Every year, the Earth travels some 940 million kilometers through space in its orbit around the Sun. Over the past years, decades, and centuries, comets and asteroids have traveled through the same region of our Solar System, leaving a trail of debris in orbit around the Sun. If the alignment is right, then once a year, the Earth will pass through that debris stream, creating a meteor shower when it does.
The most spectacular ones of all occur in August (the Perseids), in December (the Geminids), and sometimes in November (when the Leonids are favorable). What you see varies from year-to-year, but this year’s Geminids just might be the most spectacular treat you’ve ever seen. If you have clear skies and a little bit of time on the night of the 13th/morning of the 14th, the Geminids will be at their peak. Here’s the story.
It all starts with either a comet or asteroid that gets hurled into the inner Solar System, close enough to the Sun to sprout a tail. Don’t be fooled by a common misconception: the tails themselves aren’t what give rise to meteor showers at all. Because the Sun blows the tail particles directly away from where the comet/asteroid is located, they’re not coherent enough to cause a “shower” if and when they do ever collide with Earth again.
These tiny dust grains wind up as part of the micrometeroids populating interplanetary space, but play no other special role in our cosmic neighborhood. However, due to the tidal forces from the Sun and other massive bodies in the Solar System, the nucleus of the comet/asteroid gets stressed, causing tiny pieces of it to break apart. Thanks to the infrared imaging capabilities of the Spitzer Space Telescope, we’ve actually seen this in action!
The little dust grains — the particles between the major fragments — wind up getting stretched out over the entirety of the comet’s (or asteroid’s) elliptical orbit over time. In the rare occasions where the orbital path of such a comet or asteroid actually crosses the orbit of Earth, these particles will collide with our upper atmosphere. Those of you who remember your introductory physics class might recall a formula for the kinetic energy of a moving body: KE = ½mv2. Even though the masses of these individual dust grains are tiny, from about the mass of a grain of sand up to a small pebble, they’re hurtling through space at tens of thousands or even hundreds of thousands of miles-per-hour (or meters-per-second) when they strike our atmosphere. And when it comes to energy, that “squared” on the velocity makes a big difference!