Asteroids and comets can usually be distinguished by what they’re made of, and by where they’re found in space. While both are small objects orbiting the sun – both created early in the history of the solar system, 4.5 billion years ago – there are enough differences that it’s easy to tell them apart.
The majority of known asteroids orbit within the asteroid belt between Mars and Jupiter, generally with not very elongated orbits. The belt is estimated to contain between 1.1 and 1.9 million asteroids larger than 1 kilometer (0.6 mile) in diameter, and millions of smaller ones. Early in the history of the solar system, the gravity of newly formed Jupiter brought an end to the formation of planetary bodies in this region and caused the small bodies to collide with one another, fragmenting them into the asteroids we observe today.
So asteroids are little bodies that might have gone into making another planet, but didn’t. Their compositions are reminiscent of the composition of Earth itself; they’re made of metals and rocky materials.
Asteroids have lumpy shapes and are often compared to potatoes.
The International Astronomical Union has the responsibility for assigning asteroid names. The first asteroids discovered were given names from ancient mythology, while newer names can honor anything from people (such as 3505 Byrd) to bands (8749 Beatles).
In contrast to asteroids, most comets come from beyond the orbit of Pluto, in a region of the solar system called the Oort Cloud. Comets tend to have much more elongated orbits than asteroids, sometimes reaching so far out into the outer reaches of our solar system that these small bodies can be perturbed by passing stars. When that happens, a comet may plunge inward toward our sun. Because they formed in the deep freeze of the outer solar system, comets are icier than asteroids. They’re composed of rock and ice, earning them the nickname dirty snowballs.
It’s when comets sweep in close to the sun that they produce their characteristic gas and dust tails.
Comets are named for their discoverers.
Not all comets and asteroids fall into the neat categories mentioned above. Besides comets that come from the Oort Cloud, another group of comets exists that have orbits much closer to the sun. All comets with orbital periods less than 200 years are called short-period comets. The vast majority of short-period comets move in orbits that are the same direction as the planets. One notable exception to this is the most famous comet of them all, Halley’s Comet.
Here’s another strange twist on the distinctions that exist between comets and asteroids. That is, comets are the usual parent objects for meteors that come in annual showers. Comets are flimsy objects, and, when they come close to the sun, they often leave behind a trail of icy debris. Earth plows through many old comet paths in the course of every year. The cometary debris enters our atmosphere, and that’s why we experience meteor showers. For example, the Orionids and Eta Aquariids are both showers made from debris left behind by Halley’s Comet. Comet Tempel-Tuttle is the source of the Leonid meteor shower. But not all meteor showers stem from comets. The Geminid meteor shower – seen every December – has a mysterious object called 3200 Phaethon as its source. This object is sometimes called a rock-comet; in many ways, it’s like an asteroid-comet hybrid.
3200 Phaethon, the weird asteroid with a tail, isn’t the only object blurring the border between asteroids and comets. Asteroid 311P/PANSTARRS was captured by the Hubble Space Telescope and revealed to have not one tail but six! The strange asteroid is now also considered a main-belt comet, and as a bonus it may have a satellite in orbit around it.
Bottom line: Most asteroids are rocky bodies that lie within the asteroid belt while comets are dirty snowballs from the Oort Cloud, with some objects acting like a hybrid of these two types.
Eleanor Imster has helped write and edit EarthSky since 1995. She was an integral part of the award-winning EarthSky radio series almost since it began until it ended in 2013. Today, as Lead Editor at EarthSky.org, she helps present the science and nature stories and photos you enjoy. She also serves as one of the voices of EarthSky on social media platforms including Facebook, Twitter and G+. She and her husband live in Tennessee and have two grown sons.