The most sophisticated spacecraft ever sent to Mars slipped into orbit on Friday after successfully firing braking rockets and maneuvering itself into position while tense controllers monitored from Earth.

After a journey of seven months and 310 million miles, NASA's Mars Reconnaissance Orbiter, a large robotic spacecraft crammed with six scientific instruments, performed an intricate ballet of turns and engine firings to complete the crucial step of entering Mars's orbit without careering into space.

At that point, mission controllers at the space agency's Jet Propulsion Laboratory in Pasadena, Calif., announced that everything had gone as planned and that the craft was orbiting Mars.

''We've got complete control of a healthy spacecraft,'' a controller said as the room broke into applause.

The orbiter is to spend at least two years searching for signs of life on Mars and scouting possible landing spots for future spacecraft and astronauts. It is equipped to send back 10 times as much data as all previous probes combined.

But first it had to enter orbit, a goal that had eluded several missions in the past two decades. Three spacecraft are orbiting Mars now: two from the United States and one from Europe.

At a later news conference, Charles Elachi, the laboratory director, said that with Mars Reconnaissance Orbiter joining this fleet, along with two NASA rovers on the surface, the United States had established a scientific foothold on Mars similar to that on remote Antarctica.

''We really do have a permanent presence around that planet,'' Dr. Elachi said.

The Reconnaissance Orbiter's main engines ignited on schedule at 4:24 p.m., Eastern time, to begin a 27-minute firing meant to slow the craft enough to find its orbit. ''Burn, baby, burn,'' an engineer exclaimed as telemetry confirmed the start of the rocket firing.

The firing, 1,800 miles from Mars, reduced the orbiter's speed to about 8,800 miles per hour, from 11,000 m.p.h. Tension mounted 21 minutes into the rocket firing when the spacecraft slipped behind the planet, as planned, and controllers had to wait until the orbiter emerged at 5:16 p.m. to confirm that it had survived and was in orbit.

Contact with the orbiter occurred on schedule. ''Oh, look, it's right on the money!'' shouted an engineer. ''Right on the money!''

James Graf, the project manager, said controllers had had to confirm that the craft had achieved its planned initial orbit, a long ellipse that extends from 250 miles to 28,000 miles above the surface of the planet.

''I was very relieved,'' Mr. Graf said in a NASA television interview. ''It was picture-perfect.''

The spacecraft began its journey last Aug. 12, when it left Cape Canaveral, Fla., aboard an Atlas rocket. The flight path was so precise that controllers were able to cancel some rocket firings that had been planned to correct its trajectory. Howard Eisen, the flight system manager for the mission, said that the two-ton craft had saved 60 pounds of fuel by avoiding the corrections and that this could be used to extend the orbiter's mission.

Although the satellite is in orbit around Mars, scientists will have to wait until fall for it to be maneuvered into the proper lower orbit before they can begin their study of the planet.

First, the orbiter will go through a seven-month period of dipping into and out of the atmosphere to drain off energy and lower itself slowly into a working orbit less than 200 miles above the surface. This technique, called aerobraking, saved more than 1,100 pounds of launching weight, which allowed the spacecraft to be flown on a smaller rocket and saved millions of dollars, Mr. Graf said.

Before the spacecraft begins the series of 500 braking dips into the Mars atmosphere in about three weeks, Mr. Graf said, engineers will take a few images with its cameras. These will be engineering images to check the health of the systems, not the detailed scientific pictures expected when the spacecraft reaches its lower working altitude, he said.

The spacecraft's instruments will stay in hibernation during the months of aerobraking, said Michael Meyer, chief Mars scientist at NASA headquarters.

The six instruments that will study Mars during the $720 million mission are expected to revolutionize studies of the planet, Dr. Meyer said. These include the most powerful telescopic camera ever sent to another planet; an advanced mineral mapper to identify water-related areas; radar to probe beneath the surface for buried ice and water; and a weather camera that will monitor the planet daily.

The spacecraft will also serve as a communications satellite, relaying signals back to Earth from future craft that land on Mars to explore.

Chart: ''Success''
After a seven month journey, Mars Reconnaissance Orbiter successfully entered orbit around Mars.

On Friday The spacecraft fired its braking rockets just before passing behind the planet.

By October Dragging against Mars's atmosphere, or aerobraking, the spacecraft will move to an orbit 158 to 199 miles above Mars by October.

(Source by NASA)