I T TRAVELLED more than 500m kilometres simply as a passenger, sleeping dreamless, vacuum-packed. The five-and-a-half-month voyage was devoid of all interest until the very end, when the spacecraft lost 20,000kph of its velocity in just over six minutes as it plunged through an alien sky to the desert below. Parachutes spread and airbags deployed. The package bounced, rolled and came to rest.

Opportunity still had 45km to go.

Four hours after landing on January 25th 2004, it opened its eyes. Its makers, back on Earth, looked through them at a landing site as perfect as they could have wished for. Opportunity’s landing on Meridiani Planum, a little south of the Martian equator, had not been a particularly precise affair—it could have come down anywhere in an ellipse some 100km long and 18km wide. But by chance the spot where it had ended up, about 25km from the centre of the target, was inside a small crater dug out by a meteorite impact. On the side of that crater the scientists saw straight away the distinctive strata of sedimentary rock. Nothing of the sort had ever before been seen beyond the Earth. It was exactly the type of thing the robot had been sent to find.

Opportunity was officially MER-B , the second of the two rovers of the Mars Exploration Rover programme; MER-A , Spirit, had landed a few weeks earlier in the great Gusev Crater, almost half the planet away. Unofficially, it was Oppy, and often a she. The two rovers’ missions were due to last 90 sols—a sol being the 24 hours and 40 minutes it takes Mars to turn on its axis. Faced with that time limit, Opportunity’s minders were thrilled not to have to spend any of those precious sols just looking for interesting rocks.

For the first 56 sols, Opportunity never strayed more than ten metres from that propitious landing site. It took pictures with its various cameras, ground holes with its little rasping drill, dug a trench by spinning one wheel while it kept the others locked, measured the spectra of various minerals. The scientists concluded that the sediments they saw had been laid down in a salty dying sea. If that had been all Opportunity had ever told them, the mission would have been counted a great success.

But on Sol 57, it ventured out into the wider world.

Guided by pictures from satellites above, Opportunity headed for a deeper crater, Endurance, a kilometre or so away. It drove down into it and looked around further, poking and prodding for the rest of the year. A human geologist might have accomplished as much or more by way of assessment in a leisurely afternoon. But the nearest human geologists were on another planet, and likely to remain there for some decades to come.

After climbing back out of Endurance—no one had known whether it would be able to—Opportunity was sent off to inspect the jettisoned heat-shield that had protected it as it burned down through the Martian atmosphere, now a glinting monument on the pockmarked plain. It was not the only thing that had fallen from the sky. As it rolled on, Opportunity came across a meteorite, its lithology distinctively un-Martian. Later it took a little video of Phobos, the larger of Mars’s small moons, passing as a shadow across the face of the Sun.

On Sol 946, in September 2006, Opportunity reached Victoria Crater, seven kilometres from its landing site, 800 metres across. Its makers knew that the 90-sol lifetime they had promised for Opportunity and Spirit was conservative, a low bar to ensure some kudos for “mission accomplished”. But none had expected it to be surpassed by a factor of ten.

That said, it had not all been plain rolling. Opportunity had spent a few harrowing sols stuck in sand at Purgatory Ripple; later, one of its steering motors failed. While it was exploring Victoria it was caught in a global dust-storm that cut it off from Earth and covered its solar panels with dirt. Happily, though, the same winds that drove the storms also blew the panels clean, or cleaner, after their passing. A joint in the shoulder of its robot arm, dicky since the mission started, finally seized up.

On March 30th 2011, Sol 2,155 of its mission to Gusev, Spirit failed to check in with Earth by radio as scheduled; it was never heard from again. Opportunity continued its long trek to Endeavour crater, 1,000 times larger than the crater in which it had started off. The point where it reached the rim was named Spirit Point, in memoriam. On August 6th 2012 a newer, bigger rover, Curiosity, landed at the foot of Aeolis Mons, thousands of kilometres away. Opportunity looked after itself for nine days to allow the scientists and engineers to make a fuss of the newcomer, then set out to study the intriguing smectite clays that a European orbiter had detected on Cape York, a peak further along Endeavour’s rim.

In Perseverance Valley

As the unexpected years went by, Opportunity continued along the hilly rim of Endeavour until it came to Perseverance Valley, which cuts through towards the crater floor. It was heading down the valley when, on June 10th 2018—Sol 5,111, well over seven Martian years of service—another dust storm cut it off from Sun and Earth. When, after months, the global storm subsided, the rover’s minders, now family, waited for its solar panels to be blown clean. It appears they were not. After over 1,000 messages attempting to restore contact, Opportunity was declared lost on February 12th.

If more than a handful of people ever get to explore Mars in person, surely one day someone will follow its tracks down Perseverance, looking for the mast on which its cameras are mounted—separated for stereoscopy, easily anthropomorphised—to honour a pioneer who travelled far and provided insight after insight into the history of Mars. Until then, imagine her as she was photographed by the orbiting HiRise camera after first reaching the scalloped rim of Victoria, looking down onto its rolling dunes: a tiny speck perched on a promontory peak, a new planet swimming before her eagle eyes, watched in silence from the skies.