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 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.
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 touchdown, 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
onboard 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."
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