Beautiful green Comet Lovejoy discovered by Terry Lovejoy in August 2015

Comets are visitors to our skies – many of them come from the most distant parts of the outer Solar System.

Comets are small Solar System bodies that orbit the Sun. But a comet's orbit isn't like a planet's orbit. A comet comes close to the Sun from a long distance, usually in the Kuiper Belt or the Oort Cloud. After it goes around the Sun, it returns to its distant home.

The Kuiper Belt is a wide ring of small icy bodies past the orbit of Neptune. The Oort Cloud surrounds the whole Solar System at over a trillion miles from the Sun.

When a comet mysteriously appeared in the sky, people used to believe it was a sign of a great disaster.

You can see five planets without a telescope, and since ancient times people have been able to predict motions of the Sun, Moon and stars. But comets were intruders in the familiar sky and that was frightening.

Comets are small and dark and sometimes called “dirty snowballs”.

Even the biggest comets seem to be no more than a few tens of kilometers across – that's less than twenty miles. Comets are sometimes called dirty snowballs because they're a mixture of rock, dust, ice and frozen gases. Their dry crust is a few meters thick, and underneath it are frozen liquids and gases. The surface is dark, blacker than coal. The frozen comet is a nucleus.

Space probes have visited some comets.

In 1986 the European Space Agency's (ESA) Giotto spacecraft got close enough to Comet Halley to take the first picture of a comet nucleus. When we see comets they are bright, so it was a surprise to find out how dark the nucleus was. The most recent visit to a comet was in 2014 when ESA's Rosetta spacecraft met comet 67P/Churyumov-Gerasimenko out beyond Mars. Rosetta traveled with the comet into the inner Solar System and around the Sun.

A comet only has an atmosphere when it gets near the Sun.

When the dark comet nucleus gets into the inner Solar System, it warms up enough to vaporize some of its ice and gases. These come shooting out, along with some dust, to make a thin atmosphere. The atmosphere is called a coma, and it can spread out for tens of thousands of kilometers.

There are lots of comets, but most of them are only visible with binoculars or a telescope.

Sometimes we're lucky enough to have a very bright comet that anyone can look up and see in the night sky. The brilliant comet Hale-Bopp was visible in 1996-7 and probably seen by more people than any other comet in history.

People usually think of a comet as a bright body with a curved tail, but that doesn't always happen.

Some comets don't have tails. Others have a dust tail. Since dust reflects sunlight, the dust tail is the bright curved tail. But a comet can also have two tails. The second one is a faint blue tail called an ion tail. Comet Hale-Bopp (1996) had two tails. [Image: E. Kolmhofer, H. Raab; Johannes-Kepler-Observatory, Linz, Austria]

Sungrazing comets can get so close to the Sun that they break apart.

A sungrazer is a comet that comes very close to the Sun. Sometimes the forces on a sungrazer are so strong that the comet is pulled into the Sun or pulled apart. If a large comet breaks up, the pieces may become smaller comets.

Comets are usually named for their discoverers.

Before the twentieth century there were several ways of naming comets. Since then, comets have usually been named after the people who discovered them. For example, Alan Hale and Thomas Bopp were the independent discoverers of Comet Hale-Bopp. Rosetta's comet was discovered by Ukrainian astronomers Klim Churyumov and Svetlana Gerasimenko.

Some comets make regular visits to our neighborhood.

A comet that returns regularly is periodic. A periodic comet is named for the person who worked out the comet's orbit to show that it keeps coming back. The first person ever to do this was Edmond Halley.

There are exocomets in other star systems.

Astronomers have discovered thousands of planets around other stars. We call them exoplanets. They have also found some likely exocomets.