NASA’s nuclear-powered Curiosity rover is perched atop an Atlas V rocket at Cape Canaveral AFS, Fla., and ready to head for Mars at the opening of its planetary launch window on Nov. 25, managers of the Mars Science Laboratory mission said Nov. 10.

“MSL has been assembled, tested, encapsulated, and stacked on top of the Atlas, and is ready to go, 15 days to launch,” says Doug McCuistion, NASA’s Mars Exploration Program director. “It’s pretty incredible.”

Liftoff of the car-sized, six-wheeled rover and its untried “sky crane” landing system is set for 10:25 a.m. EST at the beginning of a window that extends until Dec. 18.

The 850-kg (1,900-lb.) rover is powered by a radioisotope thermoelectric generator that coverts the heat of decaying plutonium-238 to electricity. As a result, Project Manager Peter Theisinger of the Jet Propulsion Laboratory told reporters at a prelaunch press conference that Curiosity should be able to operate “for an extended period of time” as it explores the floor of Gale Crater and climbs the 3-mi.-high mound at its center.

The goal, says Deputy Project Scientist Ashwin Vasavada, will be to study whether Mars has been habitable during the 3-billion-year period represented by the terrain it will explore. Clays spotted on the crater floor by the Mars Reconnaissance Orbiter (MRO) probably formed in the presence of water, while sedimentary layers above them on the mound are believed to be made up of sulfate salts.

An early goal after landing in August 2012 will be to generate ground-truth data to validate the MRO observations.

That task, and subsequent studies, will use the rover’s 6-ft.-long robotic arm to drill into rocks and collect soil for analysis with a sophisticated robotic chemistry lab inside Curiosity’s body. McCuistion called it the “largest and most complex piece of equipment ever placed on the surface of another planet.”

But first it must get there. The rover is mounted upside down atop the Atlas V, shielded by a composite backshell and a heat shield designed to protect it from the friction of entry into the Martian atmosphere.

The United Launch Alliance Atlas V-541 carries four solid-fuel boosters and a Centaur upper stage with a single RL-10 engine. The solid boosters will burn for 1.5 min., while the main stage RD-180 engine will continue firing until 4 min., 20 sec. after liftoff. After it separates, the Centaur will fire twice, with the second burn — lasting almost 8 min. — sending the spacecraft out of Earth orbit and on to Mars.

The cruise phase of the mission will last 255 days if the launch comes at the beginning of the window, using trajectory correction maneuvers as needed to set up the most precise landing on Mars ever attempted. The landing zone on the floor of Gale Crater is an ellipse measuring only 20-by-25 km, well within the confines of the 154-km-dia. crater.

With the sky crane landing method, selected to solve the problem of getting the big rover off the lander and onto the surface, a descent stage will hover on eight downward-facing engines after separating from the backshell and heat shield at the end of a parachute descent to about 1.4 km above the surface. It will descend slowly toward the surface and lower Curiosity on nylon cables until it touches down, then fly a safe distance away and crash after the cables are cut.

“Entry, descent and landing is always a challenging time,” Theisinger says. “We’re confident in our ability to do it at the planet, but it is clearly not risk free.”