Two CubeSats fly to Mars and call the landing live

InSight touched down at 19:52:59 UTC on 26 November 2018, and the control room in Pasadena knew about it roughly eight minutes later — the time it takes light to cross from Mars, and not one second more. That is not how Mars landings normally go. Telemetry usually gets recorded by an orbiter sailing overhead and played back once the geometry allows, which means the people who built the spacecraft sit and wait to find out whether it exists. This time they did not have to, because two objects the size of briefcases were flying past the planet at the same moment.

MarCO-A and MarCO-B were 6U CubeSats: about 10 by 20 by 30 centimetres folded up, 13.5 kilograms each, built at JPL out of largely commercial parts for a reported 18.5 million dollars for the pair. They rode up with InSight on an Atlas V 401 from Vandenberg on 5 May 2018 and then flew their own trajectory, becoming the first CubeSats ever to operate beyond Earth orbit. Attitude and course control came from eight cold-gas thrusters running on R-236FA, a hydrofluorocarbon also used in fire extinguishers — which is how the engineers justified naming them Wall-E and EVE, after a film robot that flies with a fire extinguisher.

They carried no science instruments. The payload was a UHF receiver, an X-band transmitter and a flat folding reflectarray antenna, and the job was plumbing: catch InSight's 8 kbit/s UHF stream during entry, descent and landing from about 3,500 kilometres away, and push the same 8 kbit/s straight back out to the Deep Space Network without storing it. A bent pipe across 150 million kilometres. Mars Reconnaissance Orbiter recorded the descent as well but returned it afterwards, and Odyssey's confirmation that InSight's solar arrays had unfurled did not reach Earth until around five and a half hours after touchdown.

It was never a sure thing. MarCO-B had been leaking propellant almost from liftoff and needed constant fussing to stay pointed. It worked anyway. On 2 October, from 12.8 million kilometres out, its wide-angle camera returned the first image of Mars ever taken by a CubeSat — a small red dot with the spacecraft's own antenna intruding on both edges of the frame. At 12:10 p.m. PST on landing day, 7,600 kilometres past the planet and receding, it took a parting shot.

Then they went quiet, as expected. MarCO-B was last heard on 29 December 2018, MarCO-A on 4 January 2019, both drifting on solar orbits beyond Mars with nothing left to do and, most likely, nothing left to point with. The demonstration held: an interplanetary spacecraft did not have to cost nine figures or take a decade. Every cheap, small, single-purpose mission proposed for Mars since has been arguing from this flyby.