Comet

A comet is a small, icy body from within our Solar System or beyond it. When it nears the Sun, it heats up and releases gas—a process known as outgassing. This creates a large, loose atmosphere called a coma around the solid core, and sometimes a tail of gas and dust streams away from the coma. These effects are caused by solar radiation and the solar wind hitting the comet’s nucleus. The nucleus itself ranges from a few hundred meters to tens of kilometers across, made of a mix of ice, dust, and small rocky particles. The coma can grow to 15 times the width of Earth, while the tail may extend beyond one astronomical unit. If a comet is bright and close enough, it can be seen from Earth without a telescope and may span up to 30° (60 full moons) across the sky. People have watched and recorded comets for thousands of years across many cultures and religions.

Most comets follow highly stretched, elliptical orbits with periods that vary from a few years to millions of years. Short-period comets come from the Kuiper belt or the scattered disc beyond Neptune. Long-period comets likely originate in the Oort cloud, a spherical shell of icy bodies that stretches from outside the Kuiper belt to halfway to the nearest star. They are nudged toward the Sun by the gravity of passing stars and the galactic tide. Hyperbolic comets may pass through the inner Solar System just once before being flung into interstellar space. Each appearance of a comet is called an apparition.

Comets are different from asteroids. Comets contain volatile materials that sublimate near the Sun, forming their characteristic tails, while asteroids are more like rocks and lack tails. Asteroids are thought to have formed inside Jupiter’s orbit, not in the outer Solar System. However, the discovery of main-belt comets and active centaur minor planets has blurred this line. In the early 2000s, some small bodies with long-period comet orbits but asteroid-like traits were named Manx comets. They are still classified as comets, for example C/2014 S3 (PANSTARRS). Between 2013 and 2017, 27 Manx comets were found.

As of November 2021, 4,584 comets are known. This is a tiny fraction of the total population, since the Oort cloud alone may hold about one trillion comet-like bodies. Roughly one comet per year is visible to the naked eye, though most are faint. Exceptionally bright ones are called “great comets.” Uncrewed spacecraft have visited comets: NASA’s Deep Impact blasted a crater on Comet Tempel 1 to study its interior, and the European Space Agency’s Rosetta made the first landing of a robotic probe on a comet.

The word “comet” comes from Old English *cometa*, from Latin *comēta* or *comētēs*, which is a romanization of Greek *κομήτης* meaning “wearing long hair.” The Oxford English Dictionary notes that the Greek term (ἀστὴρ) κομήτης already meant “long-haired star, comet.” *Κομήτης* came from *κομᾶν* (“to wear the hair long”), from *κόμη* (“hair of the head”), which was also used for a comet’s tail. The astronomical symbol for a comet (U+2604 ☄) shows a small disc with three hairlike lines.

The solid core of a comet is the nucleus. It is made of rock, dust, water ice, and frozen carbon dioxide, carbon monoxide, methane, and ammonia. This led to the popular “dirty snowball” model by Fred Whipple. But after NASA’s Deep Impact mission hit Comet Tempel 1 in 2005, “icy dirtball” may be more accurate. Research in 2014 suggested comets are like “deep fried ice cream”: their surfaces are dense crystalline ice mixed with organic compounds, while the interior is colder and less dense. The nucleus surface is generally dry, dusty, or rocky, meaning the ices lie beneath a crust several meters thick. Nuclei contain organic compounds such as methanol, hydrogen cyanide, formaldehyde, ethanol, ethane, and possibly longer-chain hydrocarbons and amino acids. In 2009, the amino acid glycine was confirmed in comet dust from NASA’s Stardust mission. In August 2011, a NASA study of meteorites on Earth suggested that DNA and RNA components (adenine, guanine, and related molecules) may have formed on asteroids and comets.

Cometary nuclei have very low albedos, making them among the least reflective objects in the Solar System. The Giotto probe found Halley’s Comet reflects about 4% of light; Deep Space 1 found Comet Borrelly reflects less than 3%. Asphalt, for comparison, reflects 7%. The dark surface may consist of complex organic compounds. Solar heating drives off lighter volatiles, leaving behind darker, tar-like material. This low reflectivity helps the nucleus absorb heat that drives outgassing. Long-period comets tend to be larger than short-period ones, usually 4–20 km in diameter.