Where Do the Geminids Come From? (Hint: It’s Not a Comet)

Instead of watching A Charlie Brown Christmas for the umpteenth time (okay, okay, it’s timeless), try catching a holiday show guaranteed to be different each performance: the Geminid meteor shower, one of the biggest showers all year. This year’s display peaks this Friday night into Saturday morning.

Unlike most meteor showers that we stay up into the wee hours to watch, the Geminids originate not from a comet, but are presumed to come from an asteroid—Phaethon 3200. While we’ve known about Phaethon for three decades, only in the past few years have scientists found evidence to support the idea that the asteroid is tossing off space dust. The team presented their findings this past September at the European Planetary Science Congress in London.

Using images taken by NASA’s STEREO spacecraft—which are designed to take pictures of the sun—the research team noticed that as Phaethon approached its closest distance to the sun, called perihelion, it got unexpectedly brighter. A “normal” asteroid wouldn’t do that, according to Dave Jewitt, an astronomy professor at the University of California-Los Angeles, who led the team. Indeed, a typical space rock would fade as it approached our star, as the moon in its “new moon” phase would. Jewitt’s team hypothesized that Phaethon must be ejecting some sort of dust that scatters sunlight. By synthesizing data gleaned from the telescopes, the researchers found the supporting evidence they were looking for: “The shape of the image is different from a point source,” says Jewitt. In other words, they saw a tail.

How would an asteroid eject dust? Jewitt’s team ruled out the idea of vaporizing ice, because Phaethon is way too hot to contain frozen water—we’re talking nearly 1300 degrees Fahrenheit (Phaethon passes six to seven times closer to the sun than Earth does). “Even if you buried ice inside Phaethon, it would have a very, very tough time surviving because the temperatures are so incredibly high,” says Jewitt. They also ruled out debris from an impact. Why? The team had noticed that the asteroid brightened at perihelion both in 2009 and 2012, and “It’s inconceivable that Phaethon would have been impacted twice,” says Jewitt. “It would just have to be the unluckiest asteroid in the solar system to keep getting whacked like that.”

What seems plausible, says Jewitt, is that Phaethon undergoes fracturing and desiccation as it rounds the sun, thanks to temperature fluctuations and its own chemical makeup. Far away from the star, Phaethon is cold, but it warms up as it approaches the sun. Plus, Phaethon rotates in orbit every three and a half hours—it essentially bastes like a turkey at perihelion. These extreme temperatures cause it to expand and contract and eventually fracture, releasing dust, suggests Jewitt’s team. (The same thing can happen when you wash an ice-cold glass under hot water—it breaks.) Further, Phaethon might contain certain minerals whose chemical makeup includes water. Those minerals disintegrate under high temperatures, like what Phaethon experiences at perihelion. When that happens, Jewitt’s team hypothesizes, water is released, and the asteroid shrinks and cracks, expelling dust. (Think of a drying lakebed. As the sun absorbs water from the lake, the mud breaks up and releases dust.)

So far, it sounds like Phaethon could be hurling those Geminids, right? The catch is, the STEREO telescopes are only sensitive to very fine particles—on the order of microns—that cover a large surface area and scatter a lot of light, says Jewitt. In other words, the telescopes don’t detect stuff the size of what streaks across the December night sky, which is on the order of millimeters. For now, researchers can only deduce that the existence of smaller particles means there are larger ones, too. (As an analogy, think of a building exploding in an action film. There’s a billow of dust that dominates the screen. But there’s also plenty of debris with more mass, such as brick shards, that exist, even if they’re seemingly invisible to the viewer.)

Though the link between Phaethon’s dust and the Gemininds’ larger rocks isn’t completely watertight, “I’d be amazed if what we’ve seen is not connected to the Gemininds,” says Jewitt. “That would just be extraordinary.”

What’s the best way to see the Geminids? Get outside and away from city lights, which will obscure less detectable meteors, advises Dean Regas, outreach astronomer for the Cincinnati Observatory. Most of the fireballs radiate from the constellation Gemini (near Jupiter this year), which will be high up in the southern sky by 2 a.m. on Saturday morning. The picture above illustrates the point, says Regas—those three streaks in the left half all seem to originate from one place.

Working against us this week, however, is a bright gibbous moon. “That washes out a lot of the fainter meteors,” says Regas. For a better bet at spotting a few, wait until the moon goes down. “This could be a nice Saturday morning for those early birds,” says Regas. (Last year at this time, there was a new moon, allowing for ample observing—the photographer who took the picture above says he saw 250 meteors, which would have been feasible given the viewing conditions, according to Regas).

Finally, don’t get discouraged if it’s not raining fireballs when you head outside. “Don’t expect the sky to fall,” says Regas. Meteors pass in the blink of an eye, and they can be few and far between. Still, meteors are unpredictable. “You never know what they’re going to do,” says Regas. Here’s hoping they put on a good show this year, despite the moonlight.