Nicknamed Curiosity, the rover was built by the Jet Propulsion Laboratory in Pasadena and is poised to begin a risky voyage with the promise of a big scientific reward. Launch is scheduled for 7:02 a.m. PT.
A powerful Atlas V rocket will deliver the $2.5 billion science lab into Earth orbit, then the rover's own booster must launch it on a path to Mars. That's the vital step that failed on a Russian mission to Mars two weeks ago, leaving the spacecraft stranded.
Next August will be a nail-biting time for the crew at JPL. The designers hope a set of thrusters will fire to slow the rover down, then the heavy payload is detached like a sky crane and gently dropped onto the surface.
It will be pretty tricky, but it has been tested thoroughly on Earth and JPL is confident.
"If something breaks, decides to break at that point of time, we're in trouble. But we've done everything we can think of to do," said Pete Theisinger, Mars Science Laboratory project manager.
The 10-foot-long Curiosity is the largest and most complex piece of equipment ever to be placed on the surface of another planet. Its two year mission has been carefully plotted.
"Curiosity rover is really a rover on steroids," said Colleen Hartman, NASA assistant associate administrator.
JPL officials said they are looking for a place on the planet where life might have flourished early in Mars history.
Although the mission is designed for two years, it could be longer if the rover survives the rugged conditions on Mars. Its radioactive power source has a minimum lifetime of 14 years.