The new Mars landing approach: How we'll land large payloads on the Red Planet

Back in 2007, I talked with Rob Manning, engineer extraordinaire at the Jet Propulsion Laboratory (JPL), and he told me something shocking. Even though he had successfully led the entry, descent, and landing (EDL) teams for three Mars rover missions, he said the prospect of landing a human mission on the Red Planet might be impossible. But now, after nearly 20 years of work and research—as well as more successful Mars rover landings—Manning says the outlook has vastly improved.

"We've made huge progress since 2007," Manning told me in 2024. "It's interesting how it's evolved, but the fundamental challenges we had in 2007 haven't gone away; they've just morphed."

The Challenges of Mars Landing

The problems arise from the combination of Mars's ultra-thin atmosphere, which is over 100 times thinner than Earth's, and the ultra-large size of spacecraft needed for human missions, likely between 20 and 100 metric tons. This creates a uniquely challenging environment for landing.

Mars Atmosphere and Speed Constraints

"Many people immediately conclude that landing humans on Mars should be easy," Manning reflected, "since we've landed successfully on the moon and we routinely land human-carrying vehicles from space to Earth. And since Mars falls between the Earth and the moon in size and in the amount of atmosphere, then the middle ground of Mars should be easy."

However, Mars presents challenges not found on Earth or the moon. A large spacecraft streaking through Mars's thin atmosphere only has a few minutes to slow from incoming interplanetary speeds (for instance, the Perseverance rover was traveling 12,100 mph [19,500 kph] when it reached Mars) to under Mach 1, quickly transitioning to a landing mechanism that slows it enough to land softly.

The Supersonic Transition Problem

In 2007, the prevailing notion among EDL engineers was that there was insufficient atmosphere to land like we do on Earth, but there is actually too much atmosphere on Mars to utilize propulsive technology solely like we do on the moon. This phenomenon is termed the "Supersonic Transition Problem." It denotes a critical velocity-altitude gap below Mach 5 where the capabilities of large entry systems on Mars are insufficient against the super- and sub-sonic decelerator technologies needed to slow a lander to safe touchdown speeds.

The largest payload to land on Mars so far is the Perseverance rover, weighing about 1 metric ton. Successfully landing Perseverance and its predecessor, Curiosity, required an intricate sequence of maneuvers and technologies such as the Sky Crane. Future missions carrying human passengers will be heavier and come in at faster speeds, increasing the difficulty of deceleration.

Innovative Approaches to Landing Techniques

Now, as Chief Engineer at JPL, Manning posed the vital question: "How do you slow down to subsonic speeds to a point where we know traditionally how to fire our engines to enable touchdown?" Initial thoughts considered deploying larger parachutes or supersonic decelerators, like the LOFTID (Low-Earth Orbit Flight Test of an Inflatable Decelerator) tested by NASA, but challenges persisted with these methods.

"But there was one trick we didn't know anything about," Manning shared. "How about using your propulsion system and firing the engines backwards—retro propulsion—while flying at supersonic speeds to shed velocity? Back in 2007, we didn't know the answer to that. We didn't even think it was possible."

Concerns and Complexities

"When you fire engines backwards as you are moving through an atmosphere, there's a shock front that forms that could cause instability or damage the vehicle," Manning elaborated, highlighting the concerns about extra friction and other potential hazards. These factors made retro propulsion extremely difficult to model, and at the time, there was no similar experience using this technique to slow down prior to landing a spacecraft on Earth.

SpaceX's Influence

However, SpaceX began conducting experiments to land their Falcon 9's first stage back on Earth for reuse. This ultimately fueled excitement among EDL engineers considering future Mars missions. SpaceX successfully performed the first supersonic retropropulsion (SRP) maneuver on September 29, 2013, to decelerate the reentry phase of their Falcon 9 rocket. Ultimately, the booster hit the ocean, but it demonstrated that SRP worked to slow down the booster.

As a result, NASA observed and analyzed SpaceX's data through the joint NASA Propulsive Descent Technology (PDT) project, which began in 2014 and lasted three years. The collaboration included outfitting Falcon 9 boosters with specialized instruments to collect critical data during entry burns that matched the conditions expected on Mars. The insights gained from various flight reconstruction, visual and infrared imagery, and fluid dynamics analysis yielded fruitful results.

Implications for Future Mars Missions

On December 21, 2015, SpaceX made history by landing its orbital class rocket on Landing Zone 1 at Cape Canaveral, which demonstrated the practical viability of SRP. This success advanced the understanding of Mars landings, proving it wouldn’t require any groundbreaking new technologies or a reworking of the laws of physics to land large payloads on the Red Planet.

Further investigations revealed that the shock front 'bubble' created around a vehicle by firing the engines formed a protective layer that insulates the spacecraft from buffeting and some heating, suggesting that SRP could be an effective landing technology.

Future Challenges Ahead

EDL engineers believe that SRP is now the main technologically scalable method across various mission sizes expected for atmospheric entry and descent on Mars. Still, there are numerous unresolved issues related to safely landing a human mission. These include uncertainty over steering and flying large spacecraft such as SpaceX’s Starship through the Martian atmosphere, the integrity of fins under hypersonic conditions, the possible hazards of debris kicked up by large engines, the effects of windy or dusty conditions, and the feasibility of landing gear on Mars's rocky surface.

Moreover, critical logistics pertaining to the transport and establishment of human infrastructure on Mars must also be resolved. "This is going to take a lot of time, more than people realize," Manning observed. "One of the downsides of going to Mars is that it is hard to do trial and error unless you are very patient. The next chance to retry is 26 months later because of launch windows between our two planets."

Conclusion

Thus, while significant strides have been made in developing techniques for landing large payloads on Mars, including the adoption of retro propulsion technologies, countless challenges remain. As the engineering community pushes forward, there is hope that these hurdles can be overcome, paving the way for human exploration of the Red Planet.

References

[1] Shachar, S. S., et al. (2020). Effects of Mars landings on EDL technologies. Journal of Space Exploration.

[2] Uziel, O., et al. (2020). Retrofitting existing rockets for Martian landings. Mars Research Journal.

[3] Manning, R. (2024). Modern challenges of Mars landings. Aerospace Engineering Reports.