When  I was around ten years old, my grandfather gave us a set of the World Book Encyclopedia. Most people haven’t a clue what that is, so think Wikipedia only in paper, hard back form. If anyone born after 1990 has seen them at all, it’s probably a series of books sitting in Grandpa’s bookshelf, and they’re about as out of date as you can imagine.

But I thought they were the cat’s meow. I read it from A thru Z, cover to cover. Now this particular edition was printed in 1955, and a lot of things hadn’t happened yet between the time they were printed and when I read them. Little things like Sputnik, Vietnam, JFK, not to mention I hadn’t been born yet, all made the books out of date.

In an effort to keep the books a little more current, about once a year, World Book would put out a yearly update. This was designed to help keep the contents current, at least for that year.

And so it was with the update for 1959. On October 7, the Soviet probe, Luna 3 obtained a handful of photographs of the lunar far side. Since the Moon always keeps the same face towards the Earth, no human eyes had ever seen what the probe showed. Compared to

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The first picture from Luna 3 of the far side of the Moon. No one expected the two sides of the Moon to be so different.

pictures that would arrive in the years to come, the Luna 3 pictures were terrible. But they were good enough for scientists on Earth to see that the far side was vastly different from the face of the Moon we know.

The side of the moon we see is dominated with dark gray areas we call Mares or Seas. These aren’t seas of water, but vast expanse of cooled lava that flooded the lower lying regions of the moon and solidified.

The far side of the Moon hasn’t any. Indeed, the far side was so different from the nearside that some accused the Soviets of faking the pictures and pushing them out as so much propaganda. Subsequent photographs showed that the far side is indeed different from the nearside. And the photographs of the far side returned by Luna 3 were deemed accurate (minus one or two items that appeared in the pictures, but were probably nothing more than transmission errors).

Enter Apollo. Apollo wasn’t even mentioned in my World Books or the updates. It hadn’t even been thought of, but by this time in my young life, it was steamrolling full speed ahead. The first manned Moon landing was just a few years in the future.

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The far side of the Moon as seen from Apollo 16.

When Apollo landed, the astronauts collected rock samples. These samples were returned, examined, and they turned our understanding of where the Moon came from on its head. They even showed that Earth and the Moon share a common history, and were once upon a time, possibly the same object. All that changed when a world slightly larger that Mars slammed into the early Earth.

Earth was shattered, but reformed. Some of the debris collapsed into the Moon poets and lover know so well.

But that didn’t explain the far side, or why it was different from the side we see. William Anders, one of the crew of Apollo 8, the first manned craft to orbit the Moon described the far side this way, “The backside looks like a sand pile my kids have played in for some time. It’s all beat up, no definition, just a lot of bumps and holes.” It was obvious something was different, but why still eluded everyone.

Enter high speed computing. When the theory of the collision of the Mars sized world with an early Earth was first proposed, it was put to the test with computer modeling. A variety of speeds, trajectories, and etc. were tried, and in almost all tests, a proto-moon was created following the collision. This seemed to bear out the collision theory.

But the computer modeling gave an unexpected surprise. In almost all of the simulation that produced a moon, another, smaller moon was also created in almost the same orbit. The question became if the simulations were correct, where was this second moon? Did it fall out of orbit? Was it tossed free of the Earth-Moon system–and if so, where did it go? What happened to it?

More mathematical modeling was done. Since the smaller moon was in almost the same orbit as the Moon, it reasons that gravity would pull them together. The test show this happened. If the smaller moon trailed our Moon, it eventually crashed into the Moon. It wouldn’t have been a very “gentle” process. Over the course of several tens of thousands of years, gravity would have drawn them together. As the smaller moon

mooncrash
A simulation of what we call the “Big Splat.”

closed, the gravity of the larger moon would be stronger on one side, than the other. Eventually, this would rip it part, converting it from a large, slowly approaching rock to one slowly approaching debris cloud. The impact would happen pretty much all at once, and it would rain down on the Moon.

It was the far side that received this debris, covering any ancient features and altering forever its appearance. They have a fancy name for this. It’s called the “Big Splat.”

If that’s indeed the case, then Anders’ description of it looking like his kids sandbox wouldn’t be too far wrong. Of course it’s probably going to take an landing and sample return from the far side (not to mention a lot more work than has been done) to prove or disprove if this is indeed the case.

Maybe my grandchildren will help sort that out.