NASA's Mars Reconnaissance Orbiter is nearing a crucial milestone. The spacecraft is preparing to slow itself to allow the red planet's gravity to grab it into orbit on March 10.

"Once the spacecraft has successfully been placed in position, this mission will greatly expand our scientific understanding of Mars, pave the way for future robotic missions later in this decade, and help us prepare for sending humans to Mars," said NASA's Director of the Mars Exploration Program Doug McCuistion.

Designed to examine the planet in unprecedented detail, the orbiter will return more data than all previous Mars missions combined. Before the orbiter can begin its mission, it will spend approximately six months adjusting its orbit with an adventurous process called aerobraking.

The initial capture by Martian gravity will put the orbiter into an elongated, 35-hour orbit. The planned orbit for science observations is a low-altitude, nearly circular, two-hour loop. Aerobraking will use hundreds of carefully calculated dips into the upper atmosphere, deep enough to slow the spacecraft by atmospheric drag, but not deep enough to overheat the orbiter, to gain the desired orbit.

"Aerobraking is like a high-wire act in open air," said Jim Graf, orbiter project manager at NASA's Jet Propulsion Laboratory, Pasadena, Calif. "Mars' atmosphere can swell rapidly, so we need to monitor it closely to keep the orbiter at an altitude that is effective and safe."

As the orbiter nears Mars March 10, ground controllers expect a signal shortly after 4:24 p.m. EST indicating the critical engine burn to place it into low orbit started. The burn will end during a suspenseful 30 minutes, with the orbiter behind Mars and out of radio contact.

The orbiter carries six instruments that will produce data for studying Mars from underground layers to the top of the atmosphere. They include the most powerful telescopic camera ever sent to another planet; it will reveal rocks the size of a small desk. An advanced mineral-mapper will be able to identify water-related deposits in areas as small as a baseball infield. Radar will probe for buried ice and water. A weather camera will monitor the entire planet daily. An infrared sounder will monitor atmospheric temperatures and the movement of water vapor.

"We're especially interested in water, whether it's ice, liquid or vapor," said Richard Zurek, Jet Propulsion Laboratory orbiter project scientist. "Learning more about where the water is today and where it was in the past will also guide future studies about whether Mars ever supported life."

The orbiter can transmit data to Earth at approximately 10 times the rate of any previous Mars mission. It will use a 10-foot diameter dish antenna and a transmitter powered by 102 square feet of solar cells. Scientists will analyze the information to gain a better understanding of changes in Martian atmosphere and the processes that formed and modified the planet's surface.

In addition to its own investigation of Mars, the orbiter will relay information from future missions working on the surface of the planet. During its planned five-year prime mission, it will support the Phoenix Mars Scout being built to land on icy soils near the northern polar ice cap in 2008, and the Mars Science Laboratory, an advanced rover under development for launch in 2009.

For information about NASA and agency programs ion the Web, visit:

http://www.nasa.gov/home

For Information about the Mars Reconnaissance Orbiter on the Web, visit:

http://www.nasa.gov/mro

The mission is managed by the Jet Propulsion Laboratory, a division of the California Institute of Technology, Pasadena, for NASA's Science Mission Directorate. Lockheed Martin Space Systems, Denver, built the spacecraft and is the prime contractor.