When the car-sized MLS Curiosity rover arrives at Mars in August it will encounter a problem that has been the death of more than a few of the robot explorers we've sent to the Red Planet. While only 1% as dense as on Earth the atmosphere that surrounds Mars is still thick enough to burn up arriving craft from friction heating. On the other hand the atmosphere is too thin for parachutes to be of much use. As one of the speakers in the following talk explains, "It's too dense to ignore, but too thin to help."
NASA has a plan they hope will work. It's detailed in this fine little video, Challenges of Getting to Mars: Curiosity's Seven Minutes of Terror. Wicked challenge, awesome engineering, fascinating stuff. Upon their clever solution all depends.
Image: NASA/JPL-Caltech by way of http://en.wikipedia.org/wiki/Mars_Science_Laboratory