Ingenuity

Perseverance carried it to Mars strapped under its belly, lowered it onto the floor of Jezero Crater on 3 April 2021, and drove away.

A helicopter weighing 1.8 kilograms, built as a proof of concept to test if anything could fly in the Martian atmosphere. Before it could take off, its first job was simply to survive a night where the temperature falls past minus ninety, entirely on its own.

It made it through the night. Now it had to fly.

Mars has an atmosphere and it is barely there: under one per cent of the density at Earth's surface. NASA's own comparison is that flying at Jezero is flying at a hundred thousand feet here, roughly three times the altitude of an airliner and far above the ceiling of any helicopter ever built.

Gravity gives some of it back, at 38 per cent of ours. The rest has to come from the rotors, 1.2 metres across, turning at up to 2,900 revolutions a minute. About ten times an Earth machine.

Blades that fast in air that thin sit at the edge of what rotors can do.

Taped to a cable under the solar panel is a piece of muslin the size of a postage stamp, cut from the lower left wing of the aircraft the Wright brothers flew at Kitty Hawk in 1903.

It is the second time pieces of that aeroplane have gone somewhere new. In 1969 Neil Armstrong carried wood from its left propeller and fabric from its upper left wing in his personal kit, and took them out onto the surface of the Moon in a pocket of his suit.

Upper wing to the Moon, lower wing to Mars. Sixty six years, then fifty two more.

It climbed to three metres, held there, turned to face its camera, and came down. About forty seconds in total.

The strip was called Wright Brothers Field. The plan was five flights in thirty sols, after which the helicopter would be finished and the rover would get on with its own mission.

On flight 4 a microphone on Perseverance was left running, eighty metres away.

Nobody expected it to pick anything up. Sound carries badly through air that thin, and the rover's own machinery sat closer to the microphone than the helicopter did. What came back was a low hum at eighty four hertz, a bottom E, buried in the wind.

It is the first recording ever made of one spacecraft by another.

On 5 July 2021 it flew out over Séítah, a field of sand ripples the rover had no way to cross, and came back with pictures of the far side.

That was flight 9, and it changed what the helicopter was for. It stopped being a demonstration and became a scout, going ahead to find ground the rover could actually use.

Five flights became seventy two, over nearly three years.

A camera in the fuselage pointed straight down and took thirty pictures a second, each one 640 by 480, black and white.

The software picked features out of one frame, found the same features in the next, and measured how far they had shifted. That shift, thirty times a second, is how it knew how fast it was going and in which direction.

It never knew where it was. It knew only how the ground was moving underneath it, and for seventy one flights that was enough.

Flight 72 was meant to be trivial. Straight up to twelve metres, a systems check, straight back down, after the flight before it had landed earlier than planned.

Underneath was sand. Ripples in rows, evenly spaced, each one indistinguishable from the next.

About twenty seconds after takeoff the software ran out of things to hold on to.