Structure of a comet

Comets are icy objects that formed together with the planets before being flung to the outer reaches of the Sun’s realm. When comets return to the inner Solar System, the Sun’s heat and radiation brings them to life. They spew out gases and dust, enveloping the solid nucleus in a hazy coma.

Material from the coma gets swept into the comet’s tails, which get larger as the comet moves closer to the Sun. Most comets have two tails, which can extend out of the coma for many millions of kilometres. The dust tail is made of tiny particles, typically around the size of smoke particles, which tend to light up whitish yellow in the Sun’s light. The plasma tail — sometimes called the ion tail — is made up of electrically charged gas. It typically emits a blue glow thanks to the presence of carbon monoxide ions. Comet tails point away from the Sun, aligning with the solar wind, with the dust tail curving toward the comet’s path.

Surrounding the coma and part of the tails is an invisible, irregular cloud called the hydrogen envelope. The envelope is made up of neutral hydrogen atoms which most likely come from water vapour molecules that are split up by radiation from the Sun. It is only visible in ultraviolet light, which cannot pass through Earth’s atmosphere.

The right-hand side of the image shows photographs of three real comets streaking across the dark sky.