Astronomy EssentialsSpace

Comets are icy balls of gas and dust

Comets: Comet pointing down with tails up over river.
View at EarthSky Community Photos. | John Nelson took this image of Comet Neowise on July 20, 2020, over the Columbia River. John wrote: “A little over 2 hours after sunset from a rocky overlook above the Columbia River. Comet Neowise with gas and dust tails showing off in the northwest skies above Lake Wanapum, part of the Columbia River in central Washington. Above the comet you can see most of the Big Dipper except for Alkaid, the last star in the Dipper’s handle. Just to the right of the Big Dipper, a meteor entered our atmosphere leaving a telltale streak. The dotted line to the right of the comet is an airplane.” Thank you, John! Learn more about comets below.

What are comets?

Comets are icy balls of gas, dust and rock that orbit the sun. Astronomers believe most comets are leftovers from the formation of the sun and planets. In fact, comets come from the farthest reaches of the solar system, including the Kuiper Belt and the Oort Cloud. When passing stars outside our solar system jostle comets, the comets begin the long trek inward toward the sun. Then, as they come close to the sun and heat up, they release gas in a process called outgassing. This creates the long, glowing tail that stretches behind the comet and points away from the sun.

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The experts on comets

The video above is from the 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. Also, 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.

Moreover, the comets we know about are mostly potato-shaped, but some are more oddly shaped. In addition, it’s only when they come near the sun that comets heat up and spew dust and gases. In fact, they develop giant glowing heads – called a comet’s coma – that may be larger than most planets. And indeed, they sprout their long comet tails that stretch millions of miles long.

NASA says that the current number of known comets is 3,743.

The parts of a comet

The nucleus is the core of a comet. Additionally, it is also the head of the comet. When a comet nears the sun and heats up, some of its frozen surfaces begin to thaw and create the fuzzy coma that surrounds the nucleus.

Surprisingly, a comet can even have two tails. The ion tail is generally bluish in color and points away from the sun because it’s blown back by the solar wind. The ion tail is made of – you guessed it – ions. These are electrically charged gas molecules. The second tail is the dust tail. It is indeed made of dust and normally has a whiter appearance. The dust tail forms a slightly curved trail behind the path of the comet.

Diagram of a comet with labels for parts.
Artist’s concept of the parts of a comet. Image via NASA.

Observing comets

Us skywatchers 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.

Greenish comet head with streaking tail in very dens star field and faint nebulae.
View at EarthSky Community Photos. | Eliot Herman, using an iTelescope in Siding Springs, Australia, captured this image of Comet PanSTARRS on July 30, 2022. Eliot wrote: “Comet 2017 K2 is already presenting a striking image months from its predicted maximum brightness. At the time of the image, the comet was 172 million miles (277 million km) from Earth. The image shows the comet embedded in interstellar gas, providing variations of color to frame the comet. Hopefully the best is yet to come.” Thank you, Eliot!

Read more about Comet PanSTARRS 2017 K2, which reaches perihelion – closest point to the sun – on December 19, 2022.

Bottom line: Comets are diffuse balls of ice and dust orbiting the sun. They’re sometimes visible in our skies. A comet’s tail may stretch millions of miles across space.



November 24, 2022
Astronomy Essentials

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