HOUSTON — NASA’s $2.5 billion Mars Science Laboratory mission has successfully carried out the largest course refinement maneuver planned for its eight-and-a-half-month transit, a three-hr. series of cruise stage pulses setting up the rover for an Aug. 6 powered descent into Gale Crater.

At midday Thursday, MSL passed the 81.2 million-mi. mark of a 352 million-mi. journey that began with a Nov. 26 liftoff from Cape Canaveral Air Force Station.

A smaller course refinement maneuver is planned for March 26. And up to four more opportunities for small corrections in the trajectory are available prior to the spacecraft’s arrival at Mars.

“The telemetry from the spacecraft and the Doppler data show that the maneuver was completed as planned,” Brian Portock, NASA’s MSL cruise phase deputy mission manager, said in a statement following the crucial Wednesday trajectory adjustment. The operation included a single 19-min. thrust followed by more than 200 precisely timed five-sec. thruster pulses.

“The timing of the encounter is important for arriving at Mars just when the planet’s rotation puts Gale Crater in the right place,” added Tomas Martin-Mur, the chief mission navigator. The mission is managed by NASA’s Jet Propulsion Laboratory in Pasadena, Calif.

The nearly 96 mi.-wide crater with a towering, sediment-layered internal peak was selected from 30 candidate locations by scientists as the landing site for the mission’s instrument-laden rover, also known as Curiosity. Over a two-year mission, MSL will attempt to determine whether the 3.5 billion-year-old depression features environmental conditions suitable for microbial life.

As MSL lifted off atop its Atlas V rocket, the trajectory included an intentional offset to prevent the launcher’s upper stage, which did not meet planetary protection standards, from plummeting into Mars with Earthly contaminants. The series of Wednesday thruster firings placed MSL’s trajectory about 25,000 mi. closer to Mars and advanced the time of the arrival by about 14 hr.