The parts of a comet
A comet has a few distinct parts:
- Nucleus. The solid core, usually a few kilometers across. It is a loose mix of ices (mostly water, with carbon dioxide, carbon monoxide and others), dust and rock. It is often described as a dirty snowball or an icy dirtball. Comet nuclei are very dark, reflecting only a few percent of the sunlight that hits them.
- Coma. When the comet nears the Sun, heat turns ice directly into gas. The gas carries dust with it and forms a huge, fuzzy cloud around the nucleus, which can be larger than a planet.
- Dust tail. Sunlight pushes dust away from the coma. The dust tail is usually broad, curved and yellowish-white, because it reflects sunlight.
- Ion tail. Gas in the coma is ionized by ultraviolet sunlight and swept away by the solar wind. This tail is straight, narrow and often bluish.
Both tails point roughly away from the Sun, not behind the comet’s direction of travel. When a comet is moving away from the Sun, its tails actually lead the way.
Where comets come from
Comets formed in the cold outer parts of the young solar system, where ices could survive. Today they come mainly from two regions:
- The Kuiper Belt and scattered disk, beyond Neptune. These are the main source of short-period comets, which take less than 200 years to orbit the Sun. Many of these orbit in roughly the same plane as the planets.
- The Oort Cloud, a vast, roughly spherical shell of icy bodies far beyond the planets. It is thought to supply long-period comets, which can take thousands or even millions of years to complete one orbit and may arrive from any direction.
A passing star or the pull of the galaxy can nudge a distant comet into an orbit that brings it toward the Sun.
Famous comets
Halley’s Comet (1P/Halley) is the best-known short-period comet. It returns about every 76 years, last came close in 1986 and is expected back in 2061. Edmond Halley showed that comets seen in 1531, 1607 and 1682 were the same object and predicted its return.
Comet Hale–Bopp was visible to the naked eye for many months in 1996 and 1997 and was one of the most widely seen comets of the 20th century.
Comets and meteor showers
Each time a comet passes near the Sun, it leaves a trail of dust and small grains along its orbit. If Earth’s orbit crosses that trail, we see a meteor shower on about the same dates every year. The particles that hit our atmosphere create meteors.
Many well-known showers have a comet parent:
| Shower | Parent comet |
|---|---|
| Perseids | 109P/Swift–Tuttle |
| Orionids and Eta Aquariids | 1P/Halley |
| Leonids | 55P/Tempel–Tuttle |
| Draconids | 21P/Giacobini–Zinner |
| Ursids | 8P/Tuttle |
How comets are named
Modern comet names combine a code and the discoverer’s name. A prefix shows the type: P/ for a periodic comet (period under 200 years, or seen at more than one return) and C/ for a non-periodic comet. Periodic comets with confirmed returns get a number, as in 1P/Halley. Non-periodic comets get the year and a code for when in the year they were discovered, as in C/1995 O1 Hale–Bopp.
How to see a comet
When a bright comet is announced, look for it near the horizon after sunset or before sunrise, since comets are brightest near the Sun. Use binoculars to find it, then step back to enjoy the tail. Dark skies and a moonless night help a lot; check the moon phases to plan.