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.

These test flights were classic SpaceX—flying a primary mission, such as delivering cargo to the International Space Station, but also piggybacking other technology demonstration missions on top of it. The company has also found ways to build Earth-based systems that will also translate to Mars. The Dragon 2 spacecraft, built to ferry astronauts to the International Space Station, has eight SuperDraco thrusters to power its launch abort system if the capsule must quickly separate from its rocket during an emergency. But SpaceX also plans to use the same thrusters for supersonic retro-propulsion in the Martian atmosphere.

Before these recent tests, however, engineers weren't sure whether this kind of advanced propulsion would work. NASA and US universities had tested supersonic retro-propulsion in computational fluid dynamics simulations and small-scale air-in-air wind tunnel tests, but not live flights. Understandably, a lot of engineers were concerned about the stability of a vehicle during the turbulent period when its rocket engine fired directly into an atmosphere it was rushing into at supersonic speeds.

SpaceX began testing supersonic retro-propulsion as far back as September 2013, when the company first flew its upgraded Falcon 9 rocket, v1.1, which had about 60 percent more thrust than the original. But even as this vehicle made its maiden flight—a test flight really—SpaceX started collecting data on a controlled descent in the Martian-relevant conditions of the upper atmosphere. A year later, amid growing interest from NASA, a space agency WB-57 airplane and a Navy NP-3D Orion aircraft trailed the Falcon as it reentered the atmosphere to capture images and thermal data.

Among those eagerly watching the flight tests was Bobby Braun, an aerospace engineer at Georgia Institute of Technology, who has led a joint research effort with SpaceX and NASA to study supersonic retro-propulsion. "I have access to all of that data, and I’ll tell you that it's worked like a charm every time," he told Ars. "The stability was manageable, and while there are still some issues, there are no showstoppers."

Propulsive landing is key to eventual human missions to the red planet for one simple reason—it scales. In 2015 Braun, Hoppy Price and a couple other engineers wrote a paper for the American Institute of Aeronautics and Astronautics describing how supersonic retro-propulsion could be used to land up to 28 tons of useful cargo on the surface of Mars. The spacecraft and rockets would be different, but the basic landing technology is the same. "This is scalable all the way up to human Mars exploration," Braun said. "What SpaceX is doing right now is quite similar to how we might land humans on Mars."

Successfully landing Dragon on Mars would be unprecedented. It likely would enter the Martian atmosphere weighing about eight tons, and it would burn two of those tons as propellant to get down to the surface. Compare that to the largest object humans have ever landed on Mars, the Curiosity rover. It started off at 3.6 tons before entering the atmosphere, and through its sky crane and other steps, it shed weight down to 900kg by the time it reached the surface.

Braun is almost uniquely positioned to say whether SpaceX might succeed. In 2010 he was named NASA's chief technologist and formulated the Space Technology program to help NASA devise advanced technologies like entry, descent, and landing that would enable human missions to Mars. Since leaving NASA he has worked with both the space agency and SpaceX on propulsive descent technologies. And he's bullish on SpaceX's chances. "This is no stunt," he said. "It’s something they’ve been working on for a while. Don't get me wrong, it is certainly a risky proposition. But you've got to give them credit. They've been testing a lot of these Mars landing technologies already here on Earth. That certainly improves their chances of success."

Just three months after that September 2014 test flight with government planes collecting data, NASA had seen enough, too. It signed a Space Act Agreement with SpaceX, saying it would provide assistance with deep-space navigation and communications if the company would share its flight data. If NASA were to try to conduct that kind of test on its own, the cost would probably exceed $2.5 billion or $3 billion, Braun said. "It’s a great deal for NASA, in my opinion, and it’s a great deal for SpaceX."