Comet

A comet is a relatively small body of ice, dust and rocky material that travels around the Sun in a long, often highly elongated orbit. When a comet approaches the inner solar system, solar heating causes its ices to turn directly into gas, releasing dust and producing the bright features that have fascinated observers for millennia: a fuzzy head and one or more tails.

For most of a comet's life it is an unremarkable, dark object far from the Sun, invisible to all but the largest telescopes. Only during the relatively brief passage near the Sun does it become active and, occasionally, spectacular. This contrast between long dormancy and dramatic outburst is central to understanding what comets are.

What a comet is made of

The solid part of a comet is called the nucleus. It is not a single chunk of uniform ice but a mixture sometimes described as a "dirty snowball" — a fragile aggregate of water ice, frozen carbon dioxide, carbon monoxide and other volatile compounds, bound together with dust and rocky grains. Nuclei are typically only a few kilometres across, and their surfaces are often darker than charcoal because radiation has altered the outermost layers.

Because the nucleus is so small and dark, it is hard to observe directly against the glare around it. Much of what astronomers know comes from the gas and dust it releases, which can be studied through the light they emit and absorb across many wavelengths.

The coma and the tails

As the nucleus warms, the released gas and dust form an envelope around it called the coma. The coma can grow far larger than the nucleus itself, sometimes to the size of a planet or larger, though it is extraordinarily thin — closer to a vacuum than any laboratory vacuum on Earth.

Sunlight and the solar wind then act on the coma to produce tails. There are effectively two types. The dust tail is made of fine particles that lag behind the comet's motion and curve gently, reflecting sunlight so that it appears whitish or yellowish. The ion tail, or plasma tail, consists of electrically charged gas pushed almost straight away from the Sun by the solar wind, and it often glows with a faint blue tint. Both tails always point roughly away from the Sun, not opposite the direction of travel, which surprises many people seeing a comet for the first time.

Where comets come from

Most comets are thought to originate in two vast reservoirs at the edges of the solar system. The Kuiper Belt is a broad ring of icy bodies beyond Neptune, and it is the source of many short-period comets that complete an orbit in under two centuries. Much farther out lies the Oort Cloud, a roughly spherical shell of icy objects that is the likely origin of long-period comets, whose orbits can take thousands to millions of years.

These distant objects are leftovers from the formation of the solar system, material that never coalesced into planets. Gravitational nudges — from passing stars, from the giant planets, or from the overall tidal field of the galaxy — can send a body inward, where it becomes an active comet. Some comets are eventually captured into shorter orbits, while others are flung out of the solar system entirely after a close approach to a planet.

Orbits and classification

Comet orbits are classified by period. Short-period comets return in less than about two hundred years and generally move in the same direction as the planets, in orbits that lie close to the plane of the solar system. Halley's Comet, famous for its roughly 76-year cycle, is the best-known example. Long-period comets have far longer cycles and can approach from almost any direction, including steeply inclined or even retrograde paths.

A comet's orbit is not perfectly stable. The jets of gas escaping from the nucleus act like small rocket thrusters and can nudge the comet slightly, gradually changing its path. Repeated passes near the Sun also erode the nucleus, so comets have finite lifetimes on human-relevant timescales; eventually a comet may break apart, fade into an inactive rocky remnant, or fall into the Sun.

Observing and studying comets

Throughout history, comets were widely regarded as omens, and their sudden appearance often provoked fear. The realization that they are natural, predictable objects was a milestone in astronomy and helped establish the idea that the same physical laws operate throughout the heavens.

Modern study uses ground-based observatories and space telescopes to measure the composition of coma gas and the nature of dust grains. Spacecraft have flown past several comets, and one mission even landed a probe on a nucleus and returned samples of its material to Earth. These samples confirmed that comets contain organic molecules and water ice, feeding a long-running discussion about how much water and prebiotic chemistry comets may have delivered to the early Earth.

Common misconceptions

Comets are often pictured as huge glowing balls streaking across the sky. In reality the solid nucleus is tiny; the visible size comes from extremely tenuous gas and dust. Another frequent mistake is assuming a comet's tail trails behind it like smoke from a moving car. Because the tails are driven by sunlight and the solar wind, they point away from the Sun, so a departing comet can actually travel tail-first. It is also worth noting that most comets are faint, and only a small minority become bright enough to be seen easily with the unaided eye.

Key points