Comets, once considered portents of doom, have long puzzled us. They move rapidly against the starry background. They grow tails, which may explain why the ancient Greeks called them “hairy stars.” Their brightness and even their exact paths can be hard to predict. So, what are these mysterious visitors that sometimes appear in our sky?
Astronomers now use telescopes and space probes to study comets, and we’ve learned a lot about them. Comet nuclei typically range from only a few hundred yards to a few dozen miles wide. Comets have been called dirty snowballs or icy dirtballs, referring to the fact that they contain ices of water, carbon dioxide and carbon monoxide, mixed with mineral matter and organic compounds, such as methanol, ethanol, hydrogen cyanide, formaldehyde, and even amino acids.
Comets typically orbit the Sun on highly elongated, elliptical paths. Most comets come to us from the outer reaches of the solar system, regions called the Kuiper Belt and the Oort Cloud. The Kuiper Belt exists several billion