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Deep Field
29 September 2026 2518 stories tracked

Glossary · Comets & Asteroids

Coma and tail

A comet's atmosphere, and the two separate tails the Sun pushes off it.

As a comet approaches the Sun its ices turn straight to gas, forming a coma that can be larger than a planet around a nucleus a few kilometres across. Two tails follow: a dust tail curving gently along the orbit, and an ion tail of charged gas pushed dead straight away from the Sun by the solar wind. The tail always points away from the Sun, not backwards along the path, so a departing comet flies tail-first — which is the detail most illustrations get wrong.

Related terms

Near-Earth object An asteroid or comet whose orbit brings it inside 1.3 astronomical units of the Sun. Asteroid and comet Rock that stays rock, and ice that grows a tail when it gets warm. Oort cloud A shell of icy bodies at the edge of the Sun's influence, inferred rather than observed. Kuiper belt The flat ring of icy bodies beyond Neptune, where Pluto lives and short-period comets come from.

Questions

What is Coma and tail?

A comet's atmosphere, and the two separate tails the Sun pushes off it.

Why does it matter?

Two tails follow: a dust tail curving gently along the orbit, and an ion tail of charged gas pushed dead straight away from the Sun by the solar wind. The tail always points away from the Sun, not backwards along the path, so a departing comet flies tail-first — which is the detail most illustrations get wrong.

Written and computed by . Corrections and errors to hello@cantinalabs.dev.