We finally know how the mysterious Geminid meteor shower originated

Each year, skywatchers get to gaze at the spectacle of the Geminids streaking through the night sky from mid-November through late December. However, this meteor shower is highly unusual, and not only because it is one of the easiest to view.

Meteor showers usually originate from comets that fly close to the Sun. Comets are made of frozen gasses, dust, and rock, and the Sun’s heat vaporizes some of that gas and releases it into space, dislodging debris that eventually falls to Earth. But the Geminids are exceptional because they originate from an asteroid instead of a comet. Asteroid 3200 Phaeton is the source of this trail of debris, but asteroids are not affected by solar heat the same way as comets, so it’s unclear why Phaeton has left a trail of debris.

NASA scientists who analyzed data from the space agency’s Parker Solar Probe have now finally found the most likely answer to the mystery of how the Geminids formed: a catastrophic event. “The Geminids may have formed via a more violent, catastrophic destruction of bodies that transited very near to the Sun,” the scientists said in a study recently published in The Planetary Science Journal.

In pieces

So how did the Parker Solar Probe, designed to study the Sun, give hints as to how the Geminids were born? Its orbit takes it right through the core of the Geminids at perihelion, or the point where they and 3200 Phaeton come closest to the Sun. As it flew through the meteor shower, Parker was bombarded by dust grains that gave off electrical signals on impact. These signals were detected by its FIELDS instrument, which is designed to measure (among other things) electric and magnetic fields. How fast the dust grains were going and how hard they hit gave—an indication of their mass—an idea as to what might have been behind the formation of the Geminids.

The Parker data, along with simulations and Earth observations, convinced the science team, led by planetary scientist Wolf Cuvier, that the Geminids were not constantly breaking off from 3200 Phaeton. Phaeton and the debris that came from it could have resulted from a collision or explosion that broke apart a much larger body, possibly a comet. Cuvier and his team think it is also possible that the same collision also produced two nearby asteroids.

Such a collision would also explain another mystery: the mass of the Geminids. Together, they are at least as massive and possibly more massive than their parent asteroid. 3200 Phaeton does lose some material in orbit, but not nearly enough to account for the mass of the Geminids.

“The mass of the Geminids stream is estimated to be on the order of or larger than that of the parent body 3200 Phaethon, which suggests the stream was formed in a possibly catastrophic event that shed a large amount of mass in a relatively short period of time approximately 2,000 years ago,” the scientists also said in the study.

Catastrophic

Cuvier’s team used Parker data to come up with models of how the Geminids potentially formed. Taking the impacts of dust particles that crashed into the probe into account, they ran several catastrophe simulations. The first model simulated a typical catastrophic asteroid destruction event; the second involved a more violent event that would have scattered faster-moving debris over a wider area. The third model drove the formation of a meteor shower from a comet.

It turned out that the most likely scenario was also the most violent, and this was backed up by observations of the Geminids from Earth. Would an event this crushing have sent space rocks hurtling down to Earth’s surface thousands of years ago? However threatening it seems, that would have been highly unlikely. None of the three models showed any debris striking our planet.

However sensitive Parker’s FIELDS instrument is, there are still some things it cannot tell us. The type of catastrophe that formed the Geminids is still unknown. Whether it was a collision or gaseous explosion would have impacted the shape and width of the debris stream. While Parker cannot directly image its structure, further missions might. JAXA’s upcoming DESTINY+ mission will actually head directly for 3200 Phaeton after it launches in 2024. It could possibly clarify more about how the Geminids came into being by making more direct observations. Until then, we will keep gazing into the winter sky and wondering.

Planetary Science Journal, 2023.  DOI: 10.3847/PSJ/acd538 (About DOIs).