May 20, 2013
Credit: Karl Edquist/ NASA Langley Research Center
For all the attention focused on how hard it will be to keep astronauts alive while they fly from Earth to Mars, the challenge of setting them safely down on the Martian surface will be just as difficult.
Entry-descent-and-landing (EDL) experts who spoke at a Humans To Mars symposium here say the “sky crane” that landed the robotic Curiosity rover on Mars last year will not scale to the huge sizes need for humans. And even if it did, the “seven minutes of terror” controllers at the Jet Propulsion Laboratory experienced at a distance during the first sky-crane landing may be a little too tame for a human mission.
“While the Curiosity rover has been described as a small nuclear-powered car on the surface of Mars, what we're really talking about here today is landing a two-story house, and perhaps landing that two-story house right next to another two-story house that has been autonomously prepositioned and has fuel for the astronauts when they get there,” says Robert Braun, a Georgia Tech space-engineering professor who was NASA's chief technologist.
At a little less than 1 ton, Curiosity and its sky crane hardware required four distinct phases to get to the surface: atmospheric entry using a heat shield to shed hypersonic kinetic energy; parachutes for aerodynamic deceleration to speeds slow enough for propulsive deceleration of the sky-crane platform; and the final touchdown on the rover's wheels via cables lowered from that platform.
“When we got to thinking about very big objects, the size of houses, things like parachutes don't come along for the ride,” says Adam Steltzner, who headed the Curiosity EDL team at Jet Propulsion Laboratory that developed the sky crane approach. “They don't scale. A parachute the size of the Rose Bowl, which is what it would need to be for human exploration, is something that we already know from our experience on Earth, is not practically manageable.”
To land a house-sized cargo carrier or human habitat on Mars, Steltzner says, it probably will be necessary to go directly from hypersonic speeds to propulsive deceleration—essentially firing some kind of rocket to slow down enough to land. And that, the experts say, will be as difficult to accomplish as developing efficient radiation protection, the traditional long pole in the tent for a human trip to Mars.
Kendall Brown, an EDL expert in the Exploration and Mission Systems Office at Marshall Space Flight Center, said a cross-agency study using then-current design reference missions (DRMs) took parachutes entirely out of the landing sequence for a human expedition. Instead, either a rigid or inflatable aerodynamic decelerator would slow the entry vehicles from hypersonic speeds to supersonic speed in the Mach 2.5-3 range. At that point, the EDL system would shift to rocket propulsion for the remainder of the landing. It will not be easy to ignite a set of downward-facing rocket engines as they fly through the Martian atmosphere at three times the speed of sound.