Because of their high visibility and bright, long tails, nothing piques people’s interest in the universe quite like comets. With comet Tsuchinshan-Atlas reaching its peak viewing in the evening on Oct. 13, Shannon Schmoll, director of Abrams Planetarium at Michigan State University, tells us everything we need to know about these icy celestial leftovers. 

Answers are excerpts from an article originally published in The Conversation. 

What is a comet?

Comets are leftover material from the formation of the solar system. When the solar system formed about 4.5 billion years ago,most of the gas, dust, rock and metal ended up in the sun or the planets. 

Everything that was left became comets and asteroids. 

Because comets are clumps made of rock, dust, ice and the frozen forms of various gases and molecules, they are often called “dirty snowballs” or “icy dirtballs” by astronomers.  

 

Where does a comet’s tail come from?  

As a comet moves close to the sun, it heats up. The various frozen gases and molecules making up the comet change directly from solid ice to gas in a process called sublimation. This sublimation process releases dust particles trapped under the comet’s surface. 

The dust and released gas form a cloud around the comet called a coma. This gas and dust form two different tails.

The first tail, made up of gas, is called the ion tail. The sun’s radiation strips electrons from the gases in the coma, leaving them with a positive charge. These charged gases are called ions. Wind from the sun pushes these charged gas particles directly away from the sun, forming a tail that appears blue in color. The blue color comes from large numbers of carbon monoxide ions in the tail.

The second tail, called the dust tail, forms from the dust particles released during sublimation. This tail is pushed away from the sun by pressure caused by the sun’s light. The tail reflects the sunlight and swoops behind the comet as it moves, giving the comet’s tail a curve.

The closer a comet gets to the sun, the longer and brighter its tail will grow. One comet tail measured   approximately half a million miles long!

How fast do comets move? 

An object will orbit faster the closer it is to the sun, as angular momentum is conserved. Think about how an ice skater spins faster when they bring their arms in closer to their body — similarly, comets speed up when they get close to the sun. Otherwise, comets spend most of their time moving relatively slowly through the outer reaches of the solar system.

A lot of comets likely originate in a far-out region of our solar system called the Oort cloud. 

Comets from the Oort cloud take over 200 years to complete their orbits, a metric called the orbital period. Because of their long periods, they’re called long-period comets. Astronomers often don’t know much about these comets until they get close to the inner solar system. 

Short-period comets, on the other hand, have orbital periods of less than 200 years. Halley’s comet is a famous comet that comes close to the sun every 76 years.

Comets’ time in the inner solar system is relatively short, generally on the order of weeks to months. 

As they approach the sun, their tails grow, and they brighten before fading on their way back to the outer solar system. 

But even the short-period comets don’t come around often, and all of this makes their behavior difficult to predict.