Sky Crane: So Crazy it Just Might Work

Aside from the Earth, Mars is the easiest planet in our solar system on which to land; there isn't a crushing superheated atmosphere like on Venus, and there is a solid surface, unlike gas giants like Jupiter and Saturn. That said, it's still pretty tough. More than half of all missions sent to land on the red planet have merely made craters. Spacecraft on Mars have used some pretty inventive methods to reach the surface unscathed, and the next system is by far the most intricate and insane: a novel device called the Sky Crane will lower the Mars Science Laboratory's rover Curiosity to the red planet's surface next summer. It's the kind of solution that, once you really think about it, realize that it's so crazy it just might work.

The main challenge of landing on Mars is that it isn't Earth. With one-third the gravity and an atmosphere one percent as thick as Earth's, a spacecraft falling to the surface doesn't meet much resistance. As so, engineers at NASA's Jet Propulsion Laboratory have come up with some creative solutions to effect a soft landing on Mars.

In the past, spacecraft that have made it to the surface safely have used a combination of three basic technologies: parachutes to stabilize and slow the initial descent, retrorockets to slow and control the payload further, and airbags to let the payload bounce in a protective cocoon across the surface.

Landers like Vikings 1 and 2 of 1976 and the Mars Phoenix Lander of 1998 use retrorockets. Their landings were a lot like the Apollo moon landings in terms of style — small rockets provided upward thrust enabling the lander touch down gently and safely.

The three rovers on Mars have all used airbags. After a parachute slowed the initial descent, giant airbags inflated around the rover's casing to form a boucy cocoon. The cocoon rolled along the surface to a stop at which point the airbags deflated, the casing opened and the rover drove off to begin its Martian stay.

This later method has been very successful, but it has reached it's limit. The first rover to reach Mars, Sojourner, was the size of a small microwave. Spirit and Opportunity weighed in at 400 pounds each.

Curiosity dwarfs this later pair. At nearly 2,000 pounds, it is the size of an SUV. It can't fit in the casing that housed Spirit and Opportunity. The airbags needed to cushion its impact and subsequent bouncing along the surface would be four time larger than any previously sent to Mars. The weight of the fabric alone prohibits the systems used for Curiosity.

Using a parachute for landing — think NASA's use of parachutes alone to land the Mercury, Gemini, and Apollo spacecraft — is an equally unfit solution. No parachute can effectively slow Curiosity on its own. Again, the weight of the fabric needed is astronomical, not to mention a larger parachute takes a long time to inflate. A thinner atmosphere means it takes longer to inflate a parachute, and time is not a luxury on Martian landings.

Landing the rover like a lander with retrorockets is also a poor option. Retrorockets add substantial weight to the whole system and also force the rover to carry around excess hardware. It's also potentially problematic to have rockets on the underside of the rover since anything protruding from the underside could be a sticking hazard. No one wants to go all the way to Mars only to be foiled by a rock catching a rocket on the rover's belly.

Without past systems to fall back on, engineers had to develop an entirely new way to land on Mars. And so, engineers at NASA's Jet Propulsion Laboratory have developed a brilliant system to deliver the heavier rover to Mars: the Sky Crane. At first glance, it looks like a disaster waiting to happen.

Curiosity will arrive at Mars in a multi-layered delivery system. The rover is attached to a descent module, which in turn is encased in an aerodynamic casing made of a back shell on top and a rounded heat shield at the bottom.

The whole apparatus arrives at Mars and breaks into the atmosphere and begin its descent at interplanetary speeds. Four miles above the surface, the scant Martian atmosphere will have provided enough resistance against the rounded heat shield to slow Curiosity to a 1,000 miles per hour, about Mach 2. At this point, the supersonic parachute will deploy. Measuring nearly 65 feet in diameter, this is the largest chute ever sent to Mars. The chute will have 3.41 vertical miles to slow the payload's fall from 1,000 to 187 miles per hour.

According to early mission parameters, this has to happen in only 85 seconds.

Once the payload has slowed to a subsonic Mach 0.7, the heat shield on the bottom of the casing will jettison to reveal a ground sensing radar. The radar determines the payload's distance from the surface and is instrumental in ensuring the optimal timing of subsequent landing stages. Jettison of the heat shield also triggers deployment of Curiosity's wheels from their stowed position under the rover's body.

Here's where the landing gets really tricky.

About 0.6 miles from the surface, the descent vehicle springs into action. It retro rockets will fire slowing Curiosity's descent further and bringing directional control into the descent. The retrorocket's ignition triggers the jettison of the backshell and parachute. The rover, its underside exposed to the quickly-approaching Martian surface, and the descent vehicle are now On their own above the surface.

The descent vehicle's retros continue to slow the rover's descent. When the radar senses it is about 115 feet above the surface, Curiosity will fall from the descent vehicle on a 65 foot long tether. Dangling the rover, the descent vehicle will continue to descend — at this point moving at less than 1 mile an hour — until Curiosity's wheels touch the surface. If all goes according to plan, this should happen with the descent vehicle a safe 16 feet above the rover and the surface. The two will stay mated briefly before the descent vehicle severs the cord; its retros will carry it away from the rover, which will be free to begin its stay on exploration of Mars.

It sounds crazy, and it stands to reason the more moving parts means more can go wrong. But the Sky Crane system does solve the problem of getting a heavy payload safely to Mars. There are no rockets on the rover itself, there is no need for giant unwieldy parachutes that won't inflate properly, nor for cumbersome airbags.

Really, the system is no more complicated than the retrorockets that have already put landers on Mars — and men on the moon. As long as the tether does what it needs to do the landing should be smooth. And the supersonic parachute has to inflate. And the radar has to work properly to gauge altitude. And the descent vehicle's retros need to fire on time and with the necessary thrust to gently lower Curiosity.

A lot has to go right, but it's designed to work. We'll all just have to wait and see what happens.

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