Comets are a mix of gases, rocks and dust that orbit the sun. They are often called dirty snowballs (or snowy dirtballs by some astronomers) because they contain ice and liquid water along with dust and rocky material.
According to astronomers, comets formed around 4.6 billion years ago when the solar system was born. They contain residue dust and gases that didn’t become gravitationally pulled into other objects, such as asteroids and planets and moons.
Comets are usually not visible to our eyes unless they are near the sun. Most comets orbit the sun in very elongated or haphazard paths. Some of the orbits go far beyond Pluto, and the comets are only visible once in several millennia as they swing around the sun and therefore are also close enough to Earth to be visible in the night sky.
Some interesting facts about comets:
- A comet has a nucleus (made of solid ice, gas and dust), a coma (water vapor, CO2, and other gases) and a tail (dust and ionized gases). The coma and tail form only when a comet is near the sun, whose heat activates the volatile materials of the nucleus.
- When passing near the sun, comets can leave a trail of dust behind that creates stunning meteor showers years and even centuries later. The Perseid meteor shower occurs annually when Earth passes through the orbit of comet Swift-Tuttle, for example.
- In history, comets have been recorded as omens of doom. Roman Emperor Nero believed the arrival of a comet foretold his assassination and had every one of his successors killed.
- Halley’s Comet streaks by Earth every 76 years, and astronomers studied it up close when it last appeared in 1996. The comet had a potato-shaped nucleus, some 9 miles long, that contained roughly equal parts ice and dust.
Halley’s comet is classified as short-period, meaning it completes its orbit in less than 200 years. Comets can go extinct by losing all of their volatile materials, leaving behind just the rocky stuff. Some astronomers say comets brought water and even organic life to Earth.