SpaceX has already demonstrated its key Mars landing tech—with the Falcon 9

The first stage of the Falcon 9 rocket begins its reentry burn at an altitude of 70km. (credit: NASA)

In September, 2014, a Falcon 9 rocket blasted off from Florida carrying a Dragon spacecraft bound for the International Space Station. The capsule carried some notable cargo, including the first 3D printer to be tested in space as well as 20 mousetronauts to study muscle loss. Yet the most far-reaching part of that mission came after the Falcon 9 deployed its upper stage and began falling back to Earth.

As it descended into the upper levels of Earth's atmosphere, the rocket's engines fired for its "reentry burn." A few minutes later, the first stage splashed down in the Atlantic Ocean, completing one of the last flights before SpaceX began trying to land its rocket on an autonomous drone ship. But even as SpaceX was testing technology needed for terrestrial landings of its reusable Falcon 9 rocket, it was also taking some of its first steps toward landing on Mars.

That's because during that launch—and about 10 others since late 2013—SpaceX has quietly been conducting the first flight tests of a technology known as supersonic retro-propulsion—in Mars-like conditions. It did so by firing the Falcon 9's engines at an altitude of 70km down through 40km, which just happens to be where the Earth's thin upper atmosphere can act as a stand-in for the tenuous Martian atmosphere. Therefore, as the Falcon thundered toward Earth through the atmosphere at supersonic speeds and its engines fired in the opposite direction, the company might as well have been trying to land on Mars.

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