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Origin of the Geminids meteor shower has been a mystery, until now

Each winter, spectators eagerly look to the sky, their breath held in anticipation of the Geminids meteor shower – one of the most breathtaking spectacles our night sky has to offer. 

This meteor shower always raises eyebrows and many questions, chiefly because unlike other meteor showers, the Geminids don’t stem from a comet, but from an asteroid. Until recently, however, this asteroid had only been studied from our home planet, Earth.

Now, using observations from NASA’s Parker Solar Probe mission, researchers from Princeton University have ventured deeper into this mystery. 

They theorize that it was likely a violent, catastrophic event – such as a high-speed collision or a gaseous explosion – that gave birth to the Geminids. 

The findings, published in the journal Planetary Science on June 15, add more context to the asteroid’s composition and history, further explaining its unconventional behavior.

“Asteroids are like little time capsules for the formation of our solar system,” said study co-author Jamey Szalay. “They were formed when our solar system was formed, and understanding their composition gives us another piece of the story.”

Could an asteroid be responsible for the Geminids meteor shower?

Szalay’s comment touches on a significant point: unlike comets, which are made of ice and dust and are often the originators of meteor showers, the Geminids stream comes from an asteroid, specifically 3200 Phaethon, a chunk of rock and metal.

Wolf Cukier, an undergraduate from Princeton’s class of 2024 and the lead author on the paper, explained: “Most meteoroid streams are formed via a cometary mechanism, it’s unusual that this one seems to be from an asteroid.” 

“Additionally, the stream is orbiting slightly outside of its parent body when it’s closest to the sun, which isn’t obvious to explain just by looking at it,” said Cukier.

When comets come close to the Sun, they heat up, and the ice on their surface releases a tail of gas. This tail drags little pieces of ice and dust, leaving a trail behind the comet. Over time, this process fills the parent body’s orbit with material to form a meteoroid stream. 

However, asteroids like 3200 Phaethon, composed of rock and metal, don’t react to the Sun’s heat like comets do. This left scientists puzzled over what causes 3200 Phaethon’s stream across the night sky.

“What’s really weird is that we know that 3200 Phaethon is an asteroid, but as it flies by the Sun, it seems to have some kind of temperature-driven activity,” Szalay said. “Most asteroids don’t do that.”

Some scientists have speculated that 3200 Phaethon could be a comet that lost all its ice, leaving a rocky core that looks like an asteroid. But the Parker Solar Probe data suggests that while temperature might play a part in 3200 Phaethon’s activity, the creation of the Geminids stream was likely not due to a cometary mechanism. It was probably something much more catastrophic.

Combining new and old models to find origin of the Geminids

To get to the heart of the Geminids stream’s origin, Cukier and Szalay used the Parker Solar Probe data to model three possible formation scenarios. They then compared these models to existing ones created from Earth-based observations. 

“There are what’s called the ‘basic’ model of formation of a meteoroid stream, and the ‘violent’ creation model. It’s called ‘basic’ because it’s the most straightforward thing to model, but really these processes are both violent, just different degrees of violence,” explained Cukier.

The team found that the violent models matched most consistently with the Parker Solar Probe data, suggesting that the Geminids stream likely resulted from a sudden, violent event, such as a high-speed collision with another body or a gaseous explosion.

This research expands upon the work of Szalay and his colleagues from the Parker Solar Probe mission, assembled at the Johns Hopkins Applied Physics Laboratory (APL) in Maryland. Their mission was to understand the structure and behavior of the vast cloud of dust that swirls through the innermost solar system.

Tracking dust grains with the Parker probe

The team exploited the unique flight path of the Parker probe. It swings just millions of miles from the Sun, closer than any spacecraft in history. This close proximity allowed them to observe the dusty cloud of grains shed from passing comets and asteroids. Although the probe does not directly measure dust particles, it tracks dust grains in a creative way. 

As the dust grains hit the spacecraft at high velocity, they create plasma clouds. These clouds generate unique signals in electric potential that are detected by several sensors on the probe’s FIELDS instrument, designed to measure the Sun’s near electric and magnetic fields.

Nour Raouafi, Parker Solar Probe project scientist at APL, expressed his enthusiasm for the project, stating: “The first-of-its-kind data our spacecraft is gathering now will be analyzed for decades to come. And it’s exciting to see scientists of all levels and skills digging into it to shed light on the Sun, the solar system, and the universe beyond.”

