What is comet? Plain-English meaning

Short definition: A comet is a small, icy object that orbits the Sun and develops a glowing atmosphere and tail when it gets close enough to the Sun.

Plain-English explanation

Comets are sometimes called 'dirty snowballs' because they are made of ice, dust, and rocky material. They spend most of their time far from the Sun, where they are cold and dark. As a comet moves closer to the Sun, the heat causes its ices to turn into gas, releasing dust and creating a glowing coma (a cloud around the nucleus) and sometimes a tail that points away from the Sun. The tail always points away from the Sun because it is pushed by solar wind and radiation pressure. Comets can have orbits that take them from the outer solar system to the inner solar system, and some appear regularly while others take thousands of years to return.

Why it matters

Comets are important because they are leftovers from the early solar system, preserving material from when the Sun and planets formed. Studying them helps scientists learn about the history of the solar system and possibly the origins of water and organic molecules on Earth. Also, comets are visible from Earth and have fascinated people for centuries, so understanding what they are helps you interpret what you see in the night sky.

Concrete example

Halley's Comet is a famous short-period comet that orbits the Sun every 76 years. It last appeared in 1986 and will return in 2061. When it approaches the Sun, its tail becomes visible from Earth, and it is one of the few comets that can be seen with the naked eye.

Common confusion

People often confuse comets with asteroids and meteors. Asteroids are mostly rocky or metallic and do not develop tails. Meteors are flashy streaks of light in the sky caused by small particles burning up in Earth's atmosphere, not the objects themselves. Comets are icy and develop tails when near the Sun.

Related terms

asteroid, meteor, solar system, orbit, astronomy

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More context

Comets are often named after their discoverers, such as Halley's Comet, which returns every 76 years. The European Space Agency's Rosetta mission orbited comet 67P/Churyumov-Gerasimenko from 2014 to 2016, providing detailed data on its surface and composition. Comets are considered pristine remnants of the early solar system, preserving volatile materials from 4.6 billion years ago.

Additional background

Comets are classified as short-period (from the Kuiper Belt, periods under 200 years) and long-period (from the Oort Cloud, periods over 200 years). As a comet approaches the Sun, it heats up and sublimates, creating a coma and tails. The solar wind pushes the ion tail directly away from the Sun, while the dust tail is pushed more slowly, causing it to curve. Comets can also break apart or disintegrate if they pass too close to the Sun or a planet. Space missions like NASA's Deep Impact and ESA's Rosetta have visited comets, revealing that their surfaces are darker than expected and contain complex organic molecules. The study of comets helps scientists understand the early solar system and the delivery of water and organic material to Earth. Historical records of comets have been kept for thousands of years, and they have often been seen as omens.