winch and tether. As soon as the rover is ready to roll, the tether connection will be severed and the Sky Crane will fly off and crash land a short distance away.

The MSL will be the first NASA mission to employ this planetary landing scheme, but it might not be the last. Adam Steltzner, lead engineer for MSL’s entry, descent and landing system at the Pasadena, Calif.-based Jet Propulsion Laboratory, said the Sky Crane approach makes sense for any destination where the terrain is not well understood or when it is especially important not to unduly disturb the landing site. Early lunar lander missions are one possible application, Steltzner said. Mars sample return missions are another, he said.

Steltzner said NASA settled on the Sky Crane approach in 2003 after concluding that the 775-kilogram nuclear-powered MSL was too massive for the airbag landing that worked so well for the 1996 Mars Pathfinder and the 2003 Mars Exploration Rovers. Another possibility was a three- or four-legged lander assisted by parachutes and retrorockets, he said. But after the failure of the Mars Polar Lander, which employed that mode, NASA was open to trying something a little different, he said.

NASA lost contact for good with the Mars Polar Lander Dec. 3, 1999, once the probe began its descent through the Martian atmosphere. A subsequent investigation concluded that onboard computers probably misinterpreted the sudden jolt of the lander’s legs deploying as the actual landing and ordered its descent rockets shut down — even though the craft was still about 130 feet (40 meters) above the planet. Net result: a fatal 80-kilometers-per-hour impact with the Martian surface.

The experience led NASA to shelve a similar lander, the Mars Surveyor 2001, and revert to airbags for the twin Mars Exploration Rovers, Spirit and Opportunity.

NASA studied using airbags to bounce the $1 billion MSL to a safe landing, but concluded that the challenges posed by the rover’s size were insurmountable. “The airbags simply could not get the job done for the MSL rover,” he said. “They just don’t scale.”

NASA has not abandoned the concept of self-contained landers using retrorockets and shock-absorbing legs. The Mars Phoenix Lander, a stationary science platform slated to launch in August 2007 toward Mars’ northern polar plains, will employ this approach, for example. NASA made a number of changes to avoid a repeat of the Mars Polar Lander debacle. The Phoenix Lander, a Scout-class mission, was pulled together largely from Mars Surveyor 2001 hardware and spare Mars Polar Lander instruments.

Steltzner said landing on a set of legs is a tricky proposition even if everything goes right. For starters, he said, there are stability issues galore as a top-heavy lander approaches touchdown, its propulsion system on notice to shut down a second or so before the legs make contact with the ground.

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