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.
— earth is beautiful (@earth__photos) July 12, 2020
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.
Bottom line: Comets are loosely packed balls of ice and dust orbiting our sun, that sometimes become visible in Earth’s skies.