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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How much of the universe are we looking at?

Are we scanning the whole universe, or mostly only looking at one place?

At any given moment, it depends on what we’re trying to do! There are a lot of telescopes currently operating, and where we point those telescopes on is entirely dependent on what science the person using the telescope wants to do. So those telescopes wind up looking all over the place, but mostly for short periods of time.

Then there are what’s called “deep fields”, where we point a telescope at one patch of sky for a long time.  This allows us to see very faint objects, and lets us get a lot of detail or capture very distant objects.  The Hubble Deep Field (and Ultra Deep Field) is probably the most famous of these kinds of observations.

But the last kind of observation we can do is an “all sky survey”.This is where we get one telescope to observe every patch of sky available from the location of the telescope. (If the telescope is in space, then that’s ALL the patches of sky - one of many reasons we love space telescopes.)  Depending on the telescope we’re using, we get information about a different type of object, so there have been lots of these surveys to look at different colours of light.  The most recent of these to hit the news was the Planck satellite.  It conducted an all-sky survey to look at the oldest light in the universe, and in the process made a fantastic map of the Milky Way.

In fact, most of those edge-on maps of the Milky Way are made through these all-sky surveys. Some surveys want to avoid the Milky Way, so they skip the region of sky where we know the galaxy is, since getting rid of our galaxy when you’re interested in much more distant objects can be a pain.

Here’s a fun website where you can scroll through a few of the surveys that have looked at our Milky Way in different wavelengths, and see how our view of the Galaxy changes. (It’s zoomable!)

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