What is a Comet? Comet Facts

Glowing Green Comet September 9-10

December 9, 2018
Green Comet

Comets are quite the celestial spectacle. Would you believe that they are simply pieces of dusty ice? See interesting facts about comets!

Also, get ready for 2018’s brightest comet which comes closest to Earth on December 15–16. See viewing tips.

What Are Comets?

Comets are ball of frozen gases, rock, and dust—unlike asteriods which are made of rock and metal. Specifically, comets are composed of frozen gases such as carbon dioxide, methane, and ammonia, as well as water ice, in which dust particles and rocky material are embedded.

Think of comets as comic snowballs—or, some people jokingly refer to comets as dirty snowballs! They are left over from the formation of stars and planets billions of years ago.

When the gravity from a large passing body, like our Sun, becomes strong enough, the comet enters into the Sun’s orbit. As that ball of ice gets close enough to the Sun, its heat begins to melt some of the ice that makes up the comet.

Some of its ice turns into a giant glowing head larger than most planets. The gases and dust form a long, bright trail, or tail, of vapor that stretches away from the Sun for millions of kilometers, being pushed out by the Sun’s solar wind. That tail is a comet’s most amazing feature.

Comets may not be able to support life themselves, but they may have brought water and organic compounds—the building blocks of life—through collisions with Earth and other bodies in our solar system.

As Almanac astronomer Bob Berman says, “Comets are relics of the birth of the solar system around 4.6 billion years ago. Comets are part of the original material that made up the solar system.”

Why do see we green comets? They are not common, but the green is caused by dark organic matter on the comet itself! You won’t find this on an astroid.

Credit: Gerald Rhemann/NASA. This image of comet Encke was taken in Jauerling (lower Austria) in May of 2014.

Comet History and Lore

Most comets are unpredictable. Over the centuries, people were both awed and alarmed by comets because of this unpredictability—as if they were stars that suddenly appeared in the sky. Can you imagine a snowball over a mile wide appearing above your head?

Through the ages, comets were omens and portends—usually of doom and death and bad things. Comets were not good! Remember that it was a comet that probably wiped out the dinosaurs on Earth. That said, comets have also been taken as good omens by some historic figures, such a William the Conquerer.

Credit: Gerald Rhemann/NASAPhoto taken in Ebebwaldhoehe (lower Austria) with a Schmidt Camera. Exposure time:10 minutes. 

Landing on a Comet!

In 2014, European Space Agency proble Rosetta (named after the famous Rosetta Stone) landed successfully on a comet—Comet 67P/Churyumov–Gerasimenko (67P). Below are some amazing images of the comet. All credit goes to ESA/Rosetta/NAVCAMCC BY-SA IGO 3.0.

Rosetta “selfie” at Mars


Rosetta’s image of Comet 67P with a tail of gas and dust, as it orbits the comet from 162 km (101 mi) away.


Rosettta’s image of Comet 67P as it orbits the comet from 10 km (6 mi) away.


Learn more about the Rosetta mission on the European Space Agency web site here.

What are Meteor Showers? See our page with Meteor Shower facts.


The Old Farmer's Almanac for Kids, Volume 1

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We live in north east Spain

We live in north east Spain 41*16'57 N 0*39'58E and at 06.45 this morning (07/09/2014)whilst my husband was walking the dog he saw what he thought was an explosion in the sky and pieces of debris flying off of this 'object' . I was indoors but witnessed light from the ' explosion' which was bright enough to light up our bathroom. At first he thought it was an aeroplane on fire but could this have been a comet?

I saw a comet at 8:48pm in

I saw a comet at 8:48pm in seaford, Victoria, Australia. It was pretty amazing to see its tail was an awesome bright blue and looked like it was breaking off pieces as it went by. Only lasted 10 seconds before it was gone. But still something you must see.

hi,i leave in al khobar saudi

hi,i leave in al khobar saudi arabia....i saw a shooting star or a comet like.....around 7:35 pm...it was something strange...never seen before....it is true?

i never seen one and hope to

i never seen one and hope to do some day, but i have seen few moon eclipses and lots of falling stars i love watching the moon since i can remember

The first time I saw a comet

The first time I saw a comet I was 10. My friend ( a boy) invited me to watch it from it from a window in his room. it was in my home town in Central Asia. And we saw it! It had a tail. Today I have another opportunity to see meteor shower but in my transplant city- New York. I am going to the Coney Island beach....

I missed them all but hope my

I missed them all but hope my sons and grandchildren, etc watch it in the future together as a family reunion.

Wish I'd be here to see it.

Wish I'd be here to see it. But at 63 now, I don't expect to be around that long. Good luck to those who are.