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Catching the (meteor) train
In the early morning hours of April 8, 2017, astrophotographer Phil Hart was in southeastern Australia taking a series of shots to create a time-lapse video when he caught something truly weird and wonderful: a very long-lasting persistent meteor train.
Whoa! Persistent trains are the vapor trails left by meteors that usually fade in just a few minutes, but looking at the motion of the stars in Hart’s video it appears this one lasted well over an hour. That’s pretty unusual.
So, what are you seeing? Let’s back up a bit. The meteor was most likely due to a chunk of metal and rock that was once a piece of an asteroid. Perhaps it was a tiny bit of debris from a collision between two asteroids millennia or even millions of years ago, orbiting the Sun until the Earth lumbered into its way — we get hit by a hundred tons or so of this material every day! Even though this one was probably pretty small (if I had to guess from how bright it got, maybe the size of a grape or so), I suppose you could call it an asteroid in its own right, but as it passed through the atmosphere it would be called a meteoroid; the optical flash of light is the meteor.
As it entered our atmosphere a hundred or more kilometers above Earth’s surface, it was moving at a few dozen kilometers per second, so it compressed the gas in front of it violently. When you compress a gas, it heats up, so the air got very hot. This, in turn, heated the meteoroid, so much so that it became incandescent. This atmospheric drag slows the meteoroid hugely, but in this case it was small enough that it probably vaporized completely.
The train (what most people call the trail; nomenclature here is fussy) is due to rock and metal vapor from the meteoroid. Usually it fades very rapidly, sometimes in a second or less. Persistent trains last longer, and can form weird shapes as the high-atmospheric winds blow them around. You can see the train in the video twist around due to this.
At this point, I have to admit: Until recently, I wasn’t exactly sure how long-lasting trains worked. It seemed to me that the vaporized rock would cool rapidly, so it shouldn’t glow very long. However, for some reason, I never looked into it in detail until I looked at Hart’s video. Poking around turned out to be very instructive.
The glow is due to the vaporized rock and metal, but it doesn’t act alone. The meteoroid has lots of different elements in it. Two in particular are sodium and iron, and these are the keys to persistent train. Even though there’s not much air 100 km up, there’s some. There’s ozone that high, for instance, and when sodium gets near ozone, they react chemically. There are a few steps to the process, but in the end, the ozone (which is three oxygen atoms loosely bound together) gets converted to a plain old diatomic oxygen molecule (two atoms bound together), and the sodium acts as a catalyst (that is, you get the same amount of sodium out of the reaction that was put in). The sodium atoms absorb some of the energy of the reaction and then re-emit it as visible light — at a wavelength 589 nanometers, if you’re curious. That’s an orange-yellow color, uncoincidentally the same color sodium street lights emit.
This reaction can take a while to play out, which is why meteor trains can last for so long. It’s been known for a long time that the atmosphere itself glows softly at this wavelength, too, and that it was from sodium. I was rather amazed to find a paper from 1939 outlining how the source of sodium could be from meteors, though other guesses were salt from the ocean swept up by winds, volcanic dust, and particles from interstellar space.
Also, relatively recently it was found that iron behaves in a similar way around ozone; one of the steps in the process is the formation of ferrous oxide, FeO, which glows between 500-700 nm (green to red). In the end though that combines with a free oxygen atom to form a diatomic oxygen molecule and an iron atom, and the process can start again.
I have seen thousands of meteors in my time, but never one with a persistent train. Most of the meteors I’ve seen are during meteor showers, which are from bits of comets, not asteroids, so the metal content is lower (though not nonexistent; trains are seen from them as well). Maybe someday. It’s a numbers game, a matter of being out under the sky long enough.
I hope I do see a persistent train some night. There is something truly wonderful about seeing a meteor, that stochastic flash of light zipping across the sky. And when I do finally see one, the experience will be all the more sweet because now I have a fuller understanding of what I’m seeing.
Knowledge enhances beauty, especially when it comes to the heavens.