Six Months Of Adjustments Will Shrink Orbit Around Red Planet

Last Thursday, NASA's Mars Reconnaissance Orbiter began its crucial six-month campaign to gradually shrink its orbit into the best geometry for the mission's science work.

The spacecraft is now in the "aerobraking" phase of its entry into Martian orbit. The process uses friction with the upper atmosphere of Mars to transform a very elongated 35-hour orbit around the planet, to a much tighter, nearly circular two-hour orbit needed for the mission's science observations.

On March 30, Mars Reconnaissance Orbiter fired its intermediate thrusters for 58 seconds at the far point of the orbit. That maneuver lowered its altitude to 207 miles when the spacecraft next passed the near point of its orbit, at 9:46 am EST Friday.

Since its successful March 10 arrival at Mars, the MRO has been flying about 265 miles above Mars' surface at the nearest point of each loop, before swinging more than 27,000 miles away before heading in again.

While preparing for aerobraking, the flight team tested several instruments, obtaining the orbiter's first Mars pictures and demonstrating the ability of its Mars Climate Sounder instrument to track the atmosphere's dust, water vapor and temperatures. The image below shows heat emission measurements conducted by the MCS across different wavelengths.

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

Aerobraking includes about 550 dips into the atmosphere, each carefully planned for the desired amount of braking. At first, the dips will be more than 30 hours apart; by August, there will be 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."

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.

Readings from accelerometers during the passes through the atmosphere are one way the spacecraft can provide information about upward swelling of the atmosphere due to heating.

The Mars Climate Sounder instrument 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," JPL's Dr. Daniel McCleese, principal investigator for the Mars Climate Sounder, said of new test observations.

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, project manager for the Mars Reconnaissance Orbiter.

Once in its science orbit, NASA and JPL expect the Mars Reconnaissance Orbiter 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. It will also aid future missions to the surface of Mars by examining potential landing sites and providing a high-data-rate communications relay.

FMI: http://www.nasa.gov/mro