Comets and asteroids have a lot in common. Both orbit the sun. Both are far smaller than the planets. Both are remnants of the solar system's formation almost 5 billion years ago.
So what are the differences between comets and asteroids?
What is a comet?
Comets have been likened to dirty snowballs, and that's a pretty apt description. They're loosely bound masses of ice, dust and rock. The core or nucleus of a comet is relatively small — often just a few miles across. It consists mostly of water ice and frozen carbon dioxide, carbon monoxide, methane and ammonia.
Most comets circle the sun far beyond the planet Neptune and take centuries to complete a single orbit. But sometimes a comet's orbit takes it close to the sun, inside the orbits of Earth and other planets. When that happens, heat from the sun causes the comet's nucleus to form a visible cloud of vapor from the melting ice (called a coma) and a tail of gas and dust that extends for distances of up to tens of millions of miles.
In some cases, comets and their tails are so big that they're visible to the naked eye. Yet astronomer Ed Krupp, director of the Griffith Observatory in Los Angeles, says comets are "all buff and fancy. There is so little material there, a comet has been called the nearest thing to nothing anything can be and still be something."
Earth's orbit periodically takes it through the debris of a comet's tail. As the debris strikes the atmosphere, it burns up to create brief but brilliant streaks of light, known as shooting stars or meteor showers. Several meteor showers occur at regular times each year, including the Perseids each August, which are associated with Comet Swift-Tuttle.
What is an asteroid?
Asteroids are composed mainly of rock and metal — making them sources of valuable minerals that might be mined in space.
Sometimes called space rocks, asteroids range in size from several feet in diameter to the size of mountains or even continents. The largest known asteroid, Ceres, is about one-quarter of the size of the moon — and is considered a dwarf planet.
Ceres is one of an estimated 1.9 million large asteroids orbiting the sun in the asteroid belt, which lies between the orbits of Mars and Jupiter. Some asteroids make their way to the inner solar system — where they (very rarely) collide with our planet. As Krupp puts it, "The Earth is still in the line of fire from this archaic material."
Where do comets and asteroids come from — and how long have we known about them?
Both asteroids and comets are debris left over from the swirling mass of matter from which the solar system arose almost 5 billion years ago. The rocky and metallic asteroids formed closer to the sun, while the icy comets formed farther out.
Because of their dramatic, highly visible tails, comets have been known since ancient times. The name comet actually comes from an ancient Greek word for "long hair," Krupp says.
Asteroids are much harder to see. Consequently, they weren't discovered until the 19th century, when telescopes became sophisticated enough to make them out. "In a telescope, an asteroid looks like a faint star, just a dot of light, and the name means 'starlike,'" says Krupp.
Historic comets and asteroids
Comets have often appeared in the course of human history, and in ancient times they were sometimes seen as portents of disaster — or conquest.
The first known depiction of a comet appears in the Bayeux Tapestry from the 11th century, which portrays the Norman Conquest of 1066. The comet was seen a few months before the decisive battle, and was later interpreted as an omen.
In 1705, English astronomer Edmond Halley determined that a comet due to appear in 1758 had already been visible from at least three times before — in 1531, 1607 and 1682 — with about 75 years between each visit. Later studies showed that Halley's Comet, as it is now known, was the same comet seen before the Norman Conquest and shown in the Bayeux Tapestry.
Halley's Comet was last seen from Earth in 1986, and is due to appear next in 2061.
Several impact craters around the world offer dramatic evidence of large asteroids that hit Earth in the distant past. The largest, almost 200 miles across, is the Vredefort Crater in South Africa. It was formed about 2 billion years ago.
The asteroid that hit the Yucatan Peninsula around 66 million years ago was tiny by comparison — less than 10 miles across. But it's thought to have caused a sudden climatic shift that wiped out the dinosaurs.
Asteroids and comets might also sow life as well as destruction: Scientists have proposed that life on Earth could have been seeded by microbes carried on comets or asteroids, from other planets or even distant star systems, by a mechanism called panspermia.