During this approach, as the craft speeds through the atmosphere, the Curiosity rover and the descent stage are safely tucked inside the aeroshell's heat shield and backshell, depicted in this artist's rendering. The diameter of the aeroshell is 14.8 feet, the largest ever used for a mission to Mars.
Careening through the Martian atmosphere, the shell will be heated to more than 1,600 degrees Celsius by friction, which will also slow the craft significantly, to 1,000 mph. This, however, is still faster than the speed of sound, and far too fast to allow for a safe landing. The Martian atmosphere poses engineering challenges for NASA -- being 100 times thinner than Earth's, it's thick enough to destroy an improperly shielded spacecraft during entry, but not thick enough to slow the craft to subsonic speeds.