WASHINGTON — The largest heat shield ever built for a probe bound for Mars is ready for the new rover Curiosity, a massive martian robot the size of a car.

The immense heat shield will shroud the Mars Science Laboratory rover, now named Curiosity, to protect it during its deep space cruise to Mars and the searing heat of entry into the martian atmosphere. Lockheed Martin unveiled the heat shield June 16 and delivered its conical backshell to NASA last year.

“The Mars Science Laboratory aeroshell is the most complex capsule to fly to Mars,” said Rich Hund, program manager at Denver-based Lockheed Martin Space Systems. “The design had to address the large size and weight of the rover, the largest ever sent to Mars, and the requirement for landing at a more-precise point on Mars.”

Curiosity’s heat shield and conical backshell together make up the rover’s 4.5-meter-wide aero shell – larger than those used with previous Mars rovers and even larger than the shell used for NASA’s Apollo spacecraft that ferried astronauts back from the Moon. The Mars rovers Spirit and Opportunity were protected by shells measuring 2.6 meters and the Apollo capsule’s heat shield measured just less than 4 meters across.

As the rover, which is about the size of a small car, descends over the red planet the friction it creates with the thin martian atmosphere could create temperatures up to 2,100 degrees Celsius on the outside of the shield. To withstand the heat, the shield is tiled with a material called a Phenolic Impregnated Carbon Ablator. This will be the first time this heat shield type has flown on a Mars mission.

Invented at NASA’s Ames Research Center, Moffett Field, Calif., the material first flew as the heat shield for the agency’s Stardust sample return capsule, which collected particles from a comet and returned them to Earth in 2006.

NASA has an intricate landing sequence planned for the new Curiosity rover. After entering the martian atmosphere, the rover will slow itself with a parachute and jettison its heat shield and back shell before beginning a powered descent with thrusters on a so-called sky crane. The crane will lower the rover to the ground on cables and then fly itself off and crash.

Curiosity is scheduled to launch in the fall of 2011 to broaden NASA’s exploration of Mars and better understand the planet’s watery past. The evidence the rover gathers may help answer questions about life on Mars and the planet’s habitability in the past or present. The rover should also assess Mars’ climate and geology in preparation for human exploration.