Comets are quite the celestial spectacle. Would you believe that they are simply pieces of dusty ice? See interesting facts about comets—and get ready for the glowing green Comet 21P/Giacobini-Zinner to pass over our heads September 9–10, 2018! Learn more.
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.
Glowing Green Comet on September 9–10
Get ready for a famous glowing green visitor—Comet 21P/Giacobini-Zinner. Commonly called “21P,” this comet is now in our dark skies and will pass over our heads Sunday night (September 9–10, 2018) for an especially great viewing because this is the same date it reaches perihelion (closest approach to Earth).
During this flyby, Comet 21P will swing by the Sun and shed parts of its outer coating. The Sun’s intense heat and radiation will ionise the comet’s icy surface, leaving behind a bright green comet tail.
Where and When to Look
Comet 21P will be easy to see with regular binoculars or a small backyard telescope, especially because it’s close to a New Moon (so no glare from the moonlight).
Look high in the eastern skies during the dark hours between midnight and dawn. To see when dawn breaks in your location, just punch in your zip code on the Almanac rise and set page.
The very best time to look is at perihelion around 2:40 a.m. EDT (0640 GMT) on September 10 (when the comet slides through the constellation Auriga which is high in the eastern sky).
Comet 21P circles the Sun and returns every 6.5 years. It’s the parent body to the Draconids Meteor Shower which peaks every year around October 9. Normally, the Draconids are a fairly weak shower but this year falls during a new Moon phase so it may be more visible. See our Meteor Shower Calendar.
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/NASA. Photo 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/NAVCAM, CC 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.