Asteroids And Comets: Information And Facts | National Geographic

Home » Where Do Asteroids Come From » Asteroids And Comets: Information And Facts | National Geographic

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Both are remnants of a process that began some 4.6 billion years ago after a giant cloud of gas and dust collapsed and condensed to create the sun. The leftover debris orbiting the sun coalesced into planets, moons, and other objects such as steroids and comets. Here’s what you need to know about how they differ, where they come from, and whether they pose a serious threat to Earth.

(Our solar system has a new mysterious visitor—what is it?)

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What are asteroids and the asteroid belt?

Asteroids are essentially chunks of rock that measure in size from a few feet to hundreds of miles in diameter. NASA has identified more than a million asteroids, and more than 150 of them have their own moons. In 2022, astronomers published evidence that the asteroid Elektra has as many as three chunks of rock orbiting it—making it the first known quadruple asteroid.

For many years, astronomers considered Ceres to be the largest asteroid at about 590 miles wide. In 2006, however, the International Astronomical Union reclassified Ceres as a dwarf planet—making Vesta the largest asteroid at 329 miles wide. Yet this remains a point of contention among some astronomers, and the U.S. Geological Survey’s Gazetteer of Planetary Nomenclature defines Ceres as both an asteroid and a dwarf planet.

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Small asteroids are called meteoroids. If they enter Earth’s atmosphere, they are called meteors, or shooting stars—and if a meteor hits the ground, it becomes a meteorite.

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