Lovers of the night sky will soon be able to enjoy the Geminids shower, with an hourly rate of 60-75 meteors expected between December 13-14. But to what object do we owe this beautiful spectacle?
Less famous than the Perseids visible in summer, the Geminids remain one of the most anticipated meteor showers of the year. Its peak of activity usually takes place on the night of December 13-14, when Gemini is highest in the sky. The average hourly rate then peaks at between 60 and 75 visible shooting stars. There may well be more. In 2017, up to 150 meteors could indeed be observed in one hour . Where does this shower of shooting stars come from?
We owe most of these events to comets. The Perseids are the offspring of Comet 109P/Swift-Tuttle, while the Lyrids are the remnants of Comet Thatcher, for example.
When a comet's ice vaporizes in the sun, grains of dust called meteoroids get lost along its orbit. When Earth enters this long stream of dust, these particles then hit our atmosphere. The meteoroids, no bigger than a grain of sand, then move so fast that they excite the electrons of their own atoms, but also of those found in the atmosphere. As these electrons lose energy, they then emit a trail of light . You then have a shooting star.
However, the parent star of the Geminids would not be a comet, but a simple asteroid named Phaeton . Discovered in 1983, the object appears to orbit the Sun in a highly elliptical trajectory leading it inside Mercury's orbit to the asteroid belt.
How come a single asteroid could cause such a meteor shower? "It remains a mystery “, admits David Jewitt, an astronomer at UCLA. Asteroids, in fact, are not supposed to "lose" material along the way like comets when approaching the Sun. At least, that's what we thought. Hence the questioning of the researchers.
For a quarter of a century, no one has seen this object spreading dust particles capable of explaining the many meteors that make up the December spectacle. But would it be possible that as it gets closer to the Sun, the surface of this asteroid is subjected to such temperatures that particles end up being projected into space ? The calculations seemed to indicate yes, but we still had to wait for a close passage of the object near our star.
In 2009, and again in 2012, images taken by NASA's STEREO A satellite finally showed Phaethon lighting up near the sun, suggesting that the he asteroid was indeed throwing up dust particles. In 2013, researchers also noticed the presence of a short trail of dust in the data collected.
Therefore, the researchers had attributed this dust production to the extreme heat inflicted by the Sun . Phaeton also develops such low gravity that these particles can easily escape into space. In addition, its rapid rotation (Phaethon rotates on its own in three hours and 36 minutes) also causes additional stress.
More recent observations made in December 2017 then made it possible to better understand the rock. Thanks to the now-destroyed Arecibo Observatory, we know that Phaethon isn't perfectly round, looking more like a spinning top like Bennu and Ryugu, two recently visited asteroids. We also know that it has an equatorial diameter of about 6.25 kilometers, but also several craters, one of which is more than a kilometer in diameter.
Still, these findings don't solve the mystery of how such a small asteroid could produce such a heavy meteor shower. Maybe time will tell.
In 2024, Japan will indeed launch its DESTINY+ mission which will approach Phaethon a few years later. This project, which promises us sharp images, will allow researchers to better understand the geological characteristics of Phaethon . This probe could even witness a "release" of particles in real time.
DESTINY+ will also look for any scars that may indicate recent impact that could explain the flow of materials offered by the Geminids. It's also possible that the asteroid was once a larger object that at some point broke into several pieces as it approached the Sun. In fact, two other asteroids smaller than Phaethon follow similar paths around our star. Also, they could be the remains of the same parent-body .
Finally, there's another question this Japanese mission might answer:the Geminids come to us from Phaethon, okay, but where did Phaethon come from?
