If you’ve ever seen a meteor shower, you know it can be an amazing sight. You look at the sky where in a few moments there is a streak of light. Sometimes bright and in your field of vision. Sometimes it just starts out of the corner of the eye. Although meteors can appear at any time, they tend to appear at certain times of the year, such as the Perseids in August or the Orionids in October.

Meteor showers are named after the constellation they appear to originate from. It is constant from year to year, like the time of year. This is because meteor showers occur when the Earth passes through the path of a comet. The Orionids, for example, appear as we pass through the path of Halley’s comet. When dusty particles ejected from the comet hit our atmosphere, they glow as they burn up. The cometary origin of meteors has been known since the late 1800s, and while most meteor showers are caused by comets, not all are.

Geminid Meteor – George Varros (Courtesy of NASA)

The Geminids are the best example of this exception. They take place in December and can be quite strong. They seem to be growing stronger over the years as the Earth’s orbit shifts to go more strongly through the debris trail. For a while, the origin of the Geminids was a bit of a mystery, as they had no corresponding comet. But then in 1983, an asteroid called 3200 Phaethon was discovered, with an orbit directly aligned with the Geminid debris trail.

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Phaeton is a strange asteroid, only 6 kilometers in diameter. It is not icy, but it has been observed with a dusty tail, leading some to refer to it as a dusty comet. The reason it forms a trail is due to its dusty surface composition and the fact that Phaeton’s orbit takes it within 0.14 AU of the Sun, which is less than half the orbital distance of Mercury. As Phaeton approaches the Sun, its surface is baked like a dry lakebed, causing a dust trail.

Astronomers have assumed that the Geminid dust trail formed gradually as dust accumulated in each of Phaeton’s orbits, following the comet’s trajectory. But a new study finds that this is not the case. Instead, the team found that the Geminids appeared quickly.

Particle dispersion from different models. Credit: Cukier and Szalay

They used data from the Parker Solar Probe, which measured the size and composition of Geminid dust particles as well as their relative speed and direction. The team then compared the data measured using three models: a “baseline” model in which particles were simply deposited along the orbit, a comet model that gradually dispersed particles into the debris orbit, and a violent model that assumed particles were released in a single explosion.

Of the three models, the violent offending approach provided the best fit to observations. This indicates that the origin of the Geminids was not slow and steady, but rather an impact event where Phaeton was hit by a smaller asteroid. This model addresses one of the problems with the “dusty comet” model, which some argued was insufficient to explain the strength of the Geminids. But there are still factors that this study does not explain. The data and model are not precise enough to determine a specific effect source, only that the effect is a likely cause.

For all its beauty, the Geminids still hold some mysteries.

Reference: Cukier, WZ and JR Szalay. “Formation, Structure and Detectability of the Geminids Meteor Stream.” The Planetary Science Journal 4.6 (2023): 109.

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