The NASA probe’s retrorockets pressed desperately against the apricot afternoon skies of Mars. It was November 26, 2018, by Earth’s calendar. As the InSight lander worked its way down, slowing from 12,000 miles per hour to a graceful landing, overhead a pair of robots coursing through space monitored its progress. Though InSight was the size of a grand piano and the twin Mars Cube One spacecraft the size of cereal boxes, the lander was, in some sense, the easier challenge. Since the 1970s, we’ve sent a lot of big things to Mars. Until that moment, we had never sent something so small.

Engineers designed the tiny travel companions to act as radio relays, sending InSight’s telemetry back to Earth. Technically their job was a nice-to-have: InSight was landing autonomously, and it would communicate with Earth via the Mars Reconnaissance Orbiter after touching down. 

But just making it this far heralded a new age in space exploration. And engineers were only more pleased when the Deep Space Network, a global array of radio antennas, picked up the tiny explorers’ real-time signals from Mars. InSight was healthy, said MarCO. Its parachute had deployed, the cubes added. The lander had separated from the back-shell and chute; it was on rockets now. One minute later, it was done. InSight, the small spacecraft reported, had survived.

In this diminutive mission, NASA as an agency, and the community of planetary science researchers, caught a glimpse of a future long sought: a pathway to much more affordable space exploration. Each MarCO was the smallest, cheapest spacecraft ever to fly beyond the Earth-moon system. The pair cost less than $20 million to construct, launch, and operate. If engineers could build more such spacecraft—and make them even more capable in the process—they’d be an attractive alternative to multibillion-dollar flagships that launched only every 20 years or so, or even near-billion-dollar probes like InSight.

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