Asteroids vs. comets: How do they differ, and do they pose a threat to Earth?

From historical omens of doom to Hollywood blockbusters about saving the world, comets and asteroids loom large in fiction and folklore. And there’s a good reason: One of the chunky rocks or balls of ice could eventually slam into Earth and change the planet irreversibly. Such an impact 66 million years ago is widely believed to have killed off the dinosaurs.

Asteroids and comets formed 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. Asteroids and comets are the remnants of this process. Here’s what you need to know about how they differ, where they come from, and whether they pose a serious threat to Earth.

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.

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

(Here’s what to know about meteor showers.)

Today, most asteroids in our solar system orbit the sun in a region located between Mars and Jupiter called the asteroid belt. Many astronomers believe the belt is filled with primordial material that never glommed into a planet because of Jupiter’s gravitational pull. Others theorize that the belt might be “a cosmic refugee camp” for the remnants of planets that formed elsewhere in the solar system.

What are comets?

Comets are balls of ice and rock whose glowing tails can sometimes be seen from Earth as they streak through the night sky. Comets grow those tails whenever their orbits bring them close to the sun, causing the icy objects to heat up and expel a trail of gas and dust. The sun illuminates the tail, giving it a majestic glow.

Compared to asteroids, comets tend to have more elliptical, or oval-shaped, orbits. They also contain more chemical compounds that vaporize when heated, such as water. And when observed through a telescope, comets appear fuzzier than asteroids.

(11 spectacular night sky events to see in 2023.)

While there are perhaps trillions of comets ringing the outer fringes of the solar system, bright comets appear in Earth’s visible night sky only about once per decade. These are divided into two types: short-period comets and long-period comets.

Short-period comets take less than 200 years to orbit the sun, and many of these objects come from the Kuiper Belt, a ring of icy bodies beyond Neptune’s orbit. The most famous of these is Halley’s Comet, which appears every 75 to 76 years.

Most long-period comets are believed to come from the Oort Cloud, a theorized collection of objects surrounding the outer reaches of the solar system. According to NASAit can take thousands or even millions of years for these comets to orbit the sun. The appearance of these comets in the night sky is much more difficult to predict because there’s little record of their past appearances.

(One of the largest comets ever seen is headed our way.)

Do asteroids and comets pose a threat to Earth?

Gravitational tugs, orbital collisions, and outer space jostles occasionally perturb an asteroid or comet onto a wayward path—some careening close enough to Earth to pose a risk of impact. Most of them, fortunately, are too small to cause any damage. Instead, they burn up in the atmosphere and appear to us as shooting stars.

Astronomers are constantly on the lookout for larger bodies that could be on a more catastrophic trajectory. The asteroid that’s most likely to slam into Earth in the next 300 years is called Bennu, a rock that’s about a third of a mile wide. But the chances it will hit Earthin that time are slim—only 1-in-1,750—and the most likely dates for an impact wouldn’t be until the late 2100s or early 2200s.

Meanwhile, NASA is working on a plan to deflect killer asteroids. In 2022, the agency slammed a spacecraft into a 500-foot-wide asteroid. The test was a success, significantly knocking the asteroid off its trajectory and breaking off some of its rocky surface into a dusty debris tail. Although the technique would only work in specific situations, it’s reassuring to know that we might not go the way of the dinosaurs after all.

Editor’s note: This story was originally published on October 15, 2009. It has been updated.