Pasadena, Calif. ? To the great relief of NASA scientists, the Spirit rover rolled onto the surface of Mars and trundled across the salmon-colored soil Thursday for the first time since the vehicle bounced to a landing nearly two weeks ago.

The slow maneuver was a nail-biting moment for scientists who had feared that Spirit might become yet another casualty in the star-crossed history of Mars exploration.

“This is a big relief,” said Rob Manning, manager of the entry, descent and landing portion of the mission. “Our wheels are finally dirty.”

The six-wheeled vehicle had been perched atop its lander since its arrival on Mars on Jan. 3. On Thursday, it finally rolled down a ramp onto the surface of the Red Planet, covering a mere 10 feet, as planned. The trip took 78 seconds.

Engineers had worried that the golf-cart-size vehicle might become snagged on its ramp or damaged beyond repair, making it impossible to complete its mission. Scientists said the roll-off may have been the riskiest step the rover would ever take on Mars.

NASA engineers and scientists were misty-eyed and choked-up as they described the success of the maneuver, and raised a champagne toast at an early morning news conference.

“Mars now is our sandbox, and we are ready to play and learn,” said Charles Elachi, director of NASA’s Jet Propulsion Laboratory.

Spirit is to spend three to four days parked beside its lander, giving it time to find its bearings and perform some preliminary analysis of the soil and pebbles around it. Then it will set off a meandering journey to prospect for geologic evidence that the now-dry planet was once wetter and hospitable to life.

“Now we are the mission that we all envisioned 3 1/2 years ago,” said Jennifer Trosper, mission manager for surface operations.

Black-and-white pictures beamed from Spirit showed its two rear wheels on the martian soil, with its lander 32 inches behind it. Two parallel tracks led away from the lander through the cakey dust.

Originally, Spirit was supposed to roll straight off the lander on its ninth day on Mars. But the now-deflated air bags that cushioned the rover’s landing blocked the main ramp, forcing Spirit to perform a slow, 115-degree turn to line its wheels up with a different ramp.

On Friday, Spirit was expected to deploy its robotic arm and take its first photographs with its microscopic imager.

NASA then plans for Spirit to begin a meandering trip in the direction of a crater about 825 feet away. Spirit was designed to travel dozens of yards a day.

Either of two rocks dubbed Adirondack and Sashimi is expected to be Spirit’s first target once it starts rolling again. The rocks lie 2 to 3 yards from the rover.

The rover might also visit a nearby, shallow depression dubbed Sleepy Hollow — if there is enough time before its twin, Opportunity, lands Jan. 24 on the opposite side of the planet.

NASA wants to park Spirit for the three days immediately following Opportunity’s arrival, to ease the burden on members of the $820 million double mission.

Even while parked 16 inches above Mars atop its lander, Spirit kept busy. It used its nine cameras to take at least 3,900 pictures of its surroundings. Mission scientists used those images, including sweeping panoramas, to chart the rover’s next movements.