The Pathfinder spacecraft's robot rover stood up from a crouched position today and tentatively rolled down a ramp to become the first mobile entity to explore the surface of Mars.

Like a dog waking up from a nap and stretching its legs, the 23-pound Sojourner rover responded to commands from its masters on Earth at about 8 p.m. PDT (11 p.m. EDT) and rose from its bed on the foot of its mothership. Simultaneously, pyrotechnic devices released flexible metal ramps, which extended in front and behind the 2-foot-long, 1-foot-tall rover.

About 2 1/2 hours later, another set of commands sent the remote-controlled robot's six spiked metal wheels slowly venturing down the rear ramp onto the smooth sandy surface, where it made one or two practice turns and began sniffing the dusty reddish-orange dirt with a scientific probe.

"It's been another magical day on Mars," said Brian Muirhead, the Pathfinder's flight systems manager.

The rover's first tentative movement set the stage for a full-scale exploration of the rugged terrain around Pathfinder's landing site, with the lander and the rover transmitting scientific data and dozens of high-resolution color pictures of each other in the harsh Martian environment back to Earth.

Sojourner's activation came after ground controllers at NASA's Jet Propulsion Laboratory resolved a worrisome communication breakdown that had delayed the deployment. For 12 hours, the glitch had marred an otherwise near-perfect performance by the spacecraft since it bounced in a cocoon of giant air bags to a safe landing Friday to begin the first exploration of Mars in 21 years.

Exhausted but elated after the triumphant Fourth of July landing day, mission managers said they had several reasons for pushing to get the rover off the lander as rapidly as possible. Sitting on top of one solar panel, the rover was blocking the sun and draining the lander's power. And besides, "It's a lazy bum and not working," said Donna Shirley, manager of the Mars exploration program.

Members of the science team were, she said, going "bananas" waiting for the rover to move. They had already selected at least two "target rocks" for the rover to analyze out of a rich trove visible in the badlands terrain surrounding the lander in the stream of color images it has transmitted to Earth. The rocks are within 10 or 15 feet of the craft, which is sitting on an ancient flood plain called Ares Vallis.

With 3-D glasses, the rover's drivers back on Earth will conduct a "virtual" exploration of the nearby terrain, picking paths through the jagged rocks and boulders almost as if they were there by relying on images from the lander.

The rover was essentially on its own once it received the activation sequence. The ground team could not know for some time whether the orders had been executed and, once a sequence had started, would be unable to intervene in any case. That is because it takes more than 10 minutes for signals to travel from Mars to Earth.

At 10:50 p.m. PDT (1:50 a.m. EDT Sunday), the first image of the bottom of the ramp came into view with one wheel of the rover edging onto it. This triggered cheers in mission control. Nine minutes later, the picture ground controllers had waited so long for suddenly appeared on the monitors. "There it is," shouted members of the rover team. "We can report six wheels visually on soil," confirmed a NASA commentator.

Sojourner's human operators will choose its targets, laying out for its computer brain a path along the ground by means of coordinates, or "lawn darts," like a line of bread crumbs. The rover, which moves at less than a half-inch per second, is supposed to follow that trail roughly to its destination but, based on what it "sees" along the way, Sojourner can pick its own route to avoid any unacceptable hazards.

The rover has three cameras -- a forward stereo system and a rear color picture system -- that are used in conjunction with a laser system to detect rocks and avoid hazards. If Sojourner gets into a situation it thinks it can't handle, "it says, I'm just going to stay right here,' " said rover engineer Ronald Banes. There are three levels of risk it can take, and the team can raise or lower that bar depending on its analysis of the situation. The team measures more than 200 aspects of the rover's systems and general condition, including temperatures, voltages, currents and mechanical components.

The rover's aluminum wheels and suspension use a unique "rocker-bogie" system that does not use springs. Its joints rotate, following the contours of the terrain and, supposedly, remaining stable and upright as it traverses rocky and uneven surfaces. If it falls over on its back, controllers will not be able to right it, even though it weighs only nine pounds in Mars's lesser gravity.

