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.

“The rocket engine nozzles are going into a flow field that's supersonic, so you're going to set up shock fields, pressures behind the shock that the engine has to start against,” says Brown. “Those don't look like they're going to be insurmountable, but it's going to be a highly dynamic event.”

Charles Campbell, an expert in computational fluid dynamics at Johnson Space Center, is developing a sounding-rocket flight test for NASA's Space Technology Mission Directorate (STMD) to gauge just how difficult that ignition will be. At Mars multiple engines will be required, says Brown, and the flow fields of supersonic retropropulsion are likely to require some thermal protection on the body of the spacecraft, which will add more mass. There is also the question of using the system to land precisely and, in the case of the crew-habitat vehicle, ideally within walking distance of the pre-positioned cargo carrier.

The Curiosity EDL system achieved unprecedented precision in landing by jettisoning ballast as it entered the atmosphere to create enough lift to “fly” the entry vehicle toward its target, and then dropping more ballast to stabilize itself under the parachute. That technique got it down in an ellipse measuring 20 X 7 km (12 X 4 mi.), and it used all of the atmosphere to achieve it.

For human-sized landers, says Brown, “the most efficient trajectory is one that waits until almost the last minute, fires a very high thrust, and then you touch down. But you . . . have very little ability to throttle the engines to provide precision landing. And we want to start working the precision landing problem as soon as we enter the atmosphere.”

Engineers have some tricks up their sleeves as they work the precision-landing problem for human landings, according to Jim Masciarelli, a guidance, navigation and control expert at Ball Aerospace. Most of the generic hardware and software necessary for the needed level of landing accuracy is in the works and “almost ready to go,” he says. Hazard avoidance with flash lidar, radar and other sensors probably will be needed for the final few hundred meters of descent, which presents a challenge as well. Under current estimates there will only be 90 sec. from entry to landing, which will make the 7 min. of terror look like a data-processing luxury.

“You have gobs and gobs of data from these sensors to process,” Masciarelli says. “You probably have redundant sensors for reliability in case something fails, so how you process all that data is probably the biggest challenge, and get it in a package that is radiation tolerant, that can survive the trip to Mars, and is a small, lightweight package as well.”

Work on hazard avoidance under NASA's Science Mission Directorate has been halted because of budget constraints, says Doug McCuistion, until recently the head of NASA's Mars Exploration Program. Other EDL technology for human landings on Mars is just getting underway, says Mike Gazarik, the STMD associate administrator.

Campbell's supersonic retropropulsion concept will be briefed at agency headquarters this week, he says. Last year the agency ran a subscale inflatable-decelerator flight test called the Inflatable Reentry Vehicle Experiment (IRVE-3) on a sounding rocket from Wallops Flight Facility in Virginia (AW&ST July 30, 2012, p. 16).

“We're pushing on all the tools here from an entry, descent and landing viewpoint for that future mission,” Gazarik says.

One other advanced EDL technology under study by STMD is a 33.5-meter-dia. supersonic parachute with a ring sail configuration instead of the disk band gap approach used as part of Curiosity's landing. It is the largest supersonic parachute ever designed, Gazarik says, and is being considered for the Curiosity 2.0 mission under development for a 2020 launch.

While the sky crane will not scale up to the payload size that will be needed for human landings, it will remain the state of the art for landing robotic explorers for the foreseeable future. A science definition team already is at work on the instrument suite for the follow-on version of Curiosity NASA plans to launch in 2020, and the list is likely to include hardware for collecting and storing samples for eventual return to Earth (AW&ST Dec. 10, 2012, p. 32).

“I think we will see, after the science definition team comes back from their consideration on the 2020 mission, an absolutely fabulous array of measurements that need to be made on samples, decision processes on whether we keep those or not, how we keep those,” says James Green, director of NASA's Planetary Science Div. and acting director of the Mars program. “And then, of course, how we would return those is an element [in that] next decade.”

One concept just gaining public attention is using the Orion multi-purpose crew vehicle NASA is developing to bring the samples back to Earth, after picking them up in the deep retrograde lunar orbit under study for Orion's first deep space mission to a captured asteroid (AW&ST April 29, p. 36). But beyond the $100 million NASA is seeking as a down payment on the asteroid-capture mission, the agency has very little to say about its actual plans to send humans to Mars. Overall, the latest DRMs estimate a human Mars mission will require launching about 800 tons of payload with the heavy-lift Space Launch System. But estimates of just how much of that mass actually will touch down on Mars range from 20-60 tons, and the most recent DRMs NASA has developed are being held so closely that the National Academies of Science panel studying the future of human spaceflight cannot access them.

“Bottom line, as I understand it, is that the design reference missions are internal, pre-decisional studies that help inform our decision-making process,” says David S. Weaver, the agency's associate administrator for communications.