The sky is falling—twice!
Two of the more interesting meteor showers will occur as the year draws to a close: the Leonid and the Geminid showers, both of which put on a great show. Better yet, for both showers, the thin new crescent moon will slip below the horizon shortly after sunset, ushering in darker nights for better viewing.
To see the Leonids, you need to hustle a bit because they will peak on the night of November 17—that is, tonight. You can go out tomorrow and still see them, though. The Geminids will peak on December 14, so you still have time. The Geminid meteor shower is better, anyway. The Leonids can be seen at a rate of about 10 to 15 meteors per hour, while the Geminids are more ostentatious—up to 150 meteors per hour fly through our atmosphere during that shower!
Sometimes, though, on a scale of one to 10, the Leonids crank up to 11—metaphorically, at least. Historically, their peak has shown enormous upward variance, and there have been reliable reports of meteor “storms” from the Leonids that reached more than 100,000 meteors per hour! That’s about 30 per second, which would be simultaneously very exciting and extremely unsettling. It really would seem as if the sky was falling.
The cause of these storms can be traced back to the source of the Leonids themselves: the comet 55P/Tempel-Tuttle. The comet takes about 33 years to orbit the sun on a highly elliptical path that takes it as far out as three billion kilometers—roughly the same distance as Uranus from the sun—down into the inner solar system to just about Earth’s orbital distance.
Comets are a mix of ice and rock, and when Tempel-Tuttle drops closer to the sun, the warmth from sunlight vaporizes ice in its nucleus. As this vapor escapes to space, it carries away a fraction of the comet’s material—mostly as tiny grains of rock and dust but also as sizable pebbles of up to about a centimeter in diameter. All this debris orbits the sun as well, following, more or less, Tempel-Tuttle’s orbital path.
That “more or less” is important, though, because it encapsulates several quirks that can make any year’s Leonids extra prodigious—or just run-of-the-mill.
Every year in mid-November Earth passes through this cosmic flotsam. When it does, some of Tempel-Tuttle’s debris slams into our atmosphere at hypersonic speeds, heating up to glow briefly but brilliantly in the sky.
Such flashy dashes through Earth’s atmosphere are standard for most meteors, but a few things about the Leonids make their spectacle special. One is that Tempel-Tuttle orbits the sun in a so-called retrograde fashion: it moves in the opposite direction as the planets. That means we hit the Leonids head-on, which in turn means that their relative speed as they pass through the atmosphere is much higher than that of most other meteors, averaging around 70 km per second. A meteor’s brightness mostly depends on its mass and velocity, so such high speeds make the Leonids brighter than the meteors of many other showers.
As for the Leonids’ famed meteor storms, every time Tempel-Tuttle nears the sun it releases a new batch of meteoroids (the term for space-based bits of debris that subsequently become “meteors” by entering and burning up in a planet’s atmosphere). Pressure from sunlight and the planets’ gravitational pull acts on these meteoroid streams, moving them away from the comet into slightly different orbits. Sometimes Earth passes through a particularly dense stream, dramatically increasing the number of meteors. The last major storm from such a stream was in the early 2000s, and the next one is predicted to occur in 2033. Many hundreds of meteors per hour could light up Earth’s skies then.
So this year’s Leonids are not expected to be especially stormy, but there still could be surprising activity peaks. Your best bet is to go out and look!
The Geminids in December are also unusual because unlike the meteors of almost every other annual shower, their parent body is not a comet but an asteroid! In this case it’s 3200 Phaethon, a rock about 6 km across. Its orbit takes it out past Mars but drops it down to a scorching 21 million km from the sun, closer than the orbit of Mercury.
Experts assumed for many years that the intense heat of that close passage vaporized rock on Phaethon’s surface and that this was the source of the Geminid meteoroids. Research published just this year in the Planetary Science Journal, however, has a different conclusion. The scientists modeled how particles would leave Phaethon if they were produced from solar heating and found that the subsequent orbit they’d take doesn’t match what’s actually observed.
When the paper’s authors assumed that a small asteroid impacted Phaethon in the past, however, the resulting violent expulsion of rock matched the Geminid orbits much better. In fact, two other asteroids, called 1999 YC and 2005 UD, both have very similar orbits to Phaethon, implying that all three formed when a larger asteroid broke up in a big collision.
If true, this means that every Geminid meteor you see is a tiny piece of shrapnel blasted out from two asteroids that struck each other long ago! That’s seriously cool.
The Geminids orbit the sun in the same overall direction that Earth does, so they hit us at a more stately 35 km per second—still dozens of times faster than a rifle bullet. The meteor shower is known for having bigger chunks, too, so besides offering more meteors per hour, it can be very bright as well.
What do you need to do to see these two celestial performances? Unlike many other events in the sky, for the Leonid and Geminid showers (and all other meteor showers), you don’t need binoculars or a telescope. In fact, I suggest you not use them at all because meteors zip across the sky so quickly that if you’re bent over an eyepiece, you’ll likely miss them.
Instead find a spot far from human-made lights and objects that occlude the sky, such as buildings and trees. The darker and clearer your view, the better, because meteors can appear anywhere in the sky. A blanket or chaise lounge is best for repose; you want to be comfortable. Dress warmly, of course! I find that hot chocolate makes the night even better.
The best time in general for viewing showers is after midnight local time, which is when you’re on the part of Earth facing into our planet’s direction of travel. (This is like riding in a car in the rain; the raindrops mostly hit the front windshield rather than the back one.) The Leonid shower is known for its Earth grazers—meteoroids that enter the atmosphere at a very low angle—which can brightly blaze across the entire sky. These Earth grazers are best when Leo—the location in the sky from which the meteors appear to radiate, called the shower’s “radiant”—is rising on the horizon at around 11 P.M. local time.
Meteor showers are a terrific excuse to get out under the night sky and are even better in the company of friends and family. I used to wake up my daughter when she was very young so we could go out and watch them, and I still treasure those memories. If you can, spend some time watching pieces of asteroids and comets on their final journey across our sky and make some memories of your own.