NASA astronaut and geologist Harrison Schmitt stands next to the lunar roving vehicle next to Shorty Crater, during the Apollo 17 mission. To date, Schmitt is the only scientist to have set foot on the moon. (NASA)
The moon is a dusty, airless rock that we last set foot on in 1972. What's left there for us?
Plenty, it would seem.
There's been so much talk in the past decade about sending humans to Mars that one may have wondered if we would ever walk on the moon again. After all, Mars is bigger, and unlike the moon, it has an atmosphere and vast reservoirs of water ice.
But it turns out that we have room in our imaginations, and our pocketbooks, for more than one obsession in our solar system. Not only is NASA sending robotic spacecraft to explore the moon, the agency just announced 12 upcoming lunar science and technology investigations. The U.S. also plans to send astronauts back to the moon in 2024.
Other countries are also extremely keen on learning about our natural satellite. In January, China deployed a rover to the far side of the moon, a first. Within the last 15 years, India, Israel and Japan have also sent landers, probes and other devices to land on, crash into, or fly by the moon.
All these recent missions and future plans attest to enormous continued interest in the moon, as an object of scientific curiosity we're still trying to understand more fully. It is also, like Mars, an accessible proving ground where we can develop the knowledge and experience to send people to more distant worlds.
Humans and the Moon: A Love Story
From the beginning of humanity's romance with the cosmos, the moon has occupied a sweetheart position in our aspirations to explore. It is by far our most easily reached destination in the universe, only 240,000 miles away. It's close enough for us to see details of its surface features with the smallest telescopes, and even with our eyes.
In the 1600s, Galileo squinted through his small telescope at the moon and saw its craters, mountains and wide flat plains, and Shakespeare wrote about "Th'inconstant moon / That monthly changes in her circled orb." The moon has always been a tantalizing, shadowy source of mystery, familiar yet unknown territory to be explored.
From the very earliest era of telescopic observations, scientists studying the moon and its multitude of impact craters have used it as a window into our solar system's past. The fact that the moon has no erosive atmosphere, and has been largely geologically inactive for almost 4 billion years, means that the scars of past events like collisions and volcanic activity are preserved on its surface. Scientists can literally read the history of the moon's development and the conditions in our solar system from far back into its youth.
More recently, chemical analysis of rock samples brought back by the Apollo missions tells us that Earth and the moon have a common origin, as described by the Giant Impact Hypothesis. According to this moon-formation idea, over 4 billion years ago, Earth was struck by another planet about the size of Mars. The impact blasted a large amount of rock into space that eventually coalesced into the moon. This makes the moon even more personal to us Earth-dwellers, more like an extension of Mother Earth than an alien, extraterrestrial world.
Forwarding Address: Moon City
The last crewed lunar landing was Apollo 17, in 1972. Mars missions may take up the bulk of the headlines today, but we've never stopped looking to the moon as a future home base on which to build a more enduring installation, or colony, or some future lunar city.
Now the U.S., the European Space Agency, Russia and China are actively working toward establishing a permanent lunar base.
The U.S. moon effort got a big boost when the George W. Bush administration called for the development of a new human-crewed spacecraft for traveling beyond low-Earth orbit, and a return of humans to the moon with the goal of "living and working there for increasingly extended periods of time."
The Obama administration committed to increase NASA's funding to complete a heavy-lift launch vehicle that will be vital to human missions and predicted a human-crewed mission to Mars by the mid 2030s.
The Trump administration has put the moon back on the table for human flights, and NASA has scheduled the first Orion spacecraft for a quick around-the-moon-and-back trip in 2022.
Maybe a whole moon city is yet some time away, but setting up a base on the moon for astronauts to live and work is widely seen as an idea with some traction and practical applications.
Not everyone agrees with the goal of a moon-base for humanity. Buzz Aldrin, the second man ever to walk on the moon, famously believes that mankind's future lies on Mars. In 2009, he wrote an editorial in the Washington Post. "A race to the moon is a dead end. While the lunar surface can be used to develop advanced technologies, it is a poor location for homesteading," he declared.
Fueling a Mission to Mars
These initiatives for returning to and working on the moon are part of a larger plan to prepare ourselves for a much more challenging journey to Mars. Harnessing the moon's material resources to build, fuel and launch a Mars mission would come with some major advantages. The moon's surface gravity is one-sixth as strong as Earth's, and there is no atmosphere to push through when launching. Both factors reduce the need for fuel, lowering the weight and cost of the spacecraft.
It turns out that the moon is not a dusty, airless rock after all. Finding water on the moon in 2009 was a huge revelation, and a useful one. NASA turned up evidence of polar water when the impactor vehicle of its LCROSS mission was deliberately smashed into the moon's south pole, blasting out a plume of soil in the process. The LCROSS spacecraft detected water in that plume, minutes before it, too, collided with the moon.
These ancient deposits of water ice on perma-shadowed crater floors could represent a water supply for thirsty lunar astronauts, if it can be made into drinkable form. It could also supply oxygen for breathing.
But the moon has some other inherent qualities that pose a challenge to potential human colonists living there for months at a time.
One is generating power to run all of a base's systems, including life support. The easiest way to produce electricity -- the way that most current space missions, human or robotic, do -- is with solar panels, converting the light of the sun into useful electricity. Most places on the moon, however, experience nights that are two weeks long, a long time to be in the dark and running on stored battery power.
Like the water resource problem, the moon's polar regions may offer a practical solution. The peaks of some polar mountains and crater rims enjoy practically around-the-clock sunlight. Placing solar panels at these polar heights would provide almost uninterrupted solar energy, something that is impossible even on the surface of the Earth.
Moon dust is also something that astronauts will need to manage. When the Apollo astronauts walked around on the moon, their spacesuits collected a lot of dust, which was unavoidably tracked back inside the lunar landing module. Dust on the moon is very gritty and sticks to practically everything. Unlike dust and sand on Earth, which are weathered down by wind and water into smooth, round grains, moon dust has sharp edges and points. Without the effects of erosion to smooth them out, moon dust tends to act like tiny bits of broken glass. Without strict dust management, future lunar inhabitants may suffer severe health problems.
As we continue to scrutinize minute details on the moon's surface through our ongoing missions, new surprises are sure to come to light. The moon has never failed us in this.
For more on the space race, watch 'Chasing the Moon' -- a new, three-part series premiering this week on KQED 9 at 9 PM.
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