At the end of its 350-million-mile journey - which began last Aug. 12 � controllers at NASA's Jet Propulsion Laboratory received the signal at 5:25 p.m. Eastern Time that the MRO had attained a correct attitude to achieve a proper orbit.

"We now have orbit around the planet Mars," said Jim Graf, the MRO mission manager. The announcement brought exuberant cheers and applause among the mission team.

About an hour earlier, 4:24 p.m., the MRO had reported its 27-minute orbital-insertion burn had begun precisely on time. The burn slowed the spacecraft's speed to about 11,000 miles (17,700 kilometers) per hour, and swung it around Mars at an altitude of about 260 miles (420 kilometers) at its closest point.

Then at 5:16 p.m., the spacecraft's brief occultation by Mars ended and its first after-orbit signal arrived. The news brought screams of excitement in the control room. "It's right on the money," one scientist was heard to say.

Any tense moments that preceded the confirmation signal had resulted from the knowledge by the MRO team that orbital insertion presented one of the mission's most difficult challenges. Doug McCuistion, director of NASA's Mars Exploration Program, had reminded reporters last month that of all spacecraft sent to Mars, only 65 percent achieved orbit.

Before the MRO can begin its two-year science mission, however, it must spend nearly seven more months adjusting its instruments and its orbit, using an experimental process called aerobraking.

As explained by Graf, the spacecraft's initial capture by Martian gravity has placed it into an elongated, 35-hour orbit. Then, the spacecraft will use hundreds of guided dips into the upper Martian atmosphere. The maneuvers will apply atmospheric drag to slow the MRO - but not overheat it - and reshape its orbit into a nearly circular, two-hour loop at an altitude of 200 miles (321 kilometers).

This strategy allowed NASA to save 500 kilograms (1,100 lbs.) at launch, but "aerobraking is like a high-wire act in open air," Graf said. The Martian atmosphere can swell rapidly, "so we need to monitor it closely to keep the orbiter at an altitude that is effective and safe."

The orbiter is carrying six instruments to study Mars from its subsurface geology to its high thin cloudtops. They include the most powerful telescopic camera ever sent to another planet - it can image rocks the size of a small desk. Those instruments will allow the spacecraft to "return as much information as all previous Mars missions combined," said Michael Meyer, the orbiter's chief scientist.

Using the high-resolution camera, said Richard Zurek, JPL's orbiter project scientist, "you could be in New York and image people walking on the Mall in Washington, D.C." He said mission scientists as usual are "especially interested in water, whether it's ice, liquid or vapor. 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."

In addition to the camera, the orbiter carries an advanced mineral mapping instrument called PRISM, which can handle 600 channels of spectrographic information and can identify water-related deposits in areas as small as a baseball infield. Its 30-foot radar antenna will probe for ice and water buried as deep as a half-mile below the surface. Its weather camera will monitor the entire planet daily, and its infrared Climate Sounder will monitor atmospheric temperatures and the movement of water vapor.

In all, the MRO represents an extension of all of NASA's planetary exploration capabilities, including - after it finishes its science mission - communications relay. "The instrumentation is unprecedented," Meyer said, adding that the orbiter can transmit data approximately 10 times faster than any previous Mars craft, and it is expected to return more data than all previous Mars missions combined.

After it completes its data-collection mission, the orbiter will use its 10-foot dish antenna and transmitter powered by 102 square feet of solar cells to relay information - possibly from the indefatigable twin Mars Exploration Rovers, Spirit and Opportunity - but primarily in support of the Phoenix Mars Scout, scheduled to land near the northern polar ice cap in 2008, and the Mars Science Laboratory, a Volkswagen-sized rover planned for launch in 2009 and arrival in 2010.

"The MRO will play a major role in where the next rovers go," Meyer said, because it will study the overall Martian surface in greater detail than ever before.

For example, McCuistion explained, as Opportunity heads to a crater called Victoria, in Meridiani planum, the MRO is capable of taking such detailed images it can help MER mission controllers refine the craft's route to the formation.

"We're hoping we can actually image the rovers," he added.

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