Why Apollo Astronauts Trained in Nuclear Bomb Craters

On July 20th, fifty years ago, Neil Armstrong and Buzz Lightyear first set foot on the moon. But before they went there, they came here. This is Sedan Crater, and it was excavated by a nuclear bomb in 1962. [bomb explodes] It's part of the Nevada Test Site, an area of desert bigger than Rhode Island. -- [broadcast] located 70 miles [110 km] northwest of Las Vegas -- Here, the U.S. performed 928 nuclear explosions. And from 1965, they trained Apollo astronauts like Neil Armstrong and Buzz Aldrin before their trips to the moon. In the following years, astronauts returned with spacesuit mock-ups and TV cameras. They even tested out a mock-up of the lunar roving vehicle. Over a period of seven years, 11 of the 12 men who would eventually walk on the moon visited this site.

So why would you bring Apollo astronauts to a nuclear bomb test site? That's what we're gonna find out. [♫] The obvious reason is because the moon is covered in craters, so the astronauts needed experience in cratered terrain, but there were other locations available, like Barringer Crater in Arizona, claimed to be the site of a meteorite impact, though its origins were disputed until the 1960s. In another part of Arizona, scientists used conventional explosives to recreate the exact pattern of craters on the moon around the Apollo 11 landing site. Now the astronauts did train at both of these sites, but the Nevada Test Site provided something extra.

[♫ brassy fanfare ♫] -- Put on your goggles -- The Nevada Test Site was commissioned in 1951 as a place to test nuclear explosions. The goal was not only to develop better bombs, but also to understand how homes and household items could be made to withstand a nuclear blast. -- Five... -- -- Four... -- -- Three... -- -- Two... -- -- One... -- -- Zero. -- [bomb explodes] A hundred tests were conducted above ground before the Nuclear Test Ban Treaty was signed in 1963. It banned testing in the atmosphere, outer space, and underwater, leaving only underground testing.

Like the blast that formed Sedan Crater: a 1 meter wide hole was drilled down into the ground right here, 635 feet [194 m] deep, and that is where they located a nuclear bomb. It was a 104-kiloton [440 TJ] device. That means the energy released was equivalent to 104,000 tons of TNT, roughly eight times as big as the bomb dropped on Hiroshima. -- [broadcast] This close-up view was taken from a ground station three miles [5 km] from ground zero. The dome rose to a height of 290 feet [88.4 m] before it vented at three seconds. -- It excavated 12 million tons [11 Mt] of dirt and rock, forming what is the largest man-made crater in North America. In fact, it's the second biggest in the world after Chagan Crater in Russia. It's about 400 meters in diameter and nearly 100 meters deep.

This was part of an operation called "Plowshare." -- [broadcast] The United States is conducting, for the benefit of all nations, a program it calls "Plowshare." -- [♫ momentous fanfare ♫] With the idea being, if you need to form a big hole somewhere, let's say you're making a huge construction project, maybe a canal, could you use nuclear weapons to excavate that dirt? [♫] -- [broadcast] Enormous energy, relatively inexpensive, compact, and easily transportable. -- [broadcast] Enormous energy, relatively inexpensive, compact, and easily transportable. This is the new power tool that "Plowshare" would add to Man's resources of useful energy, to do jobs never before practical or even possible.

As you can see, it's a pretty good excavator. But later, they found out that it's very hard to reduce the contamination, the radioactive contamination that results from a nuclear explosion, and so the operation was effectively canceled. But what they had created was the closest thing you can get to a meteorite impact crater. And that seems a bit weird, because I mean when I think about meteor impacts, I imagine these sort of compressions of a big rock coming in and pushing the dirt out of the way.

But that's not actually how it works. Meteors are typically going incredibly fast, like 10 or 20 kilometers per second, [♫] so when one strikes the ground, it creates an incredibly hot, dense, high-pressure region at the site of the impact. This melts and vaporizes rock. A shockwave traveling out from the point of contact transforms minerals due to the extreme pressure. As the high-pressure region decompresses, it creates what is essentially a huge explosion, and it is this explosion that creates the crater.

