Several approaches to habitat design are under consideration. These fall into two general categories: rigid structures and inflatable ones. Lunar architects designing rigid habs have even more decisions to make: should the structure be cylindrical? If it is, should it lie horizontally or stand vertically? The structure might also be configured as a “tuna can,” using the full diameter of the new Ares V booster to create large, open spaces in a low-lying structure.
Cylindrical arrangements are smaller and lighter, and thus easier to transport. Once a cylinder is on the ground, the interior arrangement is fairly inflexible, with bunks, plumbing, electrical components, and shelves attached to the structure. Multiple cylinders would be chained end to end, forming long corridors of work/living areas in linear arrangements. A tuna can, on the other hand, provides a circular floor that can be reconfigured into labs, sick bays, eating areas, etcetera. The main drawback of the tuna can is mass: it's a heavy payload. And while tuna cans might accommodate individual crew quarters (as opposed to the sleeping bunks of a cylindrical hab), engineers have a high degree of confidence and experience with the cylindrical model. They've been using them for years as building blocks of the International Space Station. “We're juggling a lot of trade-offs,” says lunar architect Robert Howard. “If the interior space is too small, you have psychological problems with the crew. If it's too large, the crew is fine but missions are shorter because you can't bring as many consumables.”
To hit the right balance, Howard is in process of building several Styrofoam/wooden structures of different sizes. While the modules of the International Space Station are 4.2 meters in diameter, they are scaled to fit the Space Shuttle cargo bay. Shuttle will be long retired when the habitats reach the lunar surface, so the new designs call for something a bit smaller and more mobile. Engineers are experimenting with habs that are both three meters and 3.5 meters in diameter, roughly as far across as a Boeing 737 airliner. On paper, the two versions are so similar in scale as to suggest equality. But step inside the mockups, and the half-meter difference is remarkable. Taller ceilings give the impression of a larger floor area. It's an old trick: Architect Frank Lloyd Wright used this perceptual phenomenon to his advantage in small home designs: low-ceilinged hallways open into rooms with higher walls, giving the impression of a great expanse in a relatively modest living area. But size and carrying capacity of the Altair lunar lander are in flux, says Howard. “Habitable volume and payload capability of Altair are going to be fighting a battle for the next several years.”
Many NASA experts would like to see habitats similar to the rigid modules on the International Space Station. They are heavy, but they work. Both NASA and Russian experience with long-term space habitation is within rigid habitats in the microgravity of Earth orbit. Designing for a zero gravity environment, every surface can be used. There is no ceiling or floor, no up or down, so the physical arrangements are quite different from an environment with floor, walls, and ceiling. As Larry Toups put it, “You're not just providing volume anymore; it's a footprint.”
Another option is to scrap the idea of a rigid hab completely, in favor of an inflatable one. An inflatable habitat has a rigid core containing supplies, electronics and other equipment, cocooned within a deflated living area. Once on the lunar surface, the core would pressurize the donut-shaped habitat around it, with the core representing the donut “hole.” Floors and ceilings would unfold as the hab extends outward into a tuna-can shape. Layers on the exterior must include micrometeorite protection, thermal insulation, and restraints to hold the pressurized “balloon” in a workable form. Another configuration of an inflatable hab resembles the cylindrical shape of a rigid hab. Similar structures are in use in Antarctica.
Lunar habitats will provide the essentials of life, like food, water, and air, but they must serve another important purpose: protection from radiation. Earth is enshrouded within strong magnetic fields generated by a core of molten rock and metal. These fields form a protective barrier between our planet and the Sun's radiation. But the Moon has no such protective energy shield. Habitats must provide shelter from incoming radiation, and the prospect is not as easy as it might first appear.
Lunar explorers must concern themselves with two types of radiation. The first comes from the background radiation of cosmic rays. Long-term exposure to cosmic rays can cause cancer or other health problems. They are such high-energy particles that no artificial material significantly blocks them. But fortunately, cosmic rays provide background radiation at a fairly low rate. Additionally, from the surface of the Moon, fully half of the cosmic rays are blocked by the Moon itself.
The second type of radiation occurs less frequently, but is far more deadly. It is the high-energy radiation that explodes from the Sun's surface as solar flares. Solar radiation can be filtered out with something that is readily available on the Moon: dirt. Several meters of regolith—lunar soil—may be needed (tests are still underway), but a habitat can be designed to carry the load on its roof. Even an inflatable enclosure can use regolith for protection. NASA's Langley research center is developing a Quonset-hut-style structure that starts out flat on the lunar surface. Shackleton construction workers would scoop dirt or load sandbags onto the flat covering, then inflate it into an arch. A habitat would then be positioned underneath, an arrangement much like the sod houses of the early American West. But Johnson Space Center's lunar architect Chris Culbert warns, “The flip side of that is that moving around many tons of regolith on the lunar surface is nontrivial. We'd have to take earth-moving gear with us.” Teams at JSC and Kennedy Space Center are studying a bulldozing blade that can affix to various rovers under development.
