Why the sky crane isn’t the future for Mars landings

Sky crane delivers Curiosity to Martian surface in artist’s conception. (Credit: NASA/JPL-Caltech)

Given all the attention that it is receiving, the innovative technology that will place the Curiosity rover on Mars — the sky crane — may seem like something that we’ll be seeing much more of during future space missions. Yet it’s not. In fact, there’s good reason to suspect that it will be a long time before the sky crane is used again on Mars, if ever.

Of course, its prospects do depend on the success or failure of the Curiosity landing, but let’s hopefully assume the best. [Update, 1:36 a.m. EDT, Mon.: The best occurs! Success!] Instinctive skepticism has always greeted the plan: it is complicated and unorthodox, and a mishap anywhere along the chain of feats in involves leads to disaster. Even those of us enthusiastic about the sky crane have often conceded that it sounds crazy but might just be crazy enough to work. Even that skepticism, though, isn’t exactly why the sky crane won’t be selected for many other missions.

The absence of the sky crane might seem all the more surprising given that NASA’s rationale for using it with Curiosity has always been that it had no good alternatives. As I explained in my SmartPlanet column about it, technologies used to land other probes on Mars hit their limits with something the size and weight of the Curiosity rover. Parachutes can’t slow the craft enough in the thin Martian atmosphere for a soft landing. Airbags can absorb the force of a landing impact for the 400-lb. Spirit and Opportunity rovers but not something as big and sensitive as one-ton Curiosity. Rocket thrusters, the old reliable standby, can do it but at the cost of disrupting and polluting the landing site.

So then why wouldn’t the sky crane be the method of choice for upcoming probes? Because the sky crane is an expensive technology developed by NASA, and NASA is temporarily getting out of the Mars lander business.

Artist’s conception of the MAVEN orbiter. (Credit: NASA)

NASA expects to get years of good results out of Curiosity, so it won’t be idle on the Mars front. Nevertheless, the only mission concretely on its schedule now is the Mars Atmosphere and Volatile Evolution (MAVEN) orbiter, set to launch in 2013, which will study the planet’s upper atmosphere. It won’t be sending anything to the surface at all. NASA has long-term Mars exploration plans that would repeatedly take it to the surface — with balloons, aircraft, deep-drilling probes, more rovers, and even rockets capable of returning samples to Earth — but none of those has been scheduled or funded yet, and the cloudy condition of the economy makes it unclear when they will be.

Originally, NASA was planning to participate in the ExoMars program, a pair of joint lander missions with the European Space Agency and Russia (which was initially a minor partner) scheduled to launch in 2016 and 2018. ExoMars has been battered with financial problems and shifting plans throughout its history, and those only got worse early this past February when NASA confirmed that it was dropping out of the project because of budget constraints.

The two ExoMars missions never needed a sky crane. The one launching in 2016 was always planned to deliver a relatively small stationary instrument package. The 2018 launch will be sending a rover, but one that’s only about 200 lbs. heavier than the Spirit and Opportunity rovers. That much extra mass doesn’t justify all the complex contrivances of the sky crane, though it probably does exceed the tolerances of what airbags alone could handle.

Scale model of the proposed ExoMars landing-stage vehicle, showing the three sets of thrusters that will slow and guide its descent. (Credit: Anatoly Zak/RussianSpaceWeb.com)

The ExoMars partners have therefore developed an entry, descent, and landing system of their own. Like the Mars Science Laboratory and Curiosity, after using its heat shield to slow atmospheric entry, the ExoMars Entry, Descent and Landing Demonstrator Module will release a parachute for further deceleration. After cutting loose from the chute, the ExoMars vehicle will use small liquid thrusters to guide and slow its descent — much like the sky crane descent vehicle except that it goes all the way to the ground. That landing only qualifies as “semi-soft”: whereas Curiosity arrives on Mars with a kind of suave grace, like a tuxedoed James Bond arriving by jetpack, the ExoMars landers will hit the surface with a bit more of a thump and count on airbags to absorb the worst of it. Nevertheless, it’s a neat, elegant system that bears more than a little similarity to the arrival of a flying saucer.

The ExoMars vehicle will enter the atmosphere of Mars at a speed of about 21,000 kilometers per hour. (Credit: ESA)

The craft will deploy a parachute when it has slowed to a speed of Mach 2, and will drag it down to subsonic descent speeds. (Credit: ESA_

Three sets of hydrazine thrusters will allow the ExoMars vehicle to make a semi-soft landing on the Martian surface. (Credit: ESA)

(Sadly, Europe’s financial problems call into doubt whether ESA will be able to mount these missions at all. The ExoMars partners will need to collect commitments on all the needed money by November.)

So no other mission will use a sky crane through the end of this decade at the least. In theory, sky cranes could reappear after 2020 if Mars missions would involve payloads massing on the same order as Curiosity or larger. Any human missions would surely meet that weight requirement. Yet even for some heavy missions, a stepped-up version of the ExoMars system might turn out to be a preferable compromise.

Any decision to go with a sky crane rather than a thruster-assisted landing has to be made by assessing the risks of the more complicated procedure against the priority of not disrupting the landing site with rocket exhaust. For example, imagine that you are planning the early human missions to the Red Planet. If the lander will include rocket systems for returning the astronauts to orbit, will you care whether the sit is disturbed? Will the sky crane solution be able to scale up to handle something the size of a return vehicle? Would it be easier and safer to let the vehicle’s own rockets handle the landing?

Sky cranes may not disappear altogether from NASA’s plans, but they will always represent just one solution among a mix of several from which mission planners will choose. They may not be the future, but they will probably stay as just a part of it. And they surely are all-important to the stage of Mars exploration immediately ahead of us: much of what we hope to learn about Mars over this next decade will all depend on how well the sky crane delivering Curiosity a few hours from now performs.

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