How Can We Tell When Something Hits The Moon?

How can we tell when something hits the moon if we can’t hear it?
Artist’s conception of the March 17, 2013 lunar impact as seen from Earth. Image credit: NASA's Scientific Visualization Studio

Artist’s conception of the March 17, 2013 lunar impact as seen from Earth. Image credit: NASA's Scientific Visualization Studio

Originally posted on Forbes!

Sound notoriously is poorly transmitted in space; it’s a pressure wave, and there’s just not enough material floating around in space in order for that pressure to survive any distance in space. So anything that happens outside the confines of our little atmosphere is soundless to us. There are, of course, ways to reconstruct sounds from other information, but it’s usually not a reconstruction of what you would hear if you were there and had heard the noise through an atmosphere like our Earth’s.

But we know that the Moon should be constantly getting bombarded by small pieces of debris, because our own Earth gets hit by a considerable amount of small debris, and any dirty patches of the Earth’s orbit (such as those which are responsible for the meteor showers) are also going to be dirty patches for the Moon - it’s not really that far away from us, after all.

Before and after images taken by LRO show the location of a new 60-foot in diameter crater (right) that formed on March 17, 2013. Image credit: NASA's Goddard Space Flight Center

Before and after images taken by LRO show the location of a new 60-foot in diameter crater (right) that formed on March 17, 2013. Image credit: NASA's Goddard Space Flight Center

The main difference between a meteor shower on Earth and that same meteor shower on the Moon is that the Moon has no atmosphere. The atmosphere on Earth makes these meteors much easier to spot, because they leave a luminous trail across the sky. On the Moon, you’d expect these meteorites to make it all the way down to the surface of the Moon before there was any real observable trace of them.

Even then, they don’t make much of an announcement as to their arrival! Most impacts on the surface of the Moon are from relatively tiny pieces of grit, and so even though they hit the surface at incredible speeds, it can be hard to spot the aftermath on the surface. And even if you do spot a fresh crater, you won't know exactly how long it's been there, unless you can spot the moment of impact itself. There are a few observatories which do precisely this.

Any high speed impact is doing a lot of shifting around of energy, and even if a small fraction of that energy is converted into visible light, we can observe it. There are a few of these observatories, which look at the portion of the moon which falls in shadow, because the little blip of light will be more obvious there. NASA runs the Automated Lunar and Meteor Observatory (ALAMO) from Alabama, which is a multi-telescope setup and observes the shadowed part of the moon for small flashes of light. The multi-telescope nature of the facility means that any blip of light seen by all the telescopes isn’t very likely to be random noise.

A similar setup exists in Spain, with five telescopes working together to observe the shadowy Moon, called the Moon Impacts Detection and Analysis System (MIDAS). (Astronomers love a good acronym.) This system found a particularly bright impact flash, which was suggested to have come from a reasonably large object (a few feet across), and crashed into the surface at a whipping 38,000 miles per hour. These observatories are great for pinpointing exactly when new craters should be appearing on the Moon, and with satellites which map the Moon's surface, we can tie these flashes of light to brand new craters, and work backwards more accurately to determine what kind of object must have ended up smashing into the Moon.

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What would actually happen to the moon at the end of the movie Iron Sky?

In the movie Iron Sky, towards the end of the movie they put a massive crater in the moon. Would gravity pull the moon back into a sphere, and how long would that take? What would happen to the moon’s orbit?

I have to start by saying that, from a scientific perspective, there are a couple of really major problems with this scenario, before we even get to the physics of the energy required to make such a crater.

The first is that looking at the picture above, the moon is blocking out the sun, and yet, a quarter of the surface is illuminated. This is geometrically impossible; the sun illuminates the surface of the moon that faces the sun - there’s no way for sunlight to be reflected off the surface facing away from the sun.

Secondly, the way the crater appears, it looks like the moon is a flat disk that had a notch bitten out of it, like a cookie. The same is true if you watch the clip of the actual explosion, where they shoot a hole in the horizon so that they can see the Earth. The problem with this is that the moon is a sphere, not a disk, so any divot like that would have to be a cylindrical trench in the surface of the moon, not a spherical one like normal craters make. This cylindrical hole would only appear as a semicircular divot from a few positions in space, so that final shot would not see the chunk taken out of the moon as so clean a semicircle, since it’s looking from a different angle than the original firing.

