Comets are lumps of ice, dust, and rock that orbit the Sun. The typical comet has a core that is a few kilometers in diameter. Comets are often called the "dirty snowballs" of the Solar System.
Coma and Tail
As a comet nears the Sun its ices will begin to heat up and turn into gases and plasma. These gases form a large glowing "head" around the comet that is called a "coma". As the comet speeds through space, the gases will trail behind the comet forming a tail.
Because of their coma and tail, comets appear fuzzy as they near the Sun. This allows astronomers to easily determine comets from other space objects. Some comets can be seen with the naked eye as they pass by Earth.
The comet Hale-Bopp with coma and tail. Source: NASA.
Orbit of a Comet
Comets are usually divided into two groups determined by the type of orbit they have.
The first type of comet is the short period comet. Short period comets have orbits of less than two hundred years. Some have very short orbits of just a few years. These types of comets originate from the Kuiper belt.
The second type of comet is the long period comet. Long period comets have orbits of greater than two hundred years. Some long period comets have orbits of thousands of years. Scientists think that long period comets come from the Oort cloud.
The Kuiper Belt
The Kuiper belt is a region of the Solar System that lies beyond the planets and the orbit of Neptune. It is much larger than the asteroid belt. The Kuiper belt contains millions of icy objects including some larger objects like the dwarf planets Pluto and Eris.
The Oort Cloud
Well beyond the Kuiper belt, scientists believe there is another collection of billions of comets called the Oort cloud. This is where long orbit comets come from. The outer limit of the Oort cloud defines the outer boundary of the Solar System.
The Oort Cloud shown in relation to the rest of the Solar System. Source: NASA.
What are meteoroids, meteors, and meteorites?
A meteoroid is a small piece of rock or metal that has broken off from a comet or an asteroid. Meteoroids can form from asteroids colliding or as debris from comets speeding by the Sun.
Meteors are meteoroids that get pulled into Earth's atmosphere by Earth's gravity. When a meteor hits the atmosphere it will heat up and burn with a bright streak of light called a "falling star" or a "shooting star." If several meteors occur at the same time and near the same place in the sky, it is called a meteor shower.
A meteorite is a meteor that does not entirely burn up and makes it all the way to the ground.
Interesting Facts about Comets and Meteors
The Oort cloud is located about one light year from the Sun.
One of the most famous comets is Halley's Comet. Halley's Comet has an orbit of 76 years and is visible from Earth as it passes by.
During ancient times, people believed that the passing of a comet was an omen of doom.
Eventually the ice will burn off of a comet and it will just be a metallic rock with no coma or tail. These comets are said to have gone "extinct".
Millions of meteors enter the Earth's atmosphere every day. Most of them are about the size of a pebble.
