A planet-wide dust storm finishes Opportunity

A small dust cloud appeared in the northern hemisphere on 30 May 2018. The Mars Reconnaissance Orbiter team put the Opportunity team on notice within days. Bruce Cantor, who reads the daily weather images from the orbiter's colour imager, reported the storm officially planet-encircling by the morning of 19 June.

Martian dust storms do not knock spacecraft over — the air is too thin for wind to do real damage — they kill solar-powered spacecraft by cutting off the Sun. Atmospheric opacity is measured as tau. Opportunity had ridden out a tau of around 5 in the 2007 storm; this time tau over Endeavour Crater climbed past 10.8, the darkest sky ever measured from the martian surface. The rover's final transmission came on 10 June 2018: a power reading, and a sky too dark to charge from.

Over eight months JPL sent more than a thousand commands against every fault mode the team could construct — mission clock fault, uploss timer fault, a failed X-band radio — and waited out the windy season in case a gust cleared the panels, as had happened before. The last attempt went out on the night of 12 February 2019, and NASA announced the end of the mission the following day.

Opportunity had operated for fourteen years, driven forty-five kilometres and returned the first hard chemistry of ancient water, on a ninety-sol design life. It stopped in a valley called Perseverance.

The line "my battery is low and it's getting dark" was never transmitted. The reporter Jacob Margolis paraphrased the final telemetry that way, and the paraphrase circulated as a quotation.

MAVEN and the Trace Gas Orbiter watched the storm loft water vapour to around 80 kilometres, above where it normally freezes out and into a region where ultraviolet light splits the molecule and the hydrogen escapes to space. Planet-encircling dust storms appear to be part of the machinery by which Mars dries out.