hen NASA and SpaceX announced April 27 that they had modified an existing unfunded Space Act Agreement that involves the company’s “Red Dragon” Mars lander concept, it was, unsurprisingly, SpaceX that got all the attention. No company has ever flown a private Mars lander, and not even NASA has landed a spacecraft as large as SpaceX’s Dragon. Moreover, Red Dragon is the latest sign that SpaceX and its founder, Elon Musk, are serious about pursuing a long-term goal of Mars settlement.

But what’s in it for NASA? The answer might be summed up in two words: supersonic retropropulsion, a landing technology that the agency increasingly sees as critical to its own Mars goals.

“We’ve had some ongoing entry, descent and landing architecture studies, and they’ve concluded that supersonic retropropulsion technology is going to be required for any human mission to Mars,” said Jim Reuter, deputy associate administrator for programs in NASA’s Space Technology Mission Directorate, in a May 16 interview.

It’s necessary, he and other expert s say, because other approaches — parachutes, airbags and even the “skycrane” system used by the Curiosity rover — can only land spacecraft weighing about a ton on Mars. A human Mars lander, by comparison, is likely to weigh up to several dozen tons.

Supersonic retropropulsion offers what appears a simple solution: firing a rocket engine in the direction of travel to slow down. That becomes complicated, though, when traveling through a thin atmosphere faster than the speed of sound. The aerodynamics and stability of a vehicle during that phase aren’t well understood outside of computer simulations and small-scale wind tunnel tests.

Interest in supersonic retropropulsion isn’t new. Rob Manning, the Mars program engineering manager at NASA’s Jet Propulsion Laboratory, noted that the Viking program looked at it in the early 1970s before deciding to go with a parachute instead.

NASA revisited the concept in the mid-2000s when it started to study human Mars landing technologies and ran into limitations of existing systems. “We really discovered that we had no idea of how we were going to land,” Manning said during a panel at the Humans to Mars Summit in Washington May 18.

Supersonic retropropulsion emerged as a promising technology, but one that is both hard and expensive to test. “We have to build a complete system and fly it backwards at supersonic velocity,” Manning said.

NASA has since gotten some data on supersonic retropropulsion, thanks to SpaceX. NASA monitored several Falcon 9 first stage landing attempts, as the stage’s initial reentry burn takes place in conditions similar to the Martian atmosphere. “That was the test we always wanted,” Manning said. “Now that we have the data, we decided to get rid of parachutes.”

Red Dragon will allow a full-up test of supersonic retropropulsion on Mars. Manning said NASA hopes to get more information on just how effective that approach is, and uncover any interactions between the spacecraft and atmosphere that can’t be studied in terrestrial tests.

It is also a bargain for NASA. Phil McAlister, director of commercial spaceflight development at NASA Headquarters, said agency expects to spend “in the ballpark” of $30 million supporting its side of the agreement, mostly in the form of NASA personnel providing support to SpaceX on topics ranging from deep space communications to implementing planetary protection protocols.

That’s far less than what a dedicated technology demonstration mission would cost NASA. “It’s probably an order of magnitude less cost than if we had to do a standalone mission ourselves,” Reuter said. “And we do it much sooner.”

And that assumes there would even be funding for a dedicated mission. NASA’s space technology program has seen its budget squeezed in recent years, and those effects include Mars exploration. The day before NASA and SpaceX announced their Red Dragon agreement, Reuter told a meeting of two National Academies boards that NASA was slashing funding of another Mars landing technology, the Low Density Supersonic Decelerator, because of space technology funding cuts and a congressional requirement NASA take on a satellite servicing mission previously funded elsewhere in the agency.

NASA Administrator Charles Bolden is willing to have SpaceX help study supersonic retropropulsion to save money. “We’re not investing in that, but we don’t need to if our commercial partners are doing it,” he said during a May 18 panel on space travel organized by the Washington Post. “We’re talking about reducing the cost to the taxpayer.”

While supersonic retropropulsion is the key technology NASA hopes to learn about from Red Dragon, the agency expects to collect other data from the mission related to entry, descent and landing. There may also be the possibility to fly other NASA payloads, like science instruments, on the spacecraft.