The measurements from the rangefinding terminal descent sensor are absolutely critical to the timing of the rest of the landing events. But since Curiosity is not descending vertically, the altitudes that the terminal descent sensor are measuring are of spots at some distance away from the spot at which Curiosity will land. It could land as far as 700 meters away from the last range measurement it takes before its backshell separates. This is the reason that the Curiosity landing site has to be flat on several-hundred-meter length scales.

Another sensor that will be operating at this time is the Mars Descent Imager, whose photos will provide context for Curiosity's landing spot and help the science team figure out exactly where they are when they land.

So the terminal descent sensor is pinging away at the surface, providing continuous data on Curiosity's altitude, as it continues to descend and decelerate under parachute. The next step in the process comes when the spacecraft has slowed to about 80 meters per second, at an altitude somewhere around 1.5 kilometers. If all of this sounds rather approximate to you, it's not; there are 500,000 lines of code stored in Curiosity's electronic brains to handle every possible set of landing conditions, and the landing engineers have spent years poking and pushing at that code, throwing increasingly bizarre and challenging sets of conditions at it, including all kinds of bad-luck and low-probability failures of various components, to make sure it can handle every foreseeable contingency.

At 100 meters per second and 1500 to 2000 meters altitude, the spacecraft is ready to initiate the next step: powered descent. It prepares by opening the fuel lines to the 8 descent thrusters on its jetpack (otherwise known as the descent stage), and commanding them to 1% throttle.

Then comes what is, for me, the scariest moment in that landing animation. Pyros fire to separate the descent stage from the backshell and parachute. Suddenly lacking any means of slowing down, the spacecraft free-falls, accelerating toward the ground under Martian gravity for one heart-stopping second, speeding up from 100 to 120 meters per second. The one-second free-fall is necessary to ensure that the descent rockets don't slam the rover back into the backshell when they fire up. But: yikes.

Powered Descent

As the rover is free-falling, the rockets are already on. After that one second, they're throttled up. Still, the thrusters are not being used to decelerate the rover; at first, they are commanded to reduce any rotation of the spacecraft to zero as it continues to fall, accelerating slightly. Only when rotation has been damped, after about 400 meters of descent, do the thrusters power up to begin to slow the descent again.

At this point, the thrusters will smoothly operate to slow the spacecraft, completely halting its horizontal motion while bringing its vertical descent rate to 20 meters per second. This will take roughly 30 seconds, give or take, depending on the altitude at which the rover fell out of its backshell. Meanwhile, the backshell and parachute, equipped with no deceleration devices, will overshoot and crash ahead of the landing site.