Mars Reconnaissance Orbiter passed the biggest test of its life by safely entering orbit around the Red Planet, joining a constellation of circling spacecraft. But other challenges lie ahead.

Next month, the two-ton orbiter will begin another critical phase in its $720 million mission. It will spend seven months dipping into Mars' upper atmosphere to shrink its current elliptical orbit to a circular one, which will take it as close as 200 miles above the surface.

The purpose is to get as close to Mars as possible to beam back detailed images.

"We got the capabilities that will knock your socks off," said project scientist Richard Zurek.

The Reconnaissance Orbiter joins NASA's Mars Global Surveyor and Mars Odyssey and the European Space Agency's Mars Express, which already fly around the planet. On the surface, the NASA rovers Spirit and Opportunity continue rolling across the planet.

Unlike previous Mars missions, the Reconnaissance Orbiter is the most powerful spacecraft ever to arrive at Mars and is expected to send back more data about the Red Planet than ever before.

After adjusting its orbit, the spacecraft will begin its two-year examination of the planet in low orbit in the fall. It is expected to monitor the Martian climate and atmosphere, search for signs of ancient water on the surface and locate possible future landing sites to send the next generation of robotic rovers and possibly human explorers.

After that, it should serve as a communication relay between Earth and Mars until its primary mission ends in 2010.

Project manager Jim Graf predicted that the scientific results of the mission will be extensive.

"It will rewrite the science textbooks on Mars," Graf said.

Launched from Florida last August, the Reconnaissance Orbiter traveled 310 million miles over seven months for the risky orbit rendezvous.

It successfully circled Mars on Friday after a white-knuckle encounter in which it fired its main engines and briefly lost contact with mission control at the Jet Propulsion Laboratory in Pasadena after flying behind the planet. Engineers applauded when the orbiter came back into view and signaled that it was in position.

Project managers had been nervous about the orbiter's insertion maneuver because of Mars' reputation of producing missing-in-action space probes. In the past 15 years, two of the four orbiters that NASA flew to Mars failed before or during orbit insertion.

But the Reconnaissance Orbiter did not suffer the same fate.

"It happened right on the money," said Dan McCleese, chief scientist for the Mars program at JPL.

Later this month, engineers will send commands to the orbiter to begin the aerobraking process, in which the spacecraft will perform a series of dips into the upper atmosphere, using friction to brake and lower its altitude. Engineers estimate it will take more than 500 maneuvers to slip into a favorable orbit to collect data.

The newest orbiter is equipped with the most high-tech science instruments ever flown to another planet including a telescopic camera to photograph the surface in unprecedented detail and radar to probe underground for ice and possible evidence of liquid water.