Each winter, the Geminids meteoroids light up the sky as they race past Earth, producing one of the most intense meteor showers in the night sky. Mysteries surrounding the origin of this meteoroid stream have fascinated planetary scientists for decades. Until recently, the Geminids had only been studied from Earth.
Now, researchers using data from NASA’s Parker Solar Probe mission have deduced that it was likely a violent, catastrophic event – such as a high-speed collision with another body or a gaseous explosion – that created the Geminids. The findings, which were published in the Planetary Science Journal on June 15, narrow down hypotheses about this asteroid’s composition and history that would explain its unconventional behavior.
“Asteroids are like little time capsules for the formation of our solar system,” said Jamey Szalay, a research scholar at Princeton University and co-author on a paper covering the Geminids’ origins published today in the Planetary Science Journal. “They were formed when our solar system was formed, and understanding their composition gives us another piece of the story. However, learning what happens to these objects as they move through space tells us a great deal about how our solar system has evolved.”
An Unusual Asteroid
Unlike most known meteor showers that come from comets, which are made of ice and dust, the Geminids stream seems to originate from an asteroid – a chunk of rock and metal – called 3200 Phaethon.
“Most meteoroid streams are formed via a cometary mechanism, so it’s unusual that this one seems to be from an asteroid,” said Wolf Cukier, an undergraduate student at Princeton and the paper’s lead author.
“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,” he added, referring to recent study with Parker Solar Probe images of the Geminids led by Karl Battams of the Naval Research Laboratory.
When a comet travels really close to the Sun it gets hotter, causing the ice on its surface to release gas. As this newly formed gas moves away from the comet, it drags with it little pieces of ice and dust. This material continues to trail behind the comet as it stays within the Sun’s gravitational pull. Slowly, this repeated process fills the orbit of the parent body with material to form a meteoroid stream.
But because asteroids like 3200 Phaethon are made of rock and metal, they are not typically affected by the Sun’s heat the way comets are, leaving scientists to wonder what causes the formation of 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.”
Scientists believe some comets may have a rocky core – like a ball of snow with a rock on the inside – and the Princeton researchers wondered if perhaps 3200 Phaethon used to be a comet that lost all of its snow, leaving only a rock. But the new Parker Solar Probe data show that although there seems to be some activity related to temperature, it’s highly unlikely that this cometary mechanism created the Geminids stream. It was likely caused by something much more catastrophic.
Opening the Time Capsule
The research builds on the work of Szalay and several Parker Solar Probe mission colleagues to assemble a picture of the structure and behavior of the large cloud of dust that swirls through the innermost solar system. They took advantage of Parker’s flight path -- an orbit that swings it just millions of miles from the Sun, closer than any spacecraft in history -- to get the best direct look yet at the dusty cloud of grains shed from passing comets and asteroids.
Built and operated by the Johns Hopkins Applied Physics Laboratory (APL) in Laurel, Maryland, Parker Solar Probe does not carry a dedicated dust counter that would give it accurate readings on grain mass, composition, speed and direction. But as dust grains pelt the spacecraft along its path, the high-velocity impacts create plasma clouds. These impact clouds produce unique signals in electric potential that are picked up by several sensors on the probe's FIELDS instrument, which is designed to measure the electric and magnetic fields near the Sun.
To learn about the origin of the Geminids stream, Cukier and Szalay used this Parker data to model three possible formation scenarios, and then compared these models to existing models created from Earth-based observations.
“There are what’s called the ‘basic’ model of formation of a meteoroid stream, and the ‘violent’ creation model,” Cukier said. “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.”
The different models reflect the
chain of events that would transpire according to the laws of physics based on
different scenarios. For example, Cukier used the basic model to simulate all
of the chunks of material releasing from the asteroid with zero relative
velocity – or with no speed or direction relative to 3200 Phaethon – to see
what the resulting orbit would look like and compare it to the Parker probe
He then used the violent creation model to simulate the material releasing from the asteroid with a relative velocity of up to one kilometer per hour, as if the pieces were knocked loose by a sudden, violent event.
He also simulated the cometary model – the mechanism behind the formation of most meteoroid streams. The resulting simulated cometary orbit matched the least with the way the Geminids orbit actually appears according to the new Parker Solar Probe data, so they ruled out this scenario.
In comparing the simulated orbits from each of the models, they found that the violent models were most consistent with the way the Geminids orbit actually appears according to the Parker probe data, meaning it was likely that a sudden, violent event – such as a high-speed collision with another body or a gaseous explosion, among other possibilities – created the Geminids stream.
The Next Generation
It’s rare for an undergraduate to be the first author on a scientific paper, and Cukier said his passion for learning about outers space combined with departmental support are what motivated him to pursue this project. .
He had taken a hands-on lab class offered by Princeton’s space physics laboratory for undergraduate students. In the class, students gain practical experience building space instruments, like those currently sampling the Sun’s environment aboard Parker Solar Probe. Cukier also served as treasurer for the undergraduate astronomy club last year, where he helped organize outings and skywatching events.
When Cukier decided he wanted to pursue extracurricular research, he reached out to scientists at the Princeton space physics lab. “Everyone is very supportive of undergraduate research, especially in astrophysics, because it’s really part of the departmental culture,” he said.
"Developing and encouraging the next generation of scientists and engineers is a priority on Parker Solar Probe, and a responsibility we take seriously,” said Nour Raouafi, Parker Solar Probe project scientist at APL. “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.”
Parker Solar Probe is part of NASA's Living with a Star program to explore aspects of the Sun-Earth system that directly affect life and society. The program is managed by NASA's Goddard Space Flight Center for the Heliophysics Division of NASA's Science Mission Directorate. APL manages the Parker Solar Probe mission for NASA.
- Alaina O’Regan, Princeton University