Comets: Ice, Dust and Long Tails
Frozen travelers from the outer Solar System that grow glowing tails when the Sun warms them.
A comet is a small body of ice, dust and rock. For most of its orbit it is a dark, quiet nucleus, often only a few kilometers across. When it moves closer to the Sun, ice turns to gas and sets dust free. That forms a cloud called the coma and, often, two tails. One tail is made of electrically charged gas and points straight away from the Sun. The other is made of dust and curves gently along the orbit. Both point away from the Sun, not behind the comet, whichever way it is traveling.
Comets come from two cold reservoirs. Short-period comets, which return within about 200 years, mostly come from the Kuiper Belt beyond Neptune. Long-period comets arrive from the Oort Cloud, a far larger and much more distant shell that no spacecraft has seen directly. Halley's Comet is the best-known periodic visitor. It returns about every 76 years, and its next visit is expected in 2061.
The guides in this category explain how comets are built, why tails behave as they do, how comets differ from asteroids and which ones have earned the name great comet. They also cover Rosetta, the European Space Agency mission that orbited Comet 67P and put a lander on its surface.
A practical guide explains how to see a comet with your eyes or binoculars. One rule comes first and applies everywhere on this site: never point binoculars or a telescope at the Sun without a solar filter made for that purpose. Comets that pass very close to the Sun are best left to spacecraft.
Not sure where to begin? Read what a comet is, then how comet tails work.
The nine guides
What Is a Comet? Anatomy of a Dirty Snowball
The parts of a comet, from the frozen nucleus to the coma and two tails, and how close-up visits changed the old snowball picture.
Comet Tails Explained: Ion Tail, Dust Tail and Anti-Tail
Why a comet grows two tails of different color and shape, why both point away from the Sun, and how a tail can seem to point the wrong way.
Comet vs. Asteroid: How to Tell Them Apart
How composition, orbit and behavior separate comets from asteroids, and why active asteroids, main-belt comets and dark comets keep the line fuzzy.
The Kuiper Belt: Home of Pluto and Short-Period Comets
Where the Kuiper Belt sits, what lives in it, how it feeds short-period comets, and what New Horizons found at Pluto and the snowman-shaped Arrokoth.
The Oort Cloud: The Solar System's Distant Comet Reservoir
What the Oort Cloud is, how far it reaches, why astronomers infer it from comet orbits instead of seeing it, and how a comet gets nudged toward the Sun.
Halley's Comet: Facts, History and the Next Return
Facts about Halley's Comet: its 76-year orbit, sightings back to 240 BCE, the Giotto flyby of 1986, the May and October meteor showers, and the 2061 return.
Great Comets in History: Hale-Bopp, Hyakutake and NEOWISE
What makes a comet "great", and the stories of Hyakutake in 1996, Hale-Bopp in 1997, NEOWISE in 2020 and the uncertain forecast for Tsuchinshan-ATLAS in 2024.
How to See a Comet With Your Eyes or Binoculars
How to check forecasts, time your night, choose binoculars, read a sky chart and know what a comet looks like at the eyepiece, from NASA skywatching advice.
Rosetta and Comet 67P: The First Comet Landing
How Rosetta reached Comet 67P, what happened to the Philae lander, and what the mission learned about the comet's two-lobed shape, porous body, water and gases.