asteroid
Americannoun
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Astronomy. any of millions of small celestial objects revolving around the sun, often irregularly shaped and having a great range in size, from as small as 6 feet (2 meters) across to about 620 miles (998 kilometers) across: the vast majority of known asteroids exist within the asteroid belt.
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Zoology. a starfish; an asteroidean.
adjective
noun
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Also called: minor planet. planetoid. any of numerous small celestial bodies that move around the sun mainly between the orbits of Mars and Jupiter. Their diameters range from 930 kilometres (Ceres) to less than one kilometre
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Also called: asteroidean. any echinoderm of the class Asteroidea; a starfish
adjective
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of, relating to, or belonging to the class Asteroidea
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shaped like a star
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Any of numerous small solar system bodies that orbit the Sun primarily in the asteroid belt, a region between the orbits of Mars and Jupiter. Asteroids are intermediate in size between planets and meteoroids with diameters that measure between approximately one hundred and several hundred kilometers. While more than 1,800 asteroids have been cataloged, and as many as a million or more smaller ones may exist, their total mass has been estimated to be less than three percent of the Moon's. Asteroids are thought to be left over from the early formation of the solar system, when planetesimals in a protoplanetary disk were scattered after coming under Jupiter's gravitational influence. The continuing collision of planetesimals that remained between Jupiter and Mars caused many of them to fragment, creating the asteroids that exist today.
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Also called minor planet planetoid
Other Word Forms
Derived Forms
Inflected Forms
Nouns
Etymology
Origin of asteroid
First recorded in 1795–1805; from Greek asteroeidḗs “starry, starlike”; see star, -oid
Explanation
An asteroid is a large, irregularly shaped object in space that orbits our Sun. If one of these giant rocks ends up on a collision course with Earth, we are in for big trouble. An asteroid is like a comet. However, while comets are mostly made of ice, asteroids are made up of rock or even metal. This makes them dangerous because they can cause a lot of damage if they collide with a planet. The craters on the Moon were formed by asteroids and some think it was an asteroid that wiped out the dinosaurs. Around a million asteroids are located between Mars and Jupiter in an area called the “asteroid belt.”
Vocabulary lists containing asteroid
Over The Moon: Planetary Vocabulary
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Oh, My Stars! A Solstice Sampler
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Elements of the Universe: Aster, Astro ("Star")
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Example Sentences
Examples are provided to illustrate real-world usage of words in context. Any opinions expressed do not reflect the views of Dictionary.com.
See Examples For:
This prehistoric creature has survived ice ages, the asteroid impact that killed the dinosaurs, and continental drift.
From BBC ● Jul. 27, 2026
"Covalently bonded carbon and hydrogen in comet and asteroid material are believed to have formed in the outer envelopes of stars, in high-energy events like supernovae, and in interstellar environments," Ms. Losurdo said.
From Science Daily ● Jul. 19, 2026
It may have come from a distant region of the outer Solar System filled with rocky debris, or from the outer part of the asteroid belt near Jupiter.
From Science Daily ● Jul. 19, 2026
The asteroid strike that created it is widely associated with the extinction event that wiped out all nonavian dinosaurs and many other species.
From Science Daily ● Jul. 18, 2026
“See the stars? And that asteroid field? And that lonely satellite going by? There’s no gravity. You’re weightless. You can do anything you want with your body. You just have to think it.”
From "Everything, Everything" by Nicola Yoon
Both asteroids contain magnesium-rich clays, suggesting they experienced much longer periods of water exposure, possibly lasting millions of years while they were still parts of larger parent bodies.
From Science Daily ● Jun. 25, 2026
Even relatively small asteroids can have surprisingly eventful histories.
From Science Daily ● Jun. 25, 2026
Named after the famous fossilized human ancestor discovered in Ethiopia in 1974, Lucy is on its way to become the first mission to explore Jupiter's Trojan asteroids.
From Science Daily ● Jun. 25, 2026
Those differences may indicate that the parent asteroids formed at different times or in different regions of the solar system before eventually moving into the main asteroid belt.
From Science Daily ● Jun. 25, 2026
They spent one week each month at the Palomar Observatory in California looking for objects, asteroids primarily, whose trajectories carried them across Earth’s orbit.
From "A Short History of Nearly Everything" by Bill Bryson
Definitions and idiom definitions from Dictionary.com Unabridged, based on the Random House Unabridged Dictionary, © Random House, Inc. 2023
Idioms from The American Heritage® Idioms Dictionary copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.
