When we left our hero, Curiosity had traveled hundreds of millions of kilometers from Earth toward Mars, and had fewer than 20 meters left to go. She will be descending at 0.75 meters per second toward the surface. Fewer than 15 seconds remain in her trip. It's time for the part of the mission that people seem to be the most scared about: the skycrane maneuver.

I want to begin by reviewing why something like the skycrane maneuver is necessary. There are two main reasons. First: Curiosity is huge, nearly five times the mass of Spirit or Opportunity. It's half again larger than a Viking lander. A Mars Exploration Rover weighs 185 kilograms; to get that 185 kilograms of rover safely to the ground required a protective enclosure of a lander weighing in at 348 kilograms. Of all that 533 kilograms of landed mass, only 5 kilograms -- less than 1%! -- were science instruments.

Curiosity weighs 900 kilograms, of which 75 kilograms is science instruments. That's a much better proportion! But there is no way to wrap this thing in a lander large enough to protect it against an impact with the Martian surface. Instead, what you want to do is to go back to something more like Viking -- use a rocket-assisted descent -- except that Curiosity has to be able to drive away from the landing site, which means that the wheels have to serve in place of Viking's landing legs, and after the rover is on the ground you would really like to get the rockets and any remaining explosive fuel far away from your precious scientific exploration vehicle.

So: you need to set the rover down very gently, then sever the connection with the rockets and send them away, all without blasting the rover and its science instruments with rocket exhaust.

That's why skycrane is needed. There are five main steps in the skycrane maneuver: 1) separate the hard connection between the rocket-powered descent stage and the rover; 2) deploy the rover's landing gear (that is, its legs and wheels); 3) deposit the rover very gently on the surface; 4) sever the soft connection between the descent stage and the rover; and 5) fly the descent stage far away where its fuel can't do the rover and its instruments any harm, and won't contaminate the landing site.

Sky Crane

The first step in the Sky Crane maneuver is to separate the hard connection between the rover and the descent stage using pyros. Now all that remain are four connections: a triple bridle consisting of nylon cables, which bear all the weight, and an umbilical that supplies the electrical and electronic connections from the rover to the descent stage.

The three ropes come together at a "confluence point" underneath the descent stage, located near its center of mass; this design prevents differential loading on the bridle from tipping the descent stage. As soon as the hard connection between the rover and descent stage is severed, gravity carries the rover downward. The three ropes unwind together from an electromagnetically braked spool; it's the electromagnetic brake that controls the rate of the rover's descent. Here's what that spool looks like; there's another view here.