NASA Engineer Explains the Audacious Plan to Land Curiosity on Mars

Legged landers have the fundamental issue that your rocket engines end up very close to the ground. The rockets can dig a hole in the surface and you can get tremendous winds and turbulence as you're trying to set down very gently. You don't want to end up in a situation where the rover wakes up and it's got an inch of dust over the entire top deck.

If you want to put a 1-ton rover on top of a legged lander and keep those beefier engines from firing directly into the surface, you've got to make the legs longer. Now you have a sort of geometry problem. You've got a 1-ton rover on top of long skinny legs. You're top-heavy, and that's not good. That brings up some stability problems when you're trying to set down on surfaces where you may not know the slope exactly. There could be a rock under a leg. Even if that landing configuration works, then you have to drive the rover off the top of the platform by deploying ramps and without falling off the side. You start to get really nervous.

So then you go to airbags, which we've used on Pathfinder and the Spirit and Opportunity missions. To scale up those airbags for this size rover, you'd need some really big airbags, in which case now you're paying a mass penalty. For something as big as Curiosity, it's very hard to come up with a reliable plan that would get you clear of the giant pile of deflated airbags without getting tangled or causing significant risk to the rover. Instead, you could keep the airbags somewhat reasonably sized but just hit the ground [more slowly]. When you start doing the math and the design, you realize... why not hit the ground a little slower and forget all the airbags, and just land right on the wheels? That's sort of the high-level reasoning that went into the Sky Crane system.

Source: http://www.popularmechanics.com/science/space/moon-mars/nasa-engineer-explains-the-audacious-plan-to-land-curiosity-on-mars-11182049?src=rss

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