What are comets?

Comets are loosely packed balls of ice and dust orbiting our sun, that sometimes become visible in Earth’s skies. A comet’s tail may stretch across millions of miles of space.

From the European Space Agency. The video above is from Meet the Experts series of the European Space Agency (ESA). In it, comet scientist Charlotte Götz of ESA discusses comets, their formation and their study. She explains that comet nuclei are relatively small – about the size of a small earthly town – and that they are loosely packed balls of ice and dust. The comets we know about are mostly potato-shaped, but some are oddly shaped. It’s only when they come near the sun that comets heat up and spew dust and gases. They develop giant glowing heads – called a comet’s coma – that may be larger than most planets. And they sprout their long comet tails that stretch millions of miles long.

From NASA. This NASA Solar System exploration page is also a good place to look for information about comets. It explains that there are likely billions of comets orbiting our sun in the Kuiper Belt and even more distant Oort Cloud. And it links to individual comets that have been studied by spacecraft or from Earth, so that you can understand them better.

Glowing dot with wide white tail and narrow blue tail going off at an angle to the white one.

This 40-image conglomerate, digitally enhanced, was captured July 19, 2020, through the dark skies of the Gobi Desert in Inner Mongolia, China. This image was the Astronomy Picture of the Day for July 22, 2020. Copyrighted image via Zixuan Lin (Beijing Normal U.). Reprinted here with permission.

Comet in the sky above a lake, also reflecting in the lake.

This image is from Bob King – aka AstroBob – in Duluth, Minnesota. He wrote: “My first view of Comet NEOWISE at dusk instead of dawn from a lake near Duluth on July 11. Comets and water naturally go together as they’re thought responsible in part for delivering water to the early Earth.” Thank you, Bob!

For skywatchers. Those of us who watch the skies are, of course, most interested in comets when they appear as (sometimes unexpected, often greenish) visitors in our skies. Since comets are most active when they’re near the sun, we tend to see comets shortly after sunset or before sunrise. At such times, comets don’t sweep across the skies as meteors do. But they do move slowly, from night to night, in front of the stars. They can be very beautiful, especially in a dark sky.

We don’t get bright comets very often, which is one reason Comet NEOWISE – just visible to the eye under dark conditions, and a glorious sight through binoculars – was such a hit this past July.

Comet NEOWISE is no longer a dramatic sight for casual skywatchers. If you want to keep an eye out for comets that might be visible through your binoculars, or even to the eye alone, try this page from SkyandTelescope.com.

A composite of 3 images of the comet, one as it would look through a telescope and 2 in false color.

View these images on Facebook. | Alessandro Marchini of the Osservatorio Astronomico Università di Siena posted these images on July 26, 2020. He wrote: “Comet NEOWISE anatomy. Image of the C/2020 F3 (NEOWISE) that we made last night with our telescope, with false-color processing. The first (left) highlights gas jets coming out of the core to flow into the tail. The second (right) shows, as the artificial color changes, the different density of the material (gas and dust) that surrounds the core and forms the ‘hair’ and tail of this amazing comet.” Thank you, Alessandro!

Bottom line: Comets are loosely packed balls of ice and dust orbiting our sun, that sometimes become visible in Earth’s skies.