What are Comets?

Comet McNaught in 2007
Comet P1/McNaught, taken from Siding Spring, Australia in 2007. SOERFM/Wikimedia Commons CC BY-SA 3.0

What are Comets?

If you've ever seen a comet in the night sky or in a picture, you've probably wondered what that ghostly looking object could be. Everybody learns in school that comets are chunks of ice and dust and rocks that get close to the Sun in their orbits. Solar heating and the action of the solar wind can change the appearance of a comet drastically, which is why they are so fascinating to observe.

However, planetary scientists also treasure comets because they represent a fascinating part of our solar system's origin and evolution. They date back to the earliest epochs the history of the Sun and planets and thus contain some of the oldest materials in the solar system. 

Comets in History

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. This is relatively new knowledge, however. In antiquity, comets were seen as evil harbingers of doom, usually "foretelling" some kind of evil spirits. That changed as scientists began looking at the sky with more enlightened interest. It has only been in the past hundred years or so that the idea of comets as icy bodies was suggested and ultimately proved to be true. 

The Origins of Comets

Comets come from distant reaches of the solar system, originating in places called Kuiper belt (which extends out from the orbit of Neptune, and the Oört cloud.

which forms the outermost part of the solar system. Their 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, including the Sun.

The gravitational pull of various planets and the Sun shape their orbits, making such collisions more likely as the comet makes more orbits.

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 ices are usually water and frozen carbon dioxide (dry ice). The nucleus is 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 Tail

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 as the "head" of the comet.

The other distinctive part of a comet is the tail area. 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 is very tough to see with the naked eye, but a photograph of it shows it glowing in a brilliant blue. It 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 originated in the Kuiper Belt.

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.

It extends more than 75,000 astronomical units away from the Sun and contains millions of comets. (The term "astronomical unit" is a measurement, equivalent to the distance between Earth and the Sun.)

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.