Flashes in the Sky: The Origins of Meteors

Have you ever watched a meteor shower? They happen very frequently when Earth's orbit takes it through the debris left behind by a comet or asteroid orbits the Sun. For example, Comet Tempel-Tuttle is the parent of the November Leonid shower.

Meteor showers are made up of meteoroids, tiny bits of material that vaporize in our atmosphere and leave behind a glowing trail. Most meteoroids do not fall to Earth, although a few do. A meteor is a glowing trail left behind as the debris streaks through the atmosphere. When they hit the ground, meteoroids become meteorites. Millions of these solar system bits slam into our atmosphere (or fall to Earth) each day, which tells us that our area of space isn't exactly pristine. Meteor showers are especially concentrated meteoroid falls. These so-called "shooting stars" are actually a remnant of our solar system's history.

Where Do Meteors Come From?

Earth orbits through a surprisingly messy set of trails each year. The bits of space rock that occupy those trails are shed by comets and asteroids and can remain for quite a long time before they encounter Earth. The composition of meteoroids varies depending on their parent body, but are commonly made of nickel and iron.

A meteoroid doesn't typically just "fall off" of an asteroid; it has to be "liberated" by a collision. When asteroids slam into each other, little bits and pieces settle back onto the surfaces of the larger chunks, which then assume some kind of orbit around the Sun. That material then gets shed as the chunk moves through space, possibly through interaction with the solar wind, and forms a trail. Material from a comet is usually made up of bits of ice, specks of dust, or sand-sized grains, which are blown off the comet by the action of the solar wind. These tiny specks, too, form a rocky, dusty trail. The Stardust mission studied Comet Wild 2 and found crystalline silicate rock bits that had escaped the comet and eventually made it into Earth's atmosphere.

Everything in the solar system began in a primordial cloud of gas, dust, and ice. The bits of bits of rock, dust, and ice that stream from asteroids and comets and end up as meteoroids mostly date back to the very formation of the solar system. The ices clustered onto the grains and eventually accumulated to form the nuclei of comets. The rocky grains in asteroids clustered together to form larger and larger bodies. The biggest ones became the planets. The rest of the debris, some of which remains in orbit in the near-Earth environment, gathered into what's now known as the Asteroid Belt. The primordial cometary bodies eventually gathered in the outer regions of the solar system, in areas called the Kuiper Belt and the outermost region called the Öort Cloud. Periodically, these objects escape into orbits around the Sun. As they get closer, they shed material, forming meteoroid trails.

What You See When a Meteoroid Flares

When a meteoroid enters Earth's atmosphere, it gets heated by friction with the gases that make up our blanket of air. These gases are generally moving pretty fast, so they appear to "burn up" high in the atmosphere, 75 to 100 kilometers up. Any surviving pieces could fall to the ground, but most of these little bits of solar system history are too small for that. Larger pieces make longer and brighter trails called "bolides."

Most of the time, meteors look like white flashes of light. Occasionally you can see colors flaring in them. Those colors indicate something about the chemistry of the region in the atmosphere it flies through and the material contained in the debris. Orange-ish light indicates atmospheric sodium being heated. Yellow is from superheated iron particles likely from the meteoroid itself. A red flash comes from the heating of nitrogen and oxygen in the atmosphere, while blue-green and violet come from magnesium and calcium in the debris.

Can We Hear Meteors?

Some observers report hearing noises as a meteoroid moves across the sky. Sometimes it's a quiet hissing or swishing sound. Astronomers are still not completely sure why the hissing noises happen. Other times, there's a very obvious sonic boom, particularly with the larger bits of space debris. The folks who witnessed the Chelyabinsk meteor over Russia experienced a sonic boom and shock waves as the parent body burst apart over the ground. Meteors are fun to watch for in the nighttime skies, whether they simply flare overhead or end up with meteorites on the ground. As you watch them, remember that you're literally seeing bits of solar system history vaporize before your eyes!