To figure out what kind of damage the creation of a crater like this would do to the moon, I had to make a couple of simplifying assumptions. The major one was that the depth of the hole we see in the final shot is the depth of the crater, and that the width was equal to the width of the crater. I also assumed that the crater was round, and not a tunnel, since the explosion looked round in the clip. (It’s really hard to make cylindrical holes without a lot of prior planning.) With this information, I could calculate the volume of the crater, and using the average density of the moon, I can find the mass of the moon that had to be removed to make the crater. The above picture of the moon was very useful for this calculation, since I could measure the scale of the crater relative to the entire moon, which allowed me to work out that the crater is 1390 km wide, and 463 km deep. With the assumption of the crater being spherical, and the average density of the moon of 3.34 grams per cubic centimeter, this leads us to a total mass removed from the moon of 1.347 x 10^21 kg. This seems like a huge amount of matter (and it is), but since the entire moon weighs 7.348 x 10^22 kg, this is only a removal of about two percent of the total mass of the moon.

Since the final shot of the moon is some time after the explosion, and there’s not a lot of debris hanging around, I worked out the amount of energy required to both lift that much mass, and the amount required to fling that much mass permanently away from the moon. It’s a lot of energy. In fact, it’s more energy than could be generated by the entire world’s nuclear stockpile detonating at once, by a factor of a billion (10^9) - and that’s just to lift it. To permanently remove the mass, the current world nuclear stockpile is a factor of a quadrillion (10^15) too weak. The Götterdämmerung only fires two bullets - we can safely say that these bullets are physically impossible.

As far as whether the moon would eventually become a sphere again, the answer here is yes, it would, but it would do so in a way that’s very different from what’s depicted in the film. The film essentially depicts this massive detonation as resulting in what’s called a “simple” crater. A simple crater is just a bowl shaped cavity on the surface of a planet or moon, and the moon has plenty of them already. But when a large amount of energy is injected into the surface of a planet, a lot of the energy is lost into heating up the rock; that heat will temporarily liquify the surface. The surface will ripple, and then cool down, resulting in a series of rings and (often) a central peak - this is called a complex crater. This liquefaction and rippling means that the craters are not as deep as they would be if every crater was a simple one. The bottom of the crater, being a liquid, rebounds up towards what would have been the surface, and a lot of the material that could have been thrown out of the crater would stay on the surface as a liquid. These events are still quite destructive, and a large amount of material would be thrown out of the crater, but the moon would never have become quite as non-circular as it was depicted.

One of Saturn’s moons, Mimas, has a really massive complex crater on its surface that earned it the nickname the Death Star Moon. Mimas was thought to have been nearly shattered by this impact, and has a bunch of what is technically called “weird terrain on the exact opposite side of the moon. Weird terrain is where the seismic shock waves from the impact all meet up and collide on the opposite side of the object, releasing their energy. This fractures the surface in a particularly chaotic way, and can trigger volcanism if the planet or moon has magma hanging around under the surface. Mimas doesn’t, and our moon only has a small molten core, but an impact like this would certainly jumble the surface at the antipode of the moon.

The moon’s orbit would be remarkably not affected by this. The material thrown out (as shown in the film) from the moon is one fiftieth the mass of the remaining moon - and the laws of momentum tells us that an object fifty times larger is fifty times harder to move. Assuming that the material blown off the moon reaches escape velocity, the rest of the moon will receive a kick of 44.5 m/s. If the material doesn’t make it to escape velocity, the kick will be weaker. The moon’s average orbital speed is a touch over 1 km/s, which means that this additional velocity is only an alteration of 4% to its speed. However, we have to consider the geometry of the situation here - if the moon had been hit along the direction of travel, it would gain or lose speed in its orbit around the earth, and that might change the distance of its orbit by a small amount. But the hole is in the “top” of the moon, which means that the velocity boost to the moon is pointed “down”. This won’t change the distance of the moon’s orbit, but will make the orbit slightly more tilted. Only slightly, though, since the velocity kick still isn’t very strong.

Overall, this crater would not dramatically change the orbit of the moon, though it would make it more tilted, and if this had happened in the real universe, a lot more of the energy would have gone into melting the surface of the moon, and a lot less into making a crater.

Something here unclear, or have your own question? Feel free to ask!