Something you might notice if you look at craters is they're almost always circular. But if you think about it, meteors impact from all sorts of different angles. The reason you're seeing a circle is because really, a meteor impact is an explosion. That is what forms the crater. It's not the impact; otherwise, you would see all sorts of weird, oblong-shaped craters. Nuclear explosions are so much like meteorite impacts that craters on this test site provided the definitive evidence confirming that Barringer Crater was in fact the result of an impact.

Scientists compared samples from both sites and found the same shocked minerals, like coesite, a shocked form of quartz. This mineral can only be formed under the intense pressure of a meteorite impact, or a nuclear explosion. Further similarities with nuclear test craters allowed scientists to estimate that the energy of impact was around 10 megatons. That's the size of a fairly big thermonuclear detonation. Another similarity is in how these cratering events excavate rock.

All of this dirt got excavated out of here— 12 million tons [11 Mt]— and it was ejected out and over the rim, and what actually happens in that process is the layers of rock actually get turned over at the rim. It's called inverted stratigraphy. This is a telltale sign of meteorite impacts and nuclear explosions, and it helped the astronauts know, when they collected samples on the moon, what to look for and where. Something I was surprised to learn was that Apollo astronauts actually spent 25%, or a full quarter, of their final year before blasting off to the moon studying science, visiting sites like this one where they could learn about geology, about their rocks and minerals, and the formations that they should look for when they're on the moon.

And I guess it makes sense, because these were scientific missions. But they weren't conducted by professional scientists. By and large, the Apollo astronauts were expert pilots. As Apollo 11 blasted off, the astronauts were journeying into the unknown—literally. We knew so little about the moon, like how it formed, what it was made of, and whether or not it was volcanically active. (-- All engines run.) (We have a liftoff! --) When the Apollo astronauts were on the moon, they actually used some of their training from here. The astronauts were really excited when they found the rocks that they recognized.

In fact, it was probably due to their training here that they were able to recognize those minerals and those rocks, which were so important to bring back. And to learn not only about the moon's formation, but also about our entire solar system and the formation of all the planets. This is the actual moon dust, or lunar regolith, collected by the Apollo 11 astronauts. I'm gonna take a look at it under the microscope.

Now since the moon has basically no atmosphere, all of this material had never been exposed to oxygen. So there was some concern that once the astronauts brought it back into their spacecraft and introduced some oxygen, that it would spontaneously catch fire. So Buzz recounts the story of how they were ready to throw all the samples out if that were to happen. But, of course, thankfully nothing did happen when they brought those samples in and re-pressurized the lunar module. Amongst the dark lunar dust that Apollo 11 brought back, scientists identified tiny flecks of a light-colored rock, which they immediately identified as anorthosite. And that was an important discovery because it supported the theory that in the past the moon was entirely melted, covered in a magma ocean at least a hundred kilometers thick.

So the idea goes that when anorthosite forms, when the minerals that make it up crystallize out, they are less dense than the magma around them, so they float to the surface. So, the initial surface of the Moon, the primordial surface of the Moon, would have been made of anorthosite, and then about a billion years later, there was more volcanic activity on the Moon, leading to lava flows over these regions that are darker called the mare. And you can see they have fewer craters on them because, well, they're younger.

So how do you get anorthosite over here in the Sea of Tranquility? Well, that must have happened through giant impacts in the highlands, which would have sprayed ejecta all over the Moon. And so it got mixed in with the dark soil over here and brought back by the astronauts to my Petri dish where I can look at it today. But anorthosite was just the tip of the iceberg. Inside all of these samples was a subtle clue to how the Moon formed.

Looking at the amounts of different isotopes in a rock can tell you where that rock came from. And when scientists analyzed the Moon rocks, they found the same isotopic abundances as in Earth rocks. This is why we believe today that the Earth and Moon formed together from the same event: a giant impact between planets four-and-a-half billion years ago. Going to the Moon and knowing what to look for taught us not only how the Moon formed but also how our Earth formed.

And all this from less than 400 kilograms of Moon rock from the near side of the Moon. Clearly, there is still a lot to learn up there, and personally, I can't wait until we go back. Who knows what unexpected discoveries are waiting for us when we return.

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