At 10:31 p.m. PDT tonight, August 5 (1:31 a.m. EDT Aug. 6), after nearly 9 months of travel, NASA’s Mars Science Laboratory (and a rover named Curiosity) will arrive at the Red Planet to explore the interior of Gale Crater and hunt for the ingredients of life. Of course, between arriving and hunting, there has to be a landing… and that’s where things are going to get a little wild.

The Mars Science Laboratory landing sequence has been the topic of much discussion even before the mission launched on November 26. Nicknamed “seven minutes of terror”, the spacecraft’s entry, descent (both parachuted and powered) and unprecedented sky crane landing will require a lot of things to go perfectly right – all before anyone on Earth even receives a single signal, due to the length of time it takes for information to travel to Earth from Mars.

Recently I had a chance to talk with Steven Sell, Deputy Operations for Entry, Descent and Landing at NASA’s Jet Propulsion Laboratory in Pasadena, CA, for an article I wrote for Space.com about Curiosity’s exciting and unprecedented landing sequence.

“By the time we get the first signal that says ‘okay, I’ve now reached the top of the atmosphere’, in reality the rover has already been on the surface for seven minutes,” said Sell. “It’s like your kid playing in the big game, and you can only sit there in the stands and watch. It’s a nail-biter all the way.”

When MSL arrives at Mars it will be traveling at a velocity of 13,000 miles per hour. Within seven minutes it needs to get down to a velocity of zero miles per hour… all in one piece, of course, and in the right spot. The first step in this process will be guided entry, during which the entire MSL spacecraft (currently consisting of the rover and descent stage tucked into a protective aeroshell) will adjust its course toward its 12-by-5-mile-diameter landing ellipse – only an eighth the size of the landing targets of previous rovers.

As it impacts Mars’ atmosphere, MSL’s heat shield will take the brunt of the frictional heating generated by the deceleration, and will soon glow white-hot with temperatures reaching 1,600 degrees F. Even through this, MSL will still be physically guiding itself toward Gale Crater, firing rockets to keep it on track.

100 times thinner than Earth’s, the atmosphere on Mars isn’t dense enough to slow MSL down by itself. So at this point a parachute will be deployed – literally the largest supersonic drogue chute ever created, it will slow MSL down from 1,000 mph to about 200 mph, subjecting the spacecraft to 9 Gs of force.

Then, with the surface of Mars still approaching rapidly, the heat shield will be jettisoned, exposing the actual rover and allowing it to use its radar guiding system to determine just how high it is. This is one of the most crucial parts of the descent, since it’s not until then that MSL will know how high it really is.

“When the heat shield comes off and the radar turns on, we need to find the ground,” said Sell. “When we first eject the heat shield we’re too high for the radar to see the ground yet, so ew have to wait a very long 20 to 30 seconds, up to a minute, until the radar can get close enough to the ground to be able to see it. Without that solution it doesn’t even try to do the rest of the landing.”

And once that’s all done, it‘ll really start to get interesting.

At 2,000 pounds, Curiosity is simply too large to land with airbags like previous rovers. Instead, engineers devised a method that’s never been attempted before: a sky crane. When MSL reaches precisely the right altitude, the descent stage, gripping the Curiosity rover within the sky crane structure, will drop from the aeroshell and quickly fire its thrusters, moving it safely away from the falling back shell and slowing it further.

(Just how big is Curiosity? Find out here.)

Curiosity will be carried steadily downwards by the descent stage, which will use its Mars Descent Imager (MARDI) camera to maneuver over its target and, once at a height of 20 meters (about 65 feet) use rockets to hover in place while it lowers the rover down to the surface on bridles and an umbilical cord – all three of which are bearing the weight of the rover. Although the system has been exhaustively tested on Earth using simulations and very advanced computer models, the first time it’s going to play out in full will be during the actual landing on Mars.

“It’s the ultimate field test,” Sell said.

As soon as Curiosity has touched down, the cords will be cut and the descent stage will soar safely off to the side, crashing onto the Martian surface far enough away to pose no danger to the rover. At this point Curiosity, wheels down, is ready to begin its mission.

That is, as long as everything goes exactly and precisely as planned. On the night of August 5, the entire world – not to mention quite a few folks at JPL — will be watching to see how a rover the size of a Mini Cooper can be sent hundreds of millions of miles to touch down safely and precisely on the surface of another planet.

“We’ve been preparing for this for many, many years…we’re excited to get out there and land this thing.” Sell said. (And even after Curiosity lands, we won’t know how it’s doing for at least 14 minutes… and possibly much longer.)

Best of luck to Steve, Curiosity and the entire JPL mission team, as well as the many engineers and scientists who have been working on the various instruments elements of the MSL mission over the past several years. If you want to follow along with Curiosity’s progress tonight, you can watch online on JPL’s live broadcast here and, if you’re on Google+, you can tune in to a live Hangout featuring Fraser Cain (creator of Universe Today), Pamela Gay, the “Bad Astronomer” Phil Plait, special guests from JPL and NASA and a few easily-excitable space bloggers (such as myself!) Tune in starting at 8:00 p.m. PDT — add this event to your Google+ calendar here.

Go Curiosity!