Asteroids, Meteorites & Comets: Definitions and Characteristics

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  • 0:07 Space Debris
  • 0:52 Comet
  • 2:02 Asteroids
  • 3:08 Meteorites
  • 3:46 Impact with Earth
  • 4:38 Lesson Summary
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Lesson Transcript
Instructor: Amy Meyers

Amy holds a Master of Science. She has taught science at the high school and college levels.

This lesson will cover the definitions and characteristics of asteroids, comets and meteorites. It will also explore what impact they have had on Earth and the impact they might have in the future.

Space Debris

The bright trail following comets is made up of gas and dust.
Comet Hale Bopp

Argh! A giant asteroid, as big as the one that killed the dinosaurs, is headed towards Earth. It's going to hit us in only 18 days, and NASA is unprepared. Earth and every living thing on it could be destroyed forever!

Okay, okay. You're right. That's the plot to the movie Armageddon, starring Bruce Willis. But could it really happen? Could an asteroid hit Earth and cause the destruction of humankind?

Space is filled with rocks and other matter left over from the Big Bang. None of this debris, whether it is asteroids, comets or meteors, is currently on a collision course with Earth - but we do know stuff has hit Earth in the past and will again in the future. Let's look at what each of these things are, their characteristics and any impact they've had on Earth or might have in the future.


A comet is an icy ball of rock that grows a tail when it travels close to the Sun. It grows a tail because as comets heat up, gas and dust are forced from the body and trail behind it. The Sun illuminates this trail, which causes it to glow. The glow is visible in the night sky, and what we on Earth see is a streak.

The way a comet orbits - and the length of its orbit - varies greatly from one comet to another. They can have a short period and orbit every few years, or a very long period and orbit every 100,000 years. Most of the shorter-period comets come from the Kuiper belt, a massive region of small objects (including Pluto) out beyond Neptune. The longer-orbit comets come from the Oort cloud, a massive cloud of icy planetesimals very far out in the outer part of the solar system.

Asteroids range in size from a few feet to several miles.

Every year, Earth sees a few comets. Some of these are comets we expect because they regularly orbit the Sun. The others are new comets that have never been seen before.

The best-known comet is Halley's Comet. It last appeared in 1986 and will appear again in 2061. Halley's Comet has been seen every 76 years since 240 BC.


Asteroids are small objects that orbit the sun. They come in sizes from just a few feet across to several miles. Some of the largest asteroids are so big they've been called planetoids. The largest asteroid known so far is named Ceres and it is 950 kilometers wide.

There are millions of asteroids in the universe. The majority of them live in the asteroid belt between Mars and Jupiter. Most believe asteroids are leftover pieces of matter from the formation of our solar system that are kept in the belt area due to Jupiter's gravitation pull. Others think the asteroid belt is a planet that was broken apart during a collision. There are other small groups of asteroids in the solar system, too; one of them is near Earth.

Asteroids need to be very big and close enough to Earth to be seen with binoculars or a small telescope. Since Earth was first formed, asteroids have slammed into it. Fortunately for us, though, dangerous asteroids, like the one that probably killed off the dinosaurs, are extremely rare.

When an asteroid, or a part of it, crashes into Earth, it's called a meteorite.

An estimated 500 meteorites reach the surface of Earth annually.

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