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The rover, known as Sojourner, has been stuck on Pathfinder since its successful landing on Mars two days ago.
It is the first time a man-made craft has travelled over the surface of another planet.
Pathfinder quickly sent back the evidence: an image of the Martian surface showing the tracks made by Sojourner's six studded titanium wheels.
The problem began when a partially-deflated airbag blocked Sojourner's way out of Pathfinder. Then the computers on board the probe and the rover failed to talk to each other.
Finally, at 0646 BST (0546 GMT) there was a breakthrough.
Flight director Chris Salvo announced to the waiting team at the Jet Propulsion Laboratory in California: "Six wheels on the ground."
There was an ecstatic cheer from the 70-strong team. But they will now have to wait another day for the first high-resolution pictures because an hour after the vehicle moved off the ramp, the sun went down and the rover was left parked until the next Martian morning.
The Sojourner is a tiny robot, about the size of a bread-bin and weighing just 22 lbs (10 kg). It travels on six wheels, each of which can move independently to cope with the uneven Martian terrain.
Most of the power is provided by solar cells on the roof, and there is also a battery power pack for backup.
It is controlled remotely from California, millions of miles away.
The Pathfinder probe had a near-perfect landing on 4 July - America's Independence Day - in the Ares Vallis, an ancient channel on Mars that may once have held water.
The first reading sent back was of the temperature - a freezing minus 93 degrees C.
The probe has also sent back some astounding pictures of the barren, rock-strewn surface.
It showed massive dust storms in the pink Martian sky, one raging just 600 miles (950 km) south of the landing site.
The mission is being followed avidly by millions on the internet through the official Mars Pathfinder website.
There is particular interest in what it may find following the controversial announcement by Nasa last August that it had found evidence of life in a Martian meteorite.
Although Sojourner was designed to operate for just one week, in fact it kept exploring Mars for nearly three months.
It covered more than 50,000 square yards (42,000 square metres) of territory around Pathfinder's landing site and sent back 550 images of the Martian surface. Pathfinder itself also took thousands of pictures.
They revealed new information about geological features on Mars and provided compelling evidence that the planet once contained liquid water and was warm and wet, like the Earth.
The Mars Global Surveyor, which arrived in September 1997, was also a success, providing tantalising hints of possible water beneath the surface.
However, the next missions, Mars Climate Orbiter and the Mars Polar Lander in 1999, were costly failures, putting the future of Mars exploration in doubt.
Then in 2001, the Mars Odyssey revived Nasa's fortunes with a remarkable geological map of the planet which transformed our knowledge of what Mars is made of.
The European Space Agency put the Mars Express into orbit around Mars in December 2003.
In 2004, Nasa's Mars rovers, Spirit and Opportunity, discovered compelling evidence for the prolonged presence of water on the planet's surface.
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