Passion and love for the cosmos drove this study of the Geminids

Behind these discoveries is the dedication and curiosity of young scientists like Cukier. He credits his passion for space and the supportive environment at Princeton for motivating him to pursue this project. 

After taking a hands-on lab class at the Princeton space physics laboratory and serving as treasurer for the undergraduate astronomy club, Cukier was inspired to undertake extracurricular research. His enthusiasm was met with great support when he reached out to scientists in the Princeton Space Physics group. 

“Everyone is very supportive of undergraduate research, especially in astrophysics, because it’s really part of the departmental culture,” said Cukier.

David McComas, head of the Space Physics group and vice president for the Princeton Plasma Physics Laboratory (PPPL),  expressed his excitement as well. 

“It’s always wonderful when our students like Wolf can contribute so strongly to this sort of space research. Many of us have been in awe of the Geminids meteor displays for years and it is awesome to finally have the data and research to show how they likely formed,” said McComas.

Cukier’s love for sky-gazing started in his childhood. “Planetary science is surprisingly accessible,” he said. “For the Geminids, for instance, anyone can go outside on December 14 this year at night and look up. It’s visible from Princeton, and some of the meteors are really bright. I’d highly recommend seeing it.” 

His words remind us that the wonders of the universe, while mysterious and vast, are also within our reach – a thought as inspiring as the Geminids themselves.

More about meteor showers

Meteor showers are spectacular celestial events that captivate sky watchers and astronomers alike. They occur when Earth passes through the remnants left behind by comets and, in some cases, asteroids. Here’s an in-depth look at meteor showers:

What is a meteor shower? 

A meteor shower happens when numerous meteors, often called shooting stars, streak through the sky in a relatively short time. Each meteor is a small fragment of cosmic debris entering Earth’s atmosphere at high speed, causing it to heat up and glow, creating a bright streak of light in the sky.

Origins of meteor showers

Most meteor showers are the result of Earth passing through the dust and debris left behind by a comet. As comets orbit the Sun, they shed an icy, dusty debris stream along their orbit. If Earth travels through this stream, we see a meteor shower.

Some meteor showers, like the Geminids, originate from asteroids rather than comets. The exact mechanism by which an asteroid can produce a meteor shower is still the subject of scientific investigation.

How often do meteor showers occur?

Meteor showers occur regularly throughout the year, but there are certain annual showers that are especially notable for their frequency and brightness, such as the Perseids in August, the Geminids in December, and the Leonids in November.

The “radiant” point

Each meteor shower is named after the constellation from which the meteors seem to originate, a point known as the radiant. For instance, the Perseids meteor shower appears to come from the constellation Perseus.

Meteoroids, meteors, and meteorites

The terms meteoroids, meteors, and meteorites are often used interchangeably but have distinct meanings:

A meteoroid is a small particle from a comet or asteroid orbiting the Sun. A meteor is the light phenomenon (the glowing streak in the sky) that occurs when a meteoroid enters Earth’s atmosphere and vaporizes. This is what we commonly refer to as a “shooting star.” If a meteoroid survives its passage through the atmosphere and lands on Earth’s surface, it is called a meteorite.

Peak viewing times

While meteor showers may be visible for several days, each has a peak, where you can see the maximum number of meteors per hour. The exact date of this peak varies slightly from year to year. The best time to watch meteor showers is usually after midnight, when the part of the Earth you are on is facing into the stream of debris.

Watching meteor showers

Meteor showers are best viewed in clear, dark skies away from city lights. Patience is also key: it can take up to 30 minutes for your eyes to fully adjust to the darkness, and the best viewing is often in the pre-dawn hours, depending on the specific shower and time of year.

It’s important to note that while meteor showers can be forecasted to a degree, the exact number of meteors visible (meteor rate) is always uncertain and can vary widely from year to year. Some years may produce a spectacular show, while others are much quieter.

Meteor showers and culture

Meteor showers have been observed for thousands of years, and they have often had significant cultural and historical impacts. They have been seen as omens or signs and have been associated with significant events in various cultures.

In modern times, meteor showers are viewed with a sense of awe and are often seen as an opportunity for making wishes. They also provide a wonderful opportunity for photography and have become a popular focus for night sky and landscape photographers.


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