By Sabrina Stierwalt, Ph.D., Quick and Dirty Tips

Adding up all of the mass in every asteroid in our entire solar system totals only less than the mass of our moon. Despite their small physical size, however, these space rocks offer important clues as to how our solar system formed. The terms asteroid, meteor, meteorite, and even comet are often used interchangeably ... but what is the difference?

What is an asteroid?

Asteroids are rocky objects smaller than planets that are left over from the formation of our solar system. When the cloud of gas and dust collapsed to form our sun, much of the remaining material went into forming the rocky terrestrial and gas giant planets orbiting our star. Smaller dust fragments that never made their way into planets are left behind as asteroids.

Of the millions of known asteroids, the largest is Ceres, 584 miles (940 kilometers) wide, although Ceres has been recently reclassified as a dwarf planet. Luckily we do not expect to cross paths with this Texas-sized solar system body any time soon. NASA tracks a subset of asteroids, called "near Earth objects" or NEOs, whose trajectories have been nudged by the gravitational push and pull of nearby planets enough so that they may pass close to Earth.

Thanks to infrared surveys of the sky like NASA’s WISE and NEOWISE missions, we know of roughly 1000 near-Earth asteroids that are larger than 0.6 miles across (or 1000 meters) and 1500 more that are between a third of a mile and 0.6 miles across (from 500 to 1000 meters). Smaller near-Earth asteroids, both known to exist and predicted based on statistical analysis, number in the 18,000s.

Most are not round like planets but rather irregular in shape, sometimes due to repeated impacts over time. They are also known to orbit each other, making their way around the sun in pairs or small groups. They are not large enough to hold onto their own atmospheres and their compositions vary, mostly due to the location where they were formed, in particular how far away they were from the sun when they originated.

Most asteroids reside in the asteroid belt, the space between the orbits of Mars and Jupiter, most likely because the gravitational pull of Jupiter prevented them from accumulating into a larger planetary system. Some asteroids are also found in the orbital paths of planets like Earth. Until recently all known asteroids orbited our sun as members of our solar system, but that changed in October 2017 when astronomers discovered the first interstellar visitor just passing through our solar neighborhood. Named 'Oumuamua, which comes from the Hawaiian word for “scout,” the asteroid has an unusual elongated shape (800 by 100 feet in size) and is moving too fast to be captured by our sun’s gravitational pull. That means 'Oumuamua will eventually leave us and continue on its journey through interstellar space.

What is a comet?

Comets are also composed of material left over from the formation of our solar system and formed around the same time as asteroids. However, asteroids formed toward the inner regions of our solar system where temperatures were hotter and thus only rock or metal could remain solid without melting. Comets formed at farther distances from the Sun, beyond what we call the frost or snow line and past the orbits of Mars and Jupiter, where temperatures were low enough for water to freeze.

Comets are thus chunks of frozen gas, rock, ice, and dust that orbit our sun, earning them the nickname of dirty snowballs. They are identified by their tails which consist of trailing jets of gas and dust that has been melted off as a comet approaches too close to the sun.

What is a meteor and a meteorite?

A meteor is simply an asteroid that attempts to land on Earth but is vaporized by the Earth’s atmosphere. The resistance on the rock due to the Earth’s atmosphere causes its temperature to rise. We sometimes see the glowing hot air created by these burning meteors and dub them “shooting stars.” Meteor showers occur when the Earth passes by many meteors at once. For example, if chunks of a comet melt off as it passes close to the sun, this debris can be left behind to later dazzle us Earthlings with a meteor shower.

Meteorites are meteors that survive the dive through the Earth’s atmosphere and manage to land on the surface of our planet. They are typically composed of either iron or stone, i.e. a mix of oxygen, silicon, magnesium, iron, and other elements.

Studying asteroids helps us understand the formation of our solar system and how our planet came to be. We don’t just have to wait for meteorites to find us to know more about their composition, however. The OSIRIS-Rex mission to the asteroid Bennu aims to take samples from the asteroid and bring them back to Earth. You can learn more about why Bennu was chosen for such a special mission on the OSIRIS-Rex mission page.

A version of this article was originally published on Quick and Dirty Tips as Asteroid, Meteor, Meteorite, and Comet: What's the Difference?
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About the author

Dr. Sabrina Stierwalt earned a Ph.D. in astronomy and astrophysics from Cornell University and is now a professor of physics at Occidental College.