On 25 January 2004, a fireball streaked across the sky over Mars. If there had been anyone there to watch, they would have seen an enormous parachute open up high in the Martian atmosphere and glide down towards the ground. After a long descent, there would have been a flash and roar of rocket engines, then a crunch as the truck-sized inflatable shell protecting Mars Exploration Rover B bounced away across the rocky terrain.





It was a remarkable start to an ambitious mission. Mars Exploration Rover B, known to the public as Opportunity, was scheduled to spend 90 days wheeling across the surface of Mars, taking pictures and making scientific observations in an area called the Meridiani Planum. It landed three weeks after its twin, Spirit, touched down several thousand kilometres away in the Gusev Crater. The two rovers were record-breakers from the moment they touched down, becoming – at 185 kg (408 lb) – the heaviest spacecraft to make an airbag landing.

Encouraged by the popularity of the Mars Pathfinder mission in 1997 – which included the shoebox sized Sojourner rover (the first successful Mars rover) – mission planners at NASA and the Jet Propulsion Laboratory (JPL) had decided to release all the pictures taken by the Mars Exploration Rovers to the public. The otherworldly panoramas, which were taken with what were at the time the highest resolution cameras fitted to a planetary lander, captured by the two robots were widely shared online making them household names across the world. Opportunity got its own website and, many years later, a twitter account to show the world what it had seen.

The primary mission of the two rovers was always scientific discovery, however. Each rover was fitted with a single arm that carried a tiny grinding wheel. It could scrape away the dust on rocks, then examine what it had revealed with an array of scientific instruments. The arm carried three different spectrometers (for looking at the mineral composition of rocks and dust) as well as a microscope and a magnet array. From these small-scale observations, researchers have been able to figure out a great deal about Mars' history and current environmental conditions. Opportunity's instruments detected high concentrations of minerals such as hematite and jarosite, for example, both of which typically form in watery environments. This proved that while Mars appears to be barren now, it was once a wet hot-house world similar to the primordial Earth.





These observations continued well past the original 90-day mission plan, and the two rovers left their landing sites to explore further afield. Powered by their large solar panels, the two rovers set a new Martian land-speed record of 5 cm per second (0.18 km/h; 0.11 mph) and proved themselves able to climb steep hills and descend into treacherous, dust-filled craters. They started to show their age a little - Spirit's solar panels kept getting clogged with dust, and Opportunity lost the use of its right front wheel – but JPL's engineers figured out workarounds.

On 1 May 2009, Spirit got its wheels stuck in some very soft soil. Despite much hard work by the Robot Operations Group at JPL (which involved building a perfect replica of the Martian sand-trap on Earth), the rover eventually became covered with dust and lost power on 22 March 2010 (Spirit's mission was formally declared to have ended on 25 May 2011). Opportunity kept going, however, and on 20 June 2010 exceeded the Viking 1 lander's endurance record of 6 years 146 days (set between 1976 and 1982) – the longest time survived on Mars by a spacecraft. On 27 July 2014 the rover followed this up by breaking the record for farthest distance travelled on another world – its 40.25 km definitively surpassing the mark of 39 km set by the Soviet Union's Lunokhod 2 moon rover in 1973. It pushed on from the rocky plains of the Meridiani Planum to the rugged, wind-worn rim of the Endeavour Crater – an ancient impact crater that measures 22-km (14 miles) in width and is up to 500 m (1,640 ft) deep.





The beginning of the end for Opportunity's mission came in June 2018, when a globe-spanning dust storm blew up on Mars, blotting out the sun and cutting the power coming from the rover's solar panels to almost nothing. Opportunity had weathered storms like this before, but the storm was bigger than previous events and the rover's systems were less able to cope. With no power coming in, Opportunity dropped into low power sleep mode – shutting off all but the most important functions – and stopped communicating with Earth.



The dust storm passed in early October, but Opportunity didn't wake back up. NASA and JPL continued to try to revive the rover, sending various wake-up commands and hoping that the wind might uncover the solar panels, but on 12 February 2019, when one final communication attempt failed, it was concluded that Opportunity's enormously successful and important mission was finally over.

Opportunity 's final resting place is in the Perseverance Valley, overlooking the Endeavour Crater, some 45.16 km (28.06 miles) from where it landed 15 years and 19 days ago.



