It travelled at an average pace of 60cm an hour and it has arrived a year late. But the Mars rover Opportunity has finally approached its destination, the rim of the vast Endeavour crater.
Nasa hailed the six-wheel Opportunity's approach to the 22.5 kilometre-wide crater last night as a "tremendous scientific success".
The ageing robotic field geologist has logged more than 32kms since it was first parachuted onto the planet's surface in 2003, along with its twin rover Spirit, for a planned three-month mission. Spirit emitted its last signal a year ago after becoming trapped in sand. Opportunity crawled out from a crater in 2008 and headed south to the Endeavour crater, a two-year journey in theory, which has taken longer because Opportunity had to drive backwards to prevent its front wheel wearing out.
At the crater, Opportunity will travel south across the rim to perform a geological assessment of the location, examining the clay minerals formed under wet conditions at the oldest of the four craters it has visited.
The finishing point was nicknamed "Spirit Point" in honour of Opportunity's fallen twin. Scott Maxwell, leader of the Mars Rover driving team, tweeted: "The drive we uplink today will actually take us physically onto Cape York. So. *Freaking*. Excited."
Project manager John Callas, of the Nasa Jet Propulsion Laboratory, said that reaching the Endeavour crater was an important science target: "We will likely spend years at this location. It's not just one spot. There's kilometres of interesting geology to explore."
Endeavour is more than 25 times wider than the Victoria crater, an earlier stop that the rover examined for two years. Nasa believes the crater is much older. The minerals and sediments had previously only been viewed at a distance by orbiting spacecraft.
Mr Callas said: "It represents geology from very early Mars history. It's understanding what happened to Mars a long time ago."
Both rovers have made important discoveries about wet environments on ancient Mars. Scientists now know that Mars was at one time like Earth, with an atmosphere thick enough to support liquid water which may have been favourable for microbial life.
Mr Callas said the Spirit rover also found evidence of ancient hydrothermal systems on Mars that could support an ecosystem. He said the Opportunity was still in good health despite some "arthritis" in its joints, adding: "We're on the surface of a planet that's hundreds of millions of kilometers away in frigid cold temperatures."