Comets are fascinating celestial objects that have frightened and delighted stargazers throughout history.
There's a lot we don't know about these icy visitors, but here's the breakdown of what scientists have confirmed or strongly suspect about the different types of these so-called "dirty snowballs."
What defines a comet?
A comet is an icy ball of frozen gas, rock and dust that orbits the sun in an elliptical path. When closer to the sun in orbit, the comet's nucleus releases gasses, which form a coma (the comet's fuzzy, glowing halo) and a tail. So, when a comet is far away from the sun, it would not have a tail. Debris left behind from a comet's tail is what causes a meteor shower.
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Comets are thought to have formed 4.6 billion years ago, when the solar system was young and just after the planets formed. Because comets are so old, scientists believe they might hold the solutions to puzzles about the nature and evolution of our solar system.
This illustration shows comets racing toward the star Eta Corvi. One comet is depicted crashing into a rocky body, flinging ice- and carbon-rich dust into space and smashing water and organics into the surface of the planet. (Image: NASA/JPL-Caltech)
What are the different types of comets?
The classification of comets is an ongoing process. Comets can be distinguished by their orbits, which vary wildly. A comet can be either a long-period comet or a short-period comet, depending on whether its orbit is shorter than 200 years. Long-period comets are on paths that take them out past the solar system's planets before they return.
Scientists suspect that long-period comets originated in the Oort Cloud — located on the edge of our solar system — whereas short-period comets broke free from the Kuiper Belt, Pluto's home. Objects can break free from these areas when gravitational changes occur.
For example, Comet Hyakutake, seen in 1996, is a long-period comet. According to SPACE.com, "It will be a long, long time before Hyakutake makes the journey near Earth again; one NASA prediction from 1996 said it would be 14,000 years before the comet arrives again, but accounts vary due to the uncertainty of predicting the comet's trajectory."
Halley's Comet is a famous example of a short-period comet with an orbit of just 75 or 76 years. Speaking of Halley's Comet, there are thought to be two subgroups of short-period comets, Halley-type comets and Jupiter-type comets. According to Swinburne University in Australia, the difference between these two types of comets is that Halley-type comets have orbits that are "highly inclined to the ecliptic" and likely come from the Oort Cloud, whereas Jupiter-type comets are more affected by the gravity of Jupiter and originate from the Kuiper Belt. This suggests that long-period comets could become short-period comets depending on how planetary gravity shapes their orbits.
But wait, there are more kinds to know about
Single-apparition comets are thought to be comets that are not bound to the sun and can travel out of the solar system.
Sungrazing comets are often ill-fated comets that suffer from an Icarus problem. They are classified as comets that travel within 850,000 miles of the sun, and some of these comets burn up entirely. The Kreutz Group is a subgroup of sungrazers. According to NASA, "Many sungrazing comets follow a similar orbit, called the Kreutz Path, and collectively belong to a population called the Kreutz Group." NASA suspects that the comets currently on the Kreutz Path originated from a single comet that broke up long ago.
Dead comets, such as the recent and incorrectly named "Spooky" asteroid, are comets whose gasses have burned up. They have no tails.
Exocomets are comets that exist outside of our solar system. According to SPACE.com, scientist have identified several of these orbiting the star Beta Pictoris.
How many comets are out there?
The short answer is a whole bunch. The vast majority of them have never been seen from Earth. According to the ESA, "There are thought to be so many comets that even astronomers can’t count them all … "
While a comet might be a rare sight in the skies, they're overall quite well represented in space. Imagine the tales these icy long-tailed objects could tell, as they high-tail it through space.