Three weeks after successfully entering orbit around the red planet, the MRO began a phase called aerobraking, which uses friction with the tenuous Martian upper atmosphere to transform an elongated 35-hour orbit to a nearly circular two-hour orbit needed for detailed science observations.

The orbiter has been flying about 426 kilometers (265 miles) above Mars' surface at the nearest point of each loop since March 10, then swinging out to more than 43,000 kilometers (27,000 miles) away before heading in again. While preparing for aerobraking, the flight team at Jet Propulsion Laboratory tested several instruments, obtaining the orbiter's first Mars images and demonstrating the ability of the Mars Climate Sounder instrument to track the atmosphere's dust, water vapor and temperatures.

Controllers commanded the MRO to fire its intermediate thrusters last Thursday for 58 seconds at the far point of the orbit. That maneuver lowered its altitude to 333 kilometers (207 miles) when the spacecraft next passed the near point of its orbit, at 6:46 a.m. Pacific time Friday.

"We're not low enough to touch Mars' atmosphere yet, but we'll get to that point next week," said JPL's Daniel Kubitschek, the deputy leader for the aerobraking phase of the mission.

The phase includes about 550 dips into the atmosphere, each planned for the desired amount of braking. At first, the dips will be more than 30 hours apart, but by August, the orbiter will be doing four per day.

"We have to be sure we don't dive too deep, because that could overheat parts of the orbiter," Kubitschek said. "The biggest challenge is the variability of the atmosphere."

Readings from the onboard accelerometers during the passes through the atmosphere will provide information about upward swelling of the atmosphere due to heating.

The Mars Climate Sounder also has the capability to monitor changes in temperature that would affect the atmosphere's thickness. "We demonstrated that we're ready to support aerobraking, should we be needed," said JPL's Daniel McCleese, principal investigator for the instrument.

Infrared-sensing instruments and cameras on two other Mars orbiters are expected to be the main sources of information to the advisory team of atmospheric scientists providing day-to-day assistance to the aerobraking navigators and engineers. "There is risk every time we enter the atmosphere, and we are fortunate to have Mars Global Surveyor and Mars Odyssey with their daily global coverage helping us watch for changes that could increase the risk," said JPL's Jim Graf, the MRO's project manager.

Using aerobraking to get the spacecraft's orbit to the desired shape, instead of doing the whole job with thruster firings, reduces how much fuel a spacecraft needs to carry when launched from Earth. "It allows you to fly more science payload to Mars instead of more fuel," Kubitschek said.

As soon as it achieves its science orbit, the MRO is expected to return more data about the planet than all previous Mars missions combined. The data will help researchers decipher the processes of change on the planet, and aid future missions to the Martian surface by examining potential landing sites and - after its primary science tasks are completed - providing a high-data-rate communications relay.

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