The six-wheel configuration was selected because it affords greater stability and obstacle-crossing capability, engineers said. Six-wheeled vehicles can surmount obstacles three times larger than those that a four-wheeled vehicle can accommodate. For example, Sojourner could tip up to 45 degrees without falling over.

The $25 million Sojourner's assignment is to spend at least a week testing its mobility, taking snapshots to relay back home and deploying an instrument called an alpha proton X-ray spectrometer, which, when poked up against a rock or the surface soil, can analyze the object's composition. Sojourner will study the Martian soil by measuring its own wheel sinkage, and by sensing the wear on different thicknesses of paint on the wheels.

"There are all sorts of things out there that we are just dying to go out and look at," said Pathfinder chief scientist Matthew Golombek.

Sojourner is powered by a 1.9-square-foot solar array, enough to keep it going for several hours each day, even in the worst dust storms. As a backup, it also has lithium thionol chloride D-cell-size batteries inside a protected electronics box.

The rover's communication problem manifested itself in degraded communications whenever the rover tried to talk to the lander. It was as if only the occasional syllable, not complete sentences, was getting through, Muirhead said. The team believed the problem was in the software, rather than in any hardware damage from, say, the hard landing and repeated bounces.

Overnight, the team turned the power on and off repeatedly in hopes the two modems would get back in sync and conducted other diagnostic procedures. The team was not sure exactly how it fixed the problem, Wallace said, but it could have been "the shutting down of the rover overnight" -- the equivalent of simply rebooting a computer.

After the communications problem was corrected, Pathfinder project manager Tony Spear announced that the lander had been renamed the Carl Sagan Memorial Station, after the late author and scientist who was among the leaders of the push to study the Red Planet. "Very fitting," Spear said. "He inspired all of us rocket scientists."

During Pathfinder's 309-million-mile, seven-month cruise to Mars, Sojourner -- named after black Civil War-era abolitionist Sojourner Truth -- had been folded and crammed into a stowage space and scrunched down so that it was only seven inches tall.

The lander, which will spend at least a month studying the atmosphere and weather, will have a somewhat reduced power supply because, contrary to advance weather reports from Hubble Space Telescope scientists, the skies are not blue and clear. They are dusty and salmon colored. This means that less sunlight will fall on the solar panel on each of the lander's three petals.

The spacecraft appears to be settled on a slight "table," possibly a rock. The flight team overnight had used a winch system to further retract the spacecraft's deflated air bags, which had draped over portions of its petals and threatened to snag the ramp as it unfurled in somewhat the manner of a broad metal tape measure. When deployed, the rear ramp slants down at an angle of about 20 degrees.

Pathfinder is the first spacecraft to land on Mars since 1976. During the 21-year interim, repeated attempts to return to the planet have failed, including the devastating 1993 loss of the $1 billion U.S. Mars Observer probe. Since then, interest in exploring Mars has increased, especially after scientists announced they had found evidence in an asteroid from Mars discovered in Antarctica that microscopic life may once have existed on that planet.

Although Pathfinder is not designed specifically to identify signs of life, officials say the $267.5 million mission marks the beginning of a series of missions to Mars that they hope could eventually help determine whether life could have existed there at some time.

NASA's Mars Global Surveyor, which was launched in November, will reach the planet in September. That probe will orbit Mars for several years, mapping the planet's surface in preparation for later missions over the next several years. CAPTION: Before the rover was deployed, pictures from Pathfinder's camera were assembled into a 360-degree panoramic view of the lander's gear and the surrounding Martian landscape. The camera took 330 images to create a stereoscopic mosaic. CAPTION: A view from the Mars Pathfinder lander shows a landing petal surrounded by the deflated air bags that were used to cushion the touchdown, as well as antennae on the left and right sides of the image. A communication glitch between Sojourner and Pathfinder delayed deployment of the robot explorer. CAPTION: The 23-pound robot rover Sojourner rolls off a ramp on a Pathfinder landing petal onto Martian soil to become the first mobile entity to explore the planet.