It's the time of the year when a mysterious visitor showers all the world with wonder and joy. That's right: Asteroid 3200 Phaethon is coming to town. Its dust particles’ collision with our atmosphere will produce the Geminid meteor shower, which peaks overnight tonight, December 13. It's the only such annual shower with an asteroidal origin.
So why does this happen, and how do you see it?
The Geminid meteor shower—so named because it appeared to burst forth from the constellation Gemini—graces the night skies midway through December ever year. The Geminids are serious stargazing business: They're often one of the best meteor showers of the year. Alas, in 2016, there's a catch (because this is 2016 and nothing can go right). A supermoon will correspond to the Geminids' peak, meaning the skies will be washed out with moon glow.
This doesn't necessarily doom this year's meteor shower to oblivion. The Geminids are hearty, and the sheer volume of meteors—up to 120 an hour in an average year—promises that something is likely to break through. Should you put some time into it and abscond to a light pollution–free area, you can expect something on the order of 40 events per hour—which is still an awful lot of meteors.
MEET 3200 PHAETHON
The debris responsible for December’s light shows aren’t coming from Gemini, but rather a strange object now known as 3200 Phaethon. Astronomers aren't exactly sure what Phaethon is—an asteroid or a comet. It seems to have originated in the asteroid belt, and yet as it approaches the Sun in its orbit, it grows quite the tail. That's not typical asteroid behavior, but very comet-like. The catch: Comets hail from the Kuiper Belt, located beyond Neptune, not from the asteroid belt like Phaethon.
Since its discovery in 1983, researchers have called it a "rock comet." Over the course of Phaethon's orbit, the Sun heats it up, leading to the ejection of its debris. Comets do this because they are essentially dirty space snowballs: As their volatiles vaporize, trails of particles remain. But asteroids aren't snowballs. They're rocks. So where is Phaethon's tail coming from? One hypothesis [PDF] to explain this strange ejection of dust from a heated asteroid is that as the asteroid heats, the water bound to the rock is lost, leading to stress and cracking. (Think of how mud cracks in a drying lake.) All of this produces dust, which the Sun organizes into that comet-like tail.
Phaethon's 17-month orbit around the Sun brings it inside Mercury's orbit and then out again between Mars and Jupiter. (You can see a visualization here.) It's named after the son of Helios, the Sun god in Greek mythology. Phaethon's orbital path is highly elliptical and intersects with that of Earth every December. Earth plows through Phaeton's trail of dust particles, which collide with our atmosphere at tens of thousands of miles per hour. As they burn away, they release energy that we see as streaks of light and sometimes call shooting stars.
In a sense, Phaethon is astronomy's version of answering a question with a question. For well over a century, astronomers were vexed by the mysterious parentage of the Geminid shower. When they found Phaethon, there was much rejoicing—at first. But soon everyone realized that the answer—a bizarre "rock comet" was its source—was even weirder than the question.
HOW CAN I SEE THE SHOW?
The best time to see the show is from late tonight, December 13, through the first few hours of tomorrow the 14th. If the weather doesn't cooperate, you still have a shot at catching something the following evening. If laying outdoors on the winter ground at midnight in mid-December isn't your thing, you can crank up the heater at home, swaddle yourself in a warm blanket on the sofa, and watch the show live on Slooh.