Ensuring safe landings on Mars

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NASA to test inflatable shells for space craft.

A device that combines balloons and parachutes could help to slow craft as they descend through Mars' atmosphere. Credit: NASA/Langley Research Centre

Before NASA's Mars Science Laboratory alights on the red planet in 2012, the one-tonne rover will have fallen through Mars' thin atmosphere at hypersonic speeds and blistering temperatures.

Every inch of the craft must be protected by a rigid aeroshell, which bears the brunt of the fastest portion of the descent before parachutes and retrorockets take over. NASA engineers are confident that the craft will survive its current mission. But for missions with much heavier payloads, they will need something better. "We're kind of at the limit for what we can do," says Neil Cheatwood, of NASA's Langley Research Center in Hampton, Virginia.

The solution? Blow up the aeroshell like a balloon. By using flexible, light-weight and heat-resistant materials, NASA is creating inflatable shells that, when puffed up to many times the size of the payload, create more drag and slow spacecraft more efficiently than the rigid aeroshell.

“We're kind of at the limit for what we can do. Neil Cheatwood , Langley Research Center”

Engineers then wouldn't have to squeeze rigid aeroshells into rockets, which are limited to about 5 metres in diameter. In some of the designs, the inflatable shell would trail the payload as a 'ballute', a mixture between a balloon and a parachute. In others, the aeroshell would simply be replaced by a giant, mushroom-shaped foreshell.

On 17 August, NASA will test an inflatable aeroshell that Cheatwood, the test's project scientist, says could provide the proof of principle needed to incorporate the technology in future Mars missions. These could include a sample return or a human mission, which will need to take to Mars the means — and extra weight — of getting back to Earth.

The test launch, from the Wallops Flight Facility on Virginia's eastern shore, will take the aeroshell up on a Black Brant 9 sounding rocket. Less than four minutes after launch, the payload would reach a peak at 211 kilometres. A minute later, a cannister of nitrogen would inflate the shell to a 3-metre-wide saucer shape seven times the diameter of the rocket.

Only recently, says Cheatwood, have materials been developed that can survive the heats experienced during re-entry. This aeroshell will have a woven exterior made from a heat-resistant ceramic that caps cells made from strong Kevlar fibres.

Bulletproof idea

The idea for inflatable shells has been around for decades. It was considered as an alternative to parachutes for the Viking mission, which landed twin spacecraft on Mars in 1976. In 1996, Russia was going to use an inflatable system to land its Mars '96 mission, but the spacecraft never reached Earth orbit. Several other test flights, including a 2007 NASA test, have had malfunctions. "We've kind of been cursed," says Steve Hughes, an engineer on the project at Langley.

This slow progress means that engineers for the NASA Mars programme at the Jet Propulsion Laboratory (JPL) in Pasadena, California, have stuck to what they know best, even as the existing technology is reaching its limits. As the Mars Science Laboratory has grown in weight, for example, engineers have had to migrate to a new type of heat tile in the rigid aeroshell.

Even with the new heat tiles, however, some parts of the planet are off limits as a landing site, including one that sits in a region where methane has been observed (see 'Methane site ruled out on Mars'). There is less atmosphere to fall through at higher elevation sites — a risky proposition for an entry system that needs every kilometre of drag it can get.

With the added drag of an inflatable aeroshell, missions could decelerate more quickly, and the entire planet would be open to exploration, Cheatwood says.

Adam Steltzner, manager for the Mars Science Laboratory's entry, descent and landing systems at the JPL, says that he is aware of the inflatable aeroshell technology and is keenly awaiting its development, although the JPL Mars programme has no plans to switch to it any time soon.

The existing system, he says, should even work for a Mars sample return. But to get humans back from Mars — missions that would need to land tens of tonnes on the surface — the entry systems will need some puffing up. "North of two metric tonnes, we're going to need some help," he says.

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Hand, E. Ensuring safe landings on Mars. Nature (2009) doi:10.1038/news.2009.818

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