Terror on Mars. Source: NASA.
Terror on Mars. Source: Ghosts of Mars.
|Cuddle me! Source: Wikipedia Commons.|
That's the heaviest rover humans have ever attempted to land on Mars. While previous missions slowed on final descent with retrorockets on the craft and/or bounced to a final landing with airbags, that won't work for Curiosity.
|Apollo 11 splashdown. Source: NASA.|
A parachute will be deployed to initially slow Curiosity's landing, but that wouldn't be enough to slow the craft to a safe speed. Parachutes are enough on Earth, but the Martian atmosphere is much less dense than Earth's atmosphere. Instead, a mile from the ground, the spacecraft's cover will come off, and a "sky crane" will emerge, holding the rover below it, attached by tethers. The sky crane has retrorockets, which it will use to fly Curiosity to its landing site, and then drop the rover on the ground. Then, the sky crane tethers will detach and it will zoom off and crash at a safe distance. The rover will have been gently placed on the ground, wheels down, ready to roll.
The advantage of the sky crane system over airbags is size and weight: Curiosity can be much larger and heavier than previous mini-rovers that descended to Mars surrounded by airbags and then bounced to a landing. The advantage of the sky crane system over attached retrorockets is that an actual wheeled rover can land, and then drive around Mars without rockets attached (or without the trouble of detaching rockets from its underside after landing).
I'm not an engineer. This is how my non-engineer brain assesses the risk of the sky crane: it's better than airbags because we want to eventually land people on Mars, and we can't do that by bouncing their craft to the ground. And it's better to have the rover attached to the retrorockets with a tether that can be severed quite easily than attached to retrorockets by a more permanent, but complicated system that contains more parts that could break.
Fingers crossed for August 6!
Sources: Discovery News; NASA JPL; Scientific American.