When the wheeled robot and its twin, Spirit, first landed on the Red planet within the space of 21 days in January 2004, they were expected to operate for around 90 Martian days — the equivalent of 92 Earth days.

At the time, the Nasa team in charge of the mission regarded every additional hour as a gift.

Project manager John Callas has long since done the calculations. When Opportunity reaches its eighth anniversary on Wednesday, it will be the 2,805th day on Mars for the rover, which has more than done its name justice.

“She used every opportunity to learn about Mars,” Callas says.

For him and his team, the twin robots are one of Nasa’s outstanding achievements.

It comes as no surprise that the team at the Jet Propulsion Laboratory in Pasadena, California, intends to celebrate the event with a birthday cake — with considerable pride, but tinged with sadness.

In May last year, Nasa had to abandon Spirit, which had stuck fast in the Martian sand after a journey of 7.7 kilometres.

Attempts to free it proved fruitless, and eventually communications broke down.

Opportunity now has 34.4 kilometres on the clock, and is still going even though it is beginning to show signs of old age. One of the six wheels has ceased to function, and for this reason it travels in reverse. One of the robotic arms has the equivalent of arthritis, with its “shoulder” restricted in movement.

Some of the tools and instruments that Opportunity had on board when it left Earth are also suffering the effects of time. For example, a spectrometer provided by German scientists for investigating rocks containing iron and particles of magnetic dust no longer works as smoothly as once it did.

“But overall Opportunity is in amazingly good health,” Callas says.

The rover lifted off on July 7, 2003 headed for the Red Planet, with a mission to investigate minerals in the rocks and soil of Mars that could have been deposited by past water activity - whether through precipitation, evaporation or hydrothermal activity

From the outset, it was clear that Opportunity had luck on its side. When it landed on the Meridiani Planum it bounced on its air cushions into a small crater in a stroke of luck for the scientists and immediately began transmitting spectacular panoramic images, along with photographs of unusual rocks, back to an excited team on earth.

From then on it went from crater to crater, the trip to Victoria, which lasted almost two years, almost proving its downfall when it nearly got stuck in the sand. The rover, which is roughly the size of a golf cart, spent two years exploring the crater, both from the rim and by descending to the floor.

The team back on earth described the treasure in photographs and data it sent back as “breath-taking”.

Since August last year, Opportunity has been at Endeavour crater, a destination none of those on the team at the start would have believed within its reach. The crater is of particular interest, as the rocks there appear to be older than those at the locations previously visited.

Pictures from Nasa’s Mars Reconnaissance Orbiter indicated clay deposits that could have formed during a considerably warmer and wetter period in the planet’s previous history. Opportunity soon struck a mineral vein of potassium sulphate in all probability deposited by water.

“It’s the kind of thing that makes geologists jump out their chairs,” says Steve Squyres, a leading member of the team.

Opportunity is currently bedded down on the crater’s rim for the Martian winter, lying at an angle of 15 degrees in order to pick up as much of the sun’s energy as possible. But even during this winter break, the rover is given no opportunity to rest, instead gathering data on the planet’s rotation from its position, with the aim of drawing conclusions about the composition of Mars’ core.

A visitor will arrive in the summer in the shape of a new Nasa rover, this one the size of a car, equipped with all kinds of technical wizardry. Nevertheless, Opportunity need not fear being put on the shelf and forgotten, as the rover twins have become a pet project for Nasa over the eight years since their arrival.

“We have grown very fond of them, nearly like human beings,” Callas says. “In addition to their scientific discoveries, they have widened our view and reach. Mars is no longer a distant foreign place; it is our neighbour. This will last until long after Opportunity’s wheels have finally stopped.”  Sapa-dpa