There are millions of space rocks swirling around the solar system, most of them in the asteroid belt, but many others much closer to Earth’s orbit. If you’ve been following astronomy and space news, you’ve seen these rocks called many things, and it might not be entirely clear what the differences are between meteors, asteroids, meteorites, comets and meteoroids. If that’s the case, here’s a short primer to set you straight.
Let's begin with the one that you’re likely to have seen with your own eyes. A meteor is a light phenomenon caused by a meteoroid that enters the Earth's atmosphere and vaporizes as the air’s friction makes it rapidly heat up. The rock is the meteoroid (more on that below), and the light produced as it passes through the atmosphere is the meteor. In other words, it’s a shooting star.
Below is the famous Perseids meteor shower photographed from Black Rock Desert in Nevada. This image is actually many photos merged together, showing 29 meteors:
The Perseids are a meteor shower associated with the comet Swift–Tuttle. (Photo: Trevor Bexon [CC by 2.0]/Flickr)
The meteoroid is the source of the shooting star before it enters Earth’s atmosphere. Most are about the size of a pebble, with some as big as a meter in diameter. They’re usually rocky or metallic, and they are often pieces of bigger asteroids or comets. Meteoroids between 10 microns and 2 millimeters are usually called micrometeoroids, and anything smaller than that is just space dust. (NASA points out that every day, Earth is bombarded with more than 100 tons of dust and sand-sized particles.)
A meteorite is a meteoroid that doesn’t entirely disintegrate as it falls through the atmosphere and lands somewhere on the planet’s surface. There are three kinds of meteorites: stony meteorites, iron meteorites (usually composed of iron-nickel) and stony-iron that contain a mix of both. About 94% of meteorites are stony and 6% are are mix of iron or stony-iron.
Below is an iron meteorite:
Here is the inside of a beautiful stony-iron meteorite composed of yellow-green olivine crystals encased in the iron-nickel matrix:
Technically, asteroids are minor planets orbiting the sun. There are millions of them, mostly of rocky composition and located in the asteroid belt between Mars and Jupiter. They have neither the characteristics of full planets (not large enough to be rounded out by their own gravity) or comets (more on that below). They vary in size from 1,000 kilometers to 10 meters in diameter. "If you only consider those larger than 100 meters orbiting within the inner solar system, there’s over 150 million. Count smaller ones and you get even more," writes Universe Today.
In the future, when humanity starts sending astronauts to other planets and maybe even builds bases there, some think that asteroids could serve as "gas stations in space."
This amazing video by astronomer Scott Manley shows known asteroids in the solar system over time. Even if you don’t take the time to watch the whole thing, just have a quick look: Note the year in the bottom left corner and then jump forward near the end of the video to see the difference in the number of known objects orbiting the sun. Also note that the red dots are asteroids with orbits that come close to Earth.
Comets are icy bodies (rocky, metallic or both) that, when close enough to the sun, heat up and partially vaporize, creating a small atmosphere of dust and gas that is sometimes visible as a tail. They often have elongated elliptical orbits that will bring them closer to the sun for a while and then away from it for a long time. Some of these orbits last many years, some even millions of years.
The most famous comet is Halley’s, which is visible to the naked eye from Earth every 75-76 years. The comet's visits have been documented since 240 B.C., including by medieval observers. Don’t hold your breath waiting to see it, though, as it was last in the inner solar system in 1986 and won’t be back until 2061.
Here's a photo of Halley's comet taken in 1986:
Isn't it beautiful? Too bad it comes around these parts so rarely.