Some meteors leave trails that last up to an hour. Now we can know why

Now, the first systematic survey of persistent trains has revealed what kind of meteor is most likely to leave a train behind. Contrary to previous assumptions, the key variable in whether a meteor will leave a stable train is its height in the atmosphere, not its speed or brightness, astronomers reported in July. Journal of Geophysical Research: Space Physics.

Amateur astronomers often record such trails “like a beautiful movie,” says astrophysicist Gunter Stober of the University of Bern in Switzerland, who was not involved in the new work. “This is truly the first most comprehensive, total compilation of statistics.”

Persistence trains form when metals that have been burned from the incoming space rock react with oxygen, especially ozone, in the atmosphere. The chemical reaction emits heat and light, keeping the train going for tens of minutes or even up to an hour. They can twist and writhe like luminous serpents as the wind blows them away.

Studies from the 1940s and 1950s suggested that the trains are rare, occurring in 1 in every 750 meteors, and mostly associated with the brightest meteors. More recent studies focused on the Leonid meteor storm in the early 2000s, which was the most dramatic shower in decades (SN: 19.12.01). These studies concluded that only the fastest meteors, with speeds of about 70 kilometers per second, leave the trains.

But these surveys were either too broad, including one-off sightings of meteors by observers around the world, or too narrow, focusing on a single spectacular meteor shower.

To create a more uniform catalog, astrophysicist Logan Cordonnier and colleagues set up a camera to look at the same patch of sky over New Mexico for nearly two years. From October 2021 to July 2023, the instrument recorded every strip of light that crossed its field of view. In that time, the team recorded about 7,500 meteors, of which about 850 left stable trains. Not only were the trains more common than expected—about 1 in 8 meteors left a train, and 1 in 19 lasted more than five minutes—but the trains were showered by meteors of all speeds and luminosities.

“Some of the previously held ideas were that these stable trains were only formed by fast, bright meteors,” says Cordonnier, of the University of New Mexico in Albuquerque. “We found that it doesn’t need to be fast. Most stable trains were formed by slower meteors.”

The real determining factor was the availability of ozone, says Cordonnier. Meteors that penetrated at altitudes of 90 kilometers were much more likely to leave the trains than those that were higher. That’s above the Earth’s ozone layer, but there’s a small concentration of ozone at that altitude, Cordonnier says. While theoretically, meteors that pass through the ozone layer could also leave a trail, Cordonnier notes that few meteors make it that far without disintegrating.

Future observations of persistent trains may help probe the chemistry of this elusive atmospheric layer. The region “is the spot on your back where you can’t itch,” says Cordonnier. “It’s too high in the atmosphere for weather balloons and too low for satellites to make direct measurements. It is a difficult region to investigate.” Continuous trains, however, “happen for free, all the time. We just have to watch and see them.”

Stober wants to see the data in the new catalog applied to another question: Why do some trains retain their shapes for so long, while others quickly dissipate? Explaining the chemistry that produces trains in the first place is interesting, “but you need a force to keep the train as a train,” he says.

Atmospheric physicists have suggested that tiny charged dust grains dropped by the meteor could produce an electric field that could hold the train together. More investigations in this catalog and others may help prove this notion right or wrong.