Comets are visitors to our skies – their homes are in 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 far away in the Kuiper Belt or the Oort Cloud. After it goes around the Sun, its orbit takes it back to its distant home. The Kuiper Belt is a large circle of small icy bodies beyond 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 major disaster.
We can see five planets without a telescope and they've been well known for thousands of years. The movements of the Sun, Moon and stars are fairly predictable. Comets were intruders into the familiar sky and that made them scary.
Comets are small and dark and often called “dirty snowballs”.
Even the biggest comets seem to be no more than a few tens of kilometers (less than twenty miles) across. Comets are sometimes called dirty snowballs because they're made of a mixture of rock, dust, ice and frozen gases. They have a dry crust a few meters thick, and the frozen liquids and gases are underneath it. The surface is dark, blacker than coal. We call this frozen comet a nucleus.
Space probes have visited some comets.
Space probes make it possible to study comets close up. The first probe to see a comet nucleus was the European Space Agency's (ESA) Giotto in 1986. It studied Comet Halley. Since comets tend to be quite reflective, Giotto surprised the scientists by showing them how dark the nucleus was.
A comet gets an atmosphere – and maybe a tail – when it's getting near the Sun.
When a 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 around the nucleus. The atmosphere is called a coma and it can spread out for tens of thousands of kilometers. Material thrown out from the comet can form a tail, but not all comets have tails. Dust is good at reflecting sunlight, so if the comet gets a dust tail, we see the bright curved tail that people associate with comets. But a comet can have a faint blue tail called an ion tail. Comet Hale-Bopp (1995) had both a dust tail and an ion tail.
Sungrazing comets may 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 big that the comet is pulled into the Sun or pulled apart. If a large comet is broken up, the fragments 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.
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 had visited before. The first person ever to do this was Edmond Halley. Short-period comets take less than 200 years to return. Long-period comets take longer than 200 years to complete an orbit.
There are exocomets in other star systems.
Astronomers have discovered hundreds of planets around other stars. We call them exoplanets or extrasolar planets. They have also found some likely exocomets.
My Pinterest board "Comets" has pictures that relate to this article.