The Mars-bound Curiosity rover will be begin steering toward the red planet during a lengthy thruster firing Wednesday, erasing the launch trajectory's deliberate aim away from the destination.

This is an artist's concept of NASA's Mars Science Laboratory spacecraft during its cruise. The spacecraft includes a disc-shaped cruise stage (on the left) attached to the aeroshell that contains the rover and descent stage. Credit: NASA/JPL-Caltech

Just like all planetary launches, the rocket actually fired the payload off-center from the target on a path to miss Mars by 38,000 miles to ensure the Centaur upper stage that is following the spacecraft won't hit the planet. The rocket motor wasn't subjected to the thorough cleaning to prevent Earth's microbes from contaminating Mars, thus the purposeful effort to prevent an impact.

The car-sized rover, packed inside the protective descent capsule, will use 8 thrusters located on the donut-shaped cruise ring atop the spacecraft to conduct the trajectory correction maneuver Wednesday starting at 6 p.m. EST (2300 GMT).

The first opportunity to perform the post-launch adjustment was available 15 days into the mission. But flight controllers opted to cancel that plan and wait six weeks before doing it now.

Seen as the largest maneuver the spacecraft will carry out during its 8.5-month trek from Earth to Mars, the thrusters are fired in a pre-programmed sequence over the span of 175 minutes to impart a velocity change of about 12.3 miles per hour.

"We are well into cruise operations, with a well-behaved spacecraft safely on its way to Mars," said Arthur Amador, Mars Science Lab's cruise mission manager at NASA's Jet Propulsion Laboratory in Pasadena. "After this trajectory correction maneuver, we expect to be very close to where we ultimately need to be for our entry point at the top of the Martian atmosphere."

The mission's trajectory -- as spelled out before launch -- included windows for 6 correction maneuvers. The last could occur as late as 9 hours before landing to fix any errors prior to atmospheric entry.

A burn in June will eye the precise corridor to the landing site where scientists want to send the rover within Gale Crater to determine if the area with its layered sedimentary landscape and apparent watery past was ever hospitable to life. The landing zone is a relatively small ellipse just 12.4 miles wide and 15.5 miles long -- a mere one-third the size of previous Mars rovers.

The $2.5 billion mission is headed for a late-night landing (California time) August 5 between 10 and 10:30 p.m. PDT (1 and 1:30 a.m. EDT Aug. 6) in Gale Crater.

As of Saturday, the spacecraft has traveled 73 million miles of its 352-million-mile trek to Mars, moving at 9,500 mph relative to Earth and at 69,500 mph relative to the Sun.

Mars Science Lab is spinning at 2.04 rotations per minute, communicating with Earth at a transmission rate of 2 kilobits per second and producing 780 watts of power from the cruise ring's solar arrays.

And the mission's Radiation Assessment Detector continues collecting data on the way to Mars.

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