The National Space Council, the reconstituted White House group designed to set national space policy, reportedly is in favor of a human return to the Moon. What will stepping on the Moon be like for these next explorers? What reaction can we expect from those watching it happen again, this time using much more advanced technology, or from those who will be seeing it—experiencing it—live, for the first time? And what significance does it hold for future human expansion into the Solar System?
The Moon is a world in and of itself—not just a disc of light 250,000 miles overhead. Yet communication to and from Earth is a mere three-second round-trip. Travel time to Earth is only three days. The lunar surface spans about 38 million square kilometers, about the same size as the continent of Africa. People accustomed to living on Earth—a sphere almost four times the diameter of our Moon (an unusually large one, as moons go) might easily misjudge distances; while standing on a flat plain, the lunar horizon is only a couple of kilometers away. The Moon has mountains, many of enormous size, as high as 10-12 km (over 7 miles high). But even from vistas on these high peaks, one could not see more than a few tens of kilometers because of the abrupt curvature of its surface. With gravity 1/6 that of Earth’s, people on the Moon will experience a lighter, more buoyant sensation while walking, as their bodies are built and conditioned to living on Earth. A 150-pound person will weigh only 25 pounds on the Moon. Pictures of the Apollo astronauts walking, skipping, and falling down during their excursions give us some idea of how people will need to adapt to the reduced gravity of the Moon.
While it has no atmosphere or global magnetic field, the Moon rotates on its axis once every 28 Earth days (showing us its familiar phases, from “new” to “full”). The 28-day cycle is the same amount of time it takes the Moon to make one rotation around the Earth; this synchronous lock of rotation and revolution means we always only see one hemisphere of the Moon from Earth (imaginatively called the “near side”), while the “far side” faces away, permanently hidden from Earth’s view. The plane of the Moon’s elliptical orbit is inclined six degrees to the ecliptic and thus, we can see slightly more than 50 percent of the surface, as the limb regions (the Moon’s visible edges) wobble into and out of Earth-view. These areas of the Moon are the only surface locations where people would experience “Earthrise and Earthset,” as they twist into and out of view (“Earthrise” as seen by the Apollo astronauts, was a result of their being in orbit around the Moon). People standing near the Moon’s poles will not see sunsets at all, but instead, will watch as the Sun slowly circles around them on the horizon.
The lack of an atmosphere means a jet-black sky—no air molecules exist to scatter sunlight as on Earth. But stars in that dark sky are difficult to see when standing on the Moon’s surface in the daytime because the bright glare of reflected surface light causes your pupils to close, making the stars too faint to observe. Shadows are not perfectly dark for the same reason, as scattered ambient light from bright surrounding areas partly illuminate the dark shadows. Craters that dominate the lunar landscape formed over billions of years from impact bombardment; these holes in the surface range in size from mere pinpricks up to basins the size of terrestrial continents. Craters are both a blessing and a curse—they expose deeper layers of the Moon for geologic examination and sampling, yet at the same time, they create a rolling, rock-strewn landscape that can be difficult to navigate.
The sunlit areas near the Moon’s poles are thermally benign, with an estimated surface temperature of about -50° C, plus or minus 10°. Due to the lack of a moderating atmosphere, the remaining surface of the Moon experiences large temperature swings, ranging from -150° C during the coldest parts of the lunar night to over 100° C during the hottest portion of the lunar day. Astronauts who have walked on the Moon noticed significantly warmer temperatures as the Sun rose higher in the sky over the course of their stays on the Moon. These large temperature swings—plus the possible hazardous effects of radiation—are one of the reasons engineers like to consider the advantages of living underground on the Moon. The recent discovery of caves in some areas on the Moon (i.e., lava tubes in which the lava has evacuated and left a standing void) has given encouragement to those who consider them ideal habitats for future lunar occupants. The problem with lava tube habitation is that you must locate your outpost where the tubes are found, and all are near the lunar equator, far from the water ice deposits of the poles. A less geographically restrictive approach would be to lay habitat modules in the bottom of small crater floors and then back-fill the crater with regolith (lunar soil), creating thermally insulated, radiation-proof living quarters.
The surface of the Moon consists of soil and rock (regolith) derived from the constant micrometeorite bombardment of the surface. The fine-grained fraction (40 micron mean grain size) consists of angular broken fragments of rock, mineral and glass. When someone kicks or disturbs this soil, it does not billow up, but instead shoots out, then falls back to the surface. These dust grains are highly abrasive; if surfaces are not kept clean, moving parts will quickly grind to a halt. We must develop strategies to mitigate the negative effects of this dust. Fortunately, research has shown that much of the abrasive dust is actually attracted to strong magnetic fields. This is not because the grains are made of iron, but because a very fine layer of what is called “nano-phase” iron—material condensed from impact vapor produced by meteorite impacts—coats each glass and mineral fragment. Thus, we can remove much of the abrasive dust by using magnetic cleaning techniques. The lunar far side offers something not found anywhere else in our Solar System—a place where radio noise from the Earth is blocked. Radio telescopes placed there will be able to see back to the earliest formation stages of the Universe.
When we step onto the Moon, we are entering a natural museum of planetary processes and history. Just like all the other rocky planets of the Solar System, the Moon has a crust, mantle and core (the Moon’s crust is thick and its core is small, compared to the Earth). The Moon has experienced a bombardment of asteroids and comets for over four billion years, and because of its antiquity (and lack of wind and water erosion), this record is both accessible and recoverable on the lunar surface as on no other planet. The lunar surface has been partly resurfaced by volcanic lava over more than a billion years. These flows have locked within them buried regolith horizons whose particles contain the historical record of the ancient Sun and galaxy. This relation opens up a wholly new field of study—paleoastrophysics, the study of the universe not as it is now, but as it was in the distant past, billions of years ago. A critical question for the future of humanity in space is how the human body adapts (or does not adapt) to fractional gravity. The Moon’s one-sixth gravity is an ideal laboratory to address and answer this question, as virtually every human destination beyond Earth has less gravity.
One of the more exciting discoveries of the past decade has been the realization that the lunar polar regions are unique “micro-environments” compared to the rest of the Moon. Unlike the equatorial and mid-latitude regions, the terrain near the poles has areas that experience constant darkness, alongside some areas that are nearly always illuminated by the Sun. This simple proximity has dramatic consequences. The dark areas are extremely cold, and serve to collect volatile substances that hit the Moon over time. As shown by a variety of remote sensing techniques, these “cold traps” have accumulated significant quantities of water ice and other matter. Water is one of the most useful things found in space. It supports human life as a consumable and as protection against radiation. It’s also a medium for energy storage, and can be made into the most powerful chemical rocket propellant known. Additionally, the illuminated peaks near the poles allow us to generate electrical power almost constantly, without having to survive the two-week-long nighttime experienced everywhere else. We thus find (to the surprise of many) that the Moon is a natural logistics depot in near-Earth space, where humans can learn how to live and work and provision themselves on another world.
The Moon is unique—an achievable, worthy goal, well within reach of humanity. With its constantly open launch windows and proximity, its intellectual value and its utility, the Moon is the next logical destination for America’s national space program and an ever-growing number of commercial investors. If we take the initiative now, it won’t be long before many of us again experience the thrill of seeing people and machines step onto another world, and watch with a knowing smile as others experience it for the first time.