WASHINGTON — If NASA’s 2009 Mars Science Laboratory (MSL) 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 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, it puts its propulsion system on notice to shut down a second or so before the legs make contact with the ground.

Failure of any of the thrusters to cease firing at just the right time could send the lander hopping across the surface, as happened with NASA’s Surveyor robotic lunar lander in 1967.

Getting a rover off a legged lander after touchdown poses additional challenges, Steltzner said. Ramps are customarily used, but there is no guarantee that the martian terrain and an imprecise landing will not conspire to deny the rover a safe path to the surface. On the 1996 Mars Pathfinder mission, for example, only one of the landing platform’s two ramps opened onto a clear path for the tiny Sojourner rover. NASA could have just as well found both paths blocked.

For the MSL mission, Steltzner and his colleagues decided to bypass some of those issues by putting rover directly onto the surface.

Like NASA’s two stationary Viking landers of the 1970s, the MSL will rely on parachutes to slow its fall before the eight Viking-class thrusters on the Sky Crane landing system ignite at 1,000 meters above the surface, providing a controlled descent. At 35 meters , the Sky Crane will begin lowering the rover on a tether — similar to the way the Sikorsky S-64 delivers underslung payloads — as it continues its descent. When the rover’s wheels touch down, the tether is severed and the Sky Crane platform flies off to land 500 meters to 1000 meters away, Steltzner said.

While some scientists would like NASA to put instruments on the Sky Crane, essentially transforming it into a stationary lander, Steltzner said that is not currently in the plans. As it stands, Sky Crane would have no on board capability to process and transmit data once it has dropped off the MSL rover, which supplies the brains of the operation.

NASA has been developing the Sky Crane concept with some of the $80 million invested in MSL technology since 2002, Steltzner said. Those technologies include a landing radar and hazard avoidance system that should be better than any NASA has ever flown, and more powerful, throttle-able versions of Viking’s hydrazine thrusters, he said.

Steltzner said NASA has a vigorous validation and test plan mapped out for every element of the Sky Crane landing system. The agency ultimately must rely on a combination of integrated hardware testing and simulation to feel confident that the system will work in the only environment that counts — Mars.

“That’s par for the course in this job,” Steltzner said. “We never really get to do a full end-to-end test of any of the [entry, descent and landing] systems for Mars because we are not on Mars. This one is not any different.”

Brian Berger is editor in chief of SpaceNews.com and the SpaceNews magazine. He joined SpaceNews.com in 1998, spending his first decade with the publication covering NASA. His reporting on the 2003 Space Shuttle Columbia accident was...