Comets

A comet is a dirty snowball a few kilometres across, about the size of a town.

For most of its life it is far from the Sun, frozen and dark, and far too small to see.

Then its long orbit carries it inwards. As it warms, the ice turns straight to gas and bursts out, carrying dust with it. A glowing cloud grows around the comet, and sunlight sweeps the gas and dust back into a tail.

The tail can be longer than the distance from the Earth to the Sun. All of it comes from a lump you could walk around in a day.

A comet’s tail does not trail behind it like smoke from a train. It always points away from the Sun, whichever way the comet is going.

The parts of a comet.

Part	What it is	Size
Nucleus	The solid body: ice, dust and rock, loosely packed	Usually a few kilometres
Coma	A thin cloud of gas and dust around the nucleus	Up to hundreds of thousands of kilometres
Dust tail	Dust pushed back by sunlight; broad, curved and pale yellow	Millions of kilometres
Gas tail	Electrically charged gas carried off by the solar wind; narrow, straight and blue	Up to hundreds of millions of kilometres

The nucleus is the only solid part, and it is small and very dark. The European spacecraft Rosetta spent two years, from 2014 to 2016, alongside comet 67P/Churyumov-Gerasimenko. It found a body about 4 km long shaped like two lumps stuck together, blacker than coal, and so loosely built that it would float on water. Rosetta’s small lander, Philae, made the first landing on a comet in November 2014.

Switching on. Far from the Sun a comet is just a nucleus. Somewhere inside the orbit of Jupiter its ices begin to turn to gas. They do not melt first, because there is no air pressure to hold a liquid. The gas escapes in jets, drags dust out with it, and forms the coma.

Two tails. Sunlight itself gives a very slight push, enough to drive the dust outwards into a wide, curving tail. The gas is treated differently. Ultraviolet light strips electrons from it, and the solar wind, the stream of charged particles flowing out from the Sun, catches hold of the charged gas and carries it straight back. So a bright comet has two tails that fan apart. Both point away from the Sun. After a comet has passed the Sun and is heading out again, it travels tail first.

Where they come from. Comets come in two families, and each points back to a different store of icy bodies.

Short-period comets go round in less than 200 years, mostly close to the plane of the planets. They come from the region of the Kuiper Belt, just beyond Neptune. Halley’s Comet is the famous one: it returns about every 76 years, was last near the Sun in 1986, and is due back in 2061.

Long-period comets take thousands or millions of years and arrive from any direction, above or below the planets as often as level with them. That is the main evidence for the Oort Cloud, the distant shell of icy bodies that nobody has seen directly.

They do not last. Each time round, a comet loses a layer of itself, perhaps a metre or so deep. After some hundreds or thousands of passes the ice near the surface is gone, and what remains is a dark, dead lump or a trail of rubble. The dust a comet leaves along its orbit is what makes meteor showers, which the next lesson covers.

Visitors from elsewhere. Three objects have been caught passing through the Solar System too fast for the Sun to hold: ’Oumuamua in 2017, comet Borisov in 2019 and comet ATLAS in 2025. They formed around other stars.