What are Comets?

Comet
Comet. NASA

What are Comets?

If you've ever seen a comet in the night sky, you've probably wondered, what is that ghostly looking object? We learn in school that comets are chunks of ice and dust and rocks, but they also represent a fascinating part of our solar system's origin and evolution.

Historically, comets have been referred to as "dirty snowballs" since they were thought to simply be large chunks of ice mixed with dust and rock particles.

 While the exact nature of a comet's nucleus (see below) may not fit the mold for the dirty snowball moniker the fact remains that, according to NASA, in order for an object to officially be classified as a comet it much contain at least 85% ice (frozen water).

Comets come from distant reaches of the solar system. Their orbits usually originate in places called Kuiper belt and Oört cloud. Those orbits are highly elliptical, with one end at the Sun and the other end at a point sometimes well beyond the orbit of Uranus or Neptune. Occasionally a comet's orbit will take it directly on a collision course with one of the other bodies in our solar system.

The Comet Nucleus

The primary part of a comet is known as the nucleus. It's a mixture of mostly ice, bits of rock, dust and other frozen gases. The nucleus is usually very hard to make out when the comet is closest to the Sun because it's surrounded by a cloud of ice and dust particles called the coma.

In deep space, the "naked" nucleus reflects only a small percentage of the Sun's radiation, making it almost invisible to detectors. Typical comet nuclei vary in size from about 100 meters to more than 50 kilometers (31 miles) across.

The Comet Coma and Tailb

As comets approach the Sun, radiation begins to vaporize their frozen gases and ice, creating a cloudy glow around the object.Known formally as the coma, this cloud can extend many thousands of kilometers across.

When we observe comets from Earth, the coma is often what we see.

The other distinctive part of a comet is the tail. Radiation pressure from the Sun pushes material away from the comet forming two tails that always point away from our star. The first tail is the dust tail, while the second is the plasma tail — made up of gas that has been evaporated from the nucleus and energized by interactions with the solar wind. Dust from the tail gets left behind like a stream of bread crumbs, showing the path the comet has traveled through the solar system. The gas tail can glow in brilliant colors and often extends over a distance equal to that of the Sun to the Earth.

Short-Period Comets and the Kuiper Belt

There are generally two types of comets. Their types tell us their origin in the solar system. The first are comets that have short periods. They orbit the Sun every 200 years or less. 

Many comets of this type originate in the Kuiper Belt, which extends for about 55 astronomical units beyond the orbit of Neptune

Long-period Comets and the Oort Cloud

Some comets take more than 200 years to orbit the Sun once, sometimes millions of years.These comets come from a region outside of the Kuiper belt known as the Oort cloud.

Extending more than 75,000 AU away from the Sun, the Oört cloud contains millions of comets.

Comets and Meteor Showers:

Some comets will cross the orbit that the Earth takes around the Sun. When this happens a trail of dust is left behind. As Earth traverses this dust trail, the tiny particles enter our atmosphere. They quickly begin to glow as they are heated up during the fall to Earth and create a streak of light across the sky.When a large number of particles from a comet stream encounters Earth, we experience a meteor shower. Since the comet tails are left behind in specific locations along Earth's path, meteor showers can be predicted with great accuracy.