NASA

The moon turned one side of itself

away from the Earth 4.4 billion years ago and never looked back. This cosmic insult has turned the moon's far side into a mysterious, troublesome place where, at least in the imaginations of sci-fi writers, human-hating robot aliens crashed, and miffed Nazi colonists build spaceships to conquer Earth in 2018 (yes, you read that correctly).

That it is so close yet so unexplored, lends the far side of the moon a shadowy mystique. For example, it wasn't until this month that NASA created video footage of the far side—52 years after Soviet-era Russia snapped the first fuzzy photographs. Last week Space.com reported on NASA plans to build a waypoint in space that could act as, among other things, a way to send missions to explore the lunar far side.

Why the sudden interest, both in movies and in science? "The far side is a place we only vaguely understand. It's one of the only places in the universe you can't regularly see from Earth," planetary scientist Erik Asphaug of the University of California, Santa Cruz says. "We have no samples of it other than some meteorites, and we don't know exactly where they're from." If either robotic or human explorers could get some of these samples, they would open a door to an otherwise inaccessible epoch of Earth's history. Plus, everyone loves a good mystery.

Why Is There a Far Side, Anyway?

First of all, the far side of the moon is not "dark." It receives the same amount of sunlight month-to-month as the near side that's we see from Earth. When we're looking up at a new moon, the far side is basking in sunlight. (To be fair to Pink Floyd, though, there is a dark side of the moon—the half that's facing away from the sun at any given time. It's just not the same thing as the far side.)

But why should it be that the speed of the moon's rotation is in lock-step with its orbit of the Earth, so that the same side of our natural satellite always faces the planet?

Most lunar researchers agree that the moon formed after a Mars-size planet collided with Earth shortly after our planet's formation about 4.5 billion years ago. The splattered rock coalesced into a smoldering orb covered by lava that would become our moon; the hot oceans cooled and solidified into a crust over the next few million years.

As Earth's gravity pulled the orb into orbit, the nascent moon was sapping some of Earth's rotational energy and boosting itself into an increasingly distant orbit. In fact these tidal forces are still at work today—the moon retreats from the Earth by about an additional inch and a half each year.

Similar mechanics caused the moon's rotation to match its orbit around Earth, locking the near side into a permanent gaze. The moon is not a perfect sphere and bulges both toward and away from the Earth. So, after the lunar surface substantially cooled down, one bulge locked facing the moon like a rolling, lopsided tire that favors coming to a stop in one orientation.

Granny's Attic

Lunar geologist Noah Petro of NASA's Goddard Space Flight Center in Maryland says that the moon probably fell into this tidal lock within a few tens of millions of years after its formation. "The moon looked very, very different then than it does now," says Petro, who's a member of NASA's Lunar Reconnaissance Orbiter (LRO) mission that is mapping the moon and pounded a probe into one of its craters. "Billions of years of bombardment have made it a completely different place."

But here's the key fact: About 3.5 billion years ago, there was lava oozing on the crust of the near side of the moon, chemically altering the material there. But no such lava flowed across the far side; it remained mostly untouched, aside from collisions with asteroids and comets. A computer-simulation-powered study led by Asphaug suggests this is because of another object smacked into the moon shortly after its birth, obliterating most subterranean stores of lava that might have otherwise oozed upward.

According to Asphaug, this means that the material from early Earth that formed the moon remains mostly unchanged on its far side to this day, making the far side of the moon a mostly untapped repository of Earth history that's accessible nowhere else.

"The moon's far side is the oldest surface, and there are probably some Earth rocks over there that got transported early on. And there they still sit," Asphaug says. Locked within those samples may be direct records of conditions on Earth between 4.3 and 3.8 billion years ago—right around the time biologists think life got its start here.

"The moon is a storehouse of terrestrial evolution," Asphaug says. "It's kind of like granny's attic, where you find her old pom poms and question everything you knew before."

Far Side Dreams

For decades scientists have dreamed of visiting the far side and even setting up shop there to take advantage of its unique traits. For example, it so happens that the 8.1 million trillion tons of rock that comprise the moon make a pretty good shield for blocking humanity's busy electromagnetic chatter; as a result, some researchers have imagined building a permanent radio telescope there to create an unprecedented eye on the universe. Scientists also think the far side is relatively rich in helium-3, a potential fuel source that shows up occasionally in sci-fi.

Impact craters at the north and south poles of the moon also offer a tantalizing possibility for astronomers: There is permanent darkness down in the craters and year-round solar power at their rims. "These are places that never see sunlight. The temperatures are about 25 degrees above absolute zero," Asphaug says. Some of these shadowed craters edge toward the far side, and in them are frozen water, hydrocarbons, and other crucial resources for new lunar bases.

In a play to trump Apollo-era accomplishments, China intends to colonize such spots on the moon's surface by 2022. Republican presidential hopeful Newt Gingrich, meanwhile, garnered headlines and raised eyebrows with his plan to beat them there by two years—which most scientists have dismissed as impractical posturing.

Because of the economic chaos of the past several years, such audacious plans seem further and further away. But NASA is evaluating several mission plans. One of the more pragmatic options, in a world of shrinking budgets, is to lock a space capsule in orbit behind the moon. That way, astronauts inside the craft could control robots on the surface without having to land on the moon themselves. "This kind of orbit doesn't require much fuel to maintain," Petro says. "It's attractive from an engineering perspective."

And despite all the budgetary and political uncertainty, lunar scientists like Asphaug and Petro are holding out hope not only for direct lunar exploration, but also for missions that focus on the moon's far side. "It'd be expensive, but money is an amorphous thing," Asphaug says. "When you think of it not in terms of money, but as something humanity is destined to do, then I say yes, let's go for it. It would set new limits on what humans are capable of. It's hard to put a price on that."

Plus, establishing a lunar presence on the far side could also keep vengeful lunar Nazis away. "People like to make stuff up about what they don't understand," Asphaug said. "Until we go and explore, we'll have to keep suffering through pretty terrible movie plots."

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