...The Spirit will arrive cocooned inside four triangular petals of an 805-pound landing craft. The lander, in turn, is wrapped in air bags and encased in a protective shell.

The craft will slam into the thin Martian atmosphere at 12,000 miles per hour around 11:29 tonight.

During the next four minutes, atmospheric friction will slow it to below 1,000 miles per hour, but not before turning the exterior of its protective heat shield as hot as the surface of the sun.

Less than two minutes before landing, a parachute will pop open, slowing the spacecraft to around 220 miles per hour. The heat shield will detach and the top half of the craft, still attached to the parachute, will release the lander on a tether no thicker than a shoelace.

In the final six seconds, giant air bags will inflate around the folded lander. Retro rockets will fire, slowing the craft to near zero. Then, the tether will be cut and the lander will free-fall the last 49 feet, hitting the dusty Mars surface at 54 miles per hour.

"It's an incredibly violent process," says Steven Squyres, principal investigator for the NASA mission's science instruments. "And it's completely out of our control."[...]