A two-meter layer of regolith affords sufficient protection from the most dangerous radiation events. Some types of metals or plastics may also do the trick, as would an underground storm shelter. Water makes a good barrier to radiation and can be stored in containers within the walls of living areas.
Like McMurdo base before it, Shackleton will eventually develop into a multistructure, sprawling community with roadways, centers for specialized operations, and staging areas for expeditions to the ends of the Moon. Constellation's strategy is to open the entire lunar world to the world's explorers. Apollo mission sites did not have the range to stray far from the Moon's equator. Constellation promises an infrastructure that makes any site on the Moon accessible, from the poles to the far side.
Communications within the lunar community promise to offer some of the greatest challenges to daily living and working on the Moon. At a minimum, lunar settlers need the ability to communicate with Earth and to communicate with all the habitats, astronauts and rovers within their local community. But radio communication can be hindered by rocky embankments or valley walls. Inhabitants will need both voice and video. High-bandwidth capability is preferable for downloading scientific and engineering information. Repeater stations or communications towers might fulfill Shackleton's requirements. All elements of the local outpost itself will remain within line-of-sight, but scientific and exploratory interests will quickly take inhabitants over the hill. Sortie missions need to be able to communicate from wherever they land, and surface operations require links to orbital or incoming Altair crews. Traveling rover crews might deploy a series of towers—along with supply caches—as they roam across the lunar wilderness.
A flock of four or five Moon-orbiting satellites could provide not only services such as continuous communication, but also tracking and data, something like a lunar GPS system. International partners like the European Space Agency and India have developed complex, advanced communications satellite constellations, and they are a likely source of such a global lunar communications network.
Whatever communications system is finally used, it is likely that the system will also serve a navigation role. Without the Earthly cues of scale and landmarks, just finding one's way around is a challenge. For example, on Earth, the atmosphere tends to shift the color of objects as
they increase in distance from the observer. This phenomenon, called atmospheric perspective, is a key element used by human perception to judge distance. But the lunar environment is airless, explains Apollo 14's Ed Mitchell. “The only major problem we had was finding landmarks by eye. That turns out to be very difficult to do.”
NASA and other space agencies are exploring a balance between robotic and human-tended systems, including remotely controlled lunar rovers and automated robots programmed to assemble large structures like habitats. Astronaut Alan Bean puts it this way: “Humans have many good qualities, and one of them is adaptability and learning to adjust to the situation. Humans have [physical] limitations we have to deal with, but they also have tremendous advantages, as we've seen with the construction of the Space Station and the repair of the Hubble Space Telescope. The combination of the best automatic machinery you can build and human adaptability and flexibility make the best combination for exploration. There is no such thing as unmanned or nonhuman space flight; it just depends where you put them. If you've got a robot, you still have humans involved. They're just sitting in Mission Control somewhere in California or Houston. You're never going to do away with the human part.”
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This Chariot rover, under study at the Johnson Space Center, features cameras and lasers on the front for remote operations, as well as a gondola for a pilot. (photo by the author)
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The goal is not to displace human explorers, says the Lunar and Planetary Institute's David Kring, but to augment their work, do some operations more cheaply, and keep the humans safe. “You can spend a few hundred million dollars that will, in the end, save you billions just by building robotics.”
One such robotic brainchild is called Chariot. The vehicle has a dozen wheels arranged in six sets of two, each independently steerable. Crab mode allows all wheels to turn in the same direction concurrently, giving the vehicle a turning radius of essentially zero. The craft can also steer around a point centered under one set of wheels, or even a point somewhere off in the near distance. Chariot can be remotely piloted, but also has capacity for crews. The driver stands in a turret (gondola) at one end that spins 360 degrees. Most test drivers prefer to drive the rover with the turret in the back so they can observe all wheels. This arrangement gave rise to the vehicle's name, as it resembles a horse-drawn Roman chariot.
Most important for the early establishment of an outpost, Chariot can do teleoperated construction. The front of the craft houses laser guidance, and stereo camera systems, giving it the flexibility needed for telerobotics. Designers envision habitats that have their own platform and leg structure. The habs would be picked up from a landing area, off-loaded in a stowed position, perhaps by a remote crane, and would remain undeployed until the rover arrives at the outpost site. Designers envision a scenario in which Chariot autonomously prepares an area for a habitat, carries the hab to it and deploys it onto the site, all under human-tended remote control. Similar exercises have already been tried using Styrofoam and plywood habitat mockups.
While Chariot toils in a gravel simulation yard at Houston's Johnson Space Center, engineers send a futuristic, crablike vehicle through its paces across the deserts of California. The spidery six-legged device is called ATHLETE (for All-Terrain Hex-Legged Extra-Terrestrial Explorer). Each face of the rover's four-meter-diameter hexagonal core has a set of stereo cameras and laser rangefinders to navigate over multiple types of terrain. Each leg has a wheel so that the craft can be driven, but wheels can be locked and used as an anchor, or “foot.” Wheels can be swapped out for claws or power tools. ATHLETE's operational version will weigh in at 2500 kg, with an impressive payload capacity of nearly 15,000 kg (15 metric tons) in lunar gravity. The behemoth's arms will have a reach of eight meters, enabling ATHLETE to offload cargo from the high deck of an Altair lander.
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A recent desert test of NASA's ATHLETE rover. (photo courtesy NASA/JPL)
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Brian Wilcox has been developing ATHLETE at NASA's Jet Propulsion Laboratory. He outlines the procedure: “The current concept is that a single six-legged ATHLETE would just walk off the top deck of Altair carrying the payload. The ATHLETE limbs would be attached to a pallet that has the payload affixed to it. When the payload is freed from the top deck, ATHLETE's limbs would unfold from their stowed position and reach all the way to the ground.” Four of ATHLETE's limbs would stretch to the ground and roll while two of the limbs at the back would step on corresponding ‘hard points’ on the Altair top deck. They would support the weight of the payload while the pallet shifts forward, changing the leg pose as the pallet shifts, so as to keep the two wheels stationary on Altair's deck. “Once the pallet has shifted far enough forward on the Altair deck, the vehicle would stop rolling forward long enough for the two back limbs to readjust their wheels onto new hardpoints. Then the vehicle would roll forward again, until the back two limbs can step down onto the ground.”
ATHLETE's design also allows for transporting a pressurized crew compartment for construction or long-distance science sorties. ATHLETEs might also be stationed on high ground to act as solar-powered communications relay stations. The Chariot rover has a spring-damper suspension and so can go much faster (about 20 km/h) while the lunar version of ATHLETE is now planned to go only about 5 km/h.
ATHLETE can be commanded directly by suited EVA astronauts using an arm-mounted keypad. It can also be driven remotely by crews in the shirtsleeve safety of the outpost, using a laptop or console. ATHLETE can also be directed by controllers at Earth control centers.
If all goes according to plan, a vibrant outpost of habitats, roving vehicles, communications systems, and power infrastructures could be in place two decades after the first humans return to the Moon's surface. Chris Culbert envisions a robust international community. “You can use the Antarctic as a well-defined international model for getting people and assets there. Commercial entities are taking advantage of the infrastructure already in place; maybe commercial entities are running it. You might get stationed there for six months to finish your dissertation, for example. Just as in Antarctica, there will be people who set you up, outfit you with gear, and train you on how to do business there. Eventually you get very healthy private enterprise, perhaps setting up a hotel so you can go stay there for a week.”
NASA's Constellation plans go far beyond Earth's nearest planetary neighbor. The launch vehicles of the Constellation Architecture have capacity to transport large loads into interplanetary space, carrying their cargoes across the 50-million-mile void to Mars.
Constellation Manager Jeff Hanley believes that if Mars plays the pivotal role in informing Constellation designs, the Moon will have a lot to offer. “I think it's only critically important to send humans back to the Moon if you intend to go further. So working back from there, how can we inform ourselves along the way? The key to really getting the probability of success—and probability of not killing anybody—sufficiently high is to mature our systems and make them eminently field serviceable. Once you send somebody to Mars, light the rocket and put them on a trans-Mars trajectory, there's no turning back. You're gone for a year at least, so the spacecraft must sustain you. We need to get those reliability numbers up. That's an area of technology development we are looking to foster: making systems as robust as possible once you've committed to that long-term outbound trajectory. Eventually something's going to break. How do I make standardized components across the system such that I could keep the key systems running to keep me alive?”
NASA scientist Chris McKay agrees. A long-term advocate of sending humans to Mars, McKay has become convinced that Constellation's incremental approach is the best way to get to the red planet. “It's sort of a Zen problem: the best way of getting to Mars may be by doing other things.” McKay knows about extreme environments first-hand: his work in the Antarctic wilderness has given him insights into the operation of remote outposts. “If I was building habit
ats and rovers on Mars, I would be happy to have a team that had done it on the Moon to do my design. Using that team to design my rover on Mars would make me feel much more comfortable. There's no denying that the experience we gain in building a base and maintaining it on the Moon will be incredibly useful—if not essential—in doing that on Mars.”
McKay contends that this is not simply his own opinion. It is the perspective of engineers working on advanced designs in the Constellation infrastructure. The engineers in the trenches are the experts, McKay says. “When I went to Johnson Space Center as part of the Lunar Architecture Team, it was clear that the systems engineers had come to several conclusions based on their work. One was that ‘We think we need to design around the Moon before we design around Mars.’ So I've got to say that these are the people we're relying on to do that. Their opinion, uniformly, is that we need to do it on the Moon first. I think we, the scientists who are interested in Mars, should put a lot of weight on that.”
Eventually, the lessons learned on the Moon's outpost at Shackleton Crater promise to teach us how to live—permanently—on the most Earthlike world in our solar system. With its vast natural resources and keys to planetary evolution and history, Mars beckons.
Copyright © 2009 Michael Carroll
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About the author: Michael Carroll is an astronomical artist and science writer. His next book, The Seventh Landing: How We'll Return to the Moon to Stay, is due out in the summer of 2009 through Springer Publications.
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Novelette: CHAIN
by Stephen L. Burns
Analog SFF, June 2009 Page 6