Comets: Ghostly Visitors from the Solar System's Frontier

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

Comets are fascinating objects in the sky. Up until a couple of hundred years ago, people thought they were ghostly sky visitors. In early times, no one could explain these strange sky apparitions that came and went without warning. They seemed mysterious and even frightening. Some cultures associated them with evil omens, while others saw them as spirits in the sky. All those ideas fell by the wayside once astronomers figured out what these ghostly things are. It turns out they're not frightening at all, and in fact can tell us something about the most distant reaches of the solar system. 

We now know that comets are dirty-ice leftovers from the formation of our solar system. Some of their ices and dust are thought to be older than the solar system, which means they were part of the birth nebula of the Sun and planets. In short, comets are old, and they are among the least-changed objects in our solar system and, as such, may yield important clues about what conditions were like at that time. Think of them as icy repositories of chemical information from the earliest epochs of our solar system. 

Where Do Comets Originate?

There are two main types, designed by their orbital periods — that is, the length of time they take to make a trip around the Sun. Short-period comets take less than 200 years to orbit the Sun and long-period comets, which can take thousands or even millions of years to complete one orbit. 

Short-period Comets

Generally, these objects are sorted into two categories based on where they first started out in the solar system: short-period and long-period comets. All comets originate in two regions: an area beyond the planet Neptune (called the Kuiper Belt) and the Oört Cloud. The Kuiper Belt is where objects such as Pluto orbit, and is home to potentially hundreds of thousands of objects both large and small.

Out there, despite the large number of planetary nuclei, dwarf planets, and other small worlds, there is a lot of empty space, lessening the possibility of random collisions. But occasionally something occurs that will send a comet hurtling toward the Sun. When this happens, it begins a journey that can slingshot it around the Sun and back out to the Kuiper Belt. It stays on this path until the Sun's immense heat erodes it away or the comet is "perturbed" into a new orbit, or onto collision course with a planet or moon.

Short-period comets have orbits under 200 years long. That's why some, such as Comet Halley, are so familiar. They approach Earth frequently enough that their orbits are well understood.

Long-period Comets

On the other end of the scale, long-period comets can have orbital periods up to thousands of years long. They come from the Oört Cloud, a loosely distributed sphere of comets and other icy bodies thought to extend nearly a light-year away from the Sun; reaching nearly a quarter of the way to our Sun's nearest neighbor: the stars of the Alpha Centauri system. As many as a trillion comets may reside in the Oort cloud, orbiting the Sun near the edge of the Sun's influence.

Studying comets from this region is difficult because most of the time they are so distant that we can rarely see them from Earth, even with the most powerful telescopes. When they do venture into the solar system's inner sanctum, they disappear back to the farthest depths of the solar system; gone from our view for thousands of years. Sometimes comets are ejected completely out of the solar system. 

The Formation of Comets

Most comets originated in the cloud of gas and dust that formed the Sun and planets. Their materials existed in the cloud, and as things heated up with the birth of the Sun, these icy objects migrated out to cooler regions. They are easily influenced by the gravity of nearby planets, and so many of the cometary nuclei that exist in the Kuiper Belt and Oort Clouds were "slingshotted" out to those regions after gravitational interactions with the gas giants (which also migrated out to their present positions). 

What Are Comets Made Of?

Each comet has only a tiny solid part, called a nucleus, often no bigger than a few kilometers across. The nucleus contains icy chunks and frozen gases with bits of embedded rock and dust. At its center, the nucleus may have a small, rocky core. Some comets, such as Comet 67P/Churyumov-Gerasimenko, which was studied by the Rosetta spacecraft for more than a year, appear to be made of smaller pieces somehow "cemented" together. 

Growing a Coma and a Tail

As a comet nears the Sun, it begins to warm up. The comet gets bright enough to see from Earth while its atmosphere — the coma — grows larger. The Sun's heat causes ice on and beneath the comet's surface to change to gases. The atoms of gas are energized by interactions with the solar wind, and they start to glow like a neon sign. "Vents" on the Sun-warmed side may release fountains of dust and gas that stretch out across tens of thousands of kilometers. 

The pressure of sunlight and flow of electrically charged particles that stream from the Sun, called the solar wind, blow coma materials away from the comet, forming its long, bright tail. One is a "plasma tail" made of electrically charged ions of gas from the comet. The other is an arching tail of dust. 

The closest point that a comet gets to the Sun is called its perihelion point. For some comets that point can be fairly close to the Sun; for others, it might be well beyond the orbit of Mars. For example, Comet Halley comes no closer than 89 million kilometers, which is closer than Earth gets. However, some comets, called sun-grazers, crash straight into the Sun or get so close that they break up and vaporize. If a comet survives its trip around the Sun, it moves out to the farthest point in its orbit, called aphelion, and then begins the long trip back sunward. 

Comets Affecting Earth

Impacts from comets played a major role in the evolution of the Earth, primarily during its early history billions of years ago. Some scientists suggest that they contributed their water and a variety of organic molecules to the infant Earth, just as early planetesimals did.

Earth passes through the trails of comets each year, sweeping up the debris they leave behind. The result of each passage is a meteor shower. One of the most famous of these is the Perseid shower, which is made up of material from Comet Swift-Tuttle. Another well-known shower called the Orionids, peaks in October, and is made up of debris from Comet Halley. 

Edited and updated by Carolyn Collins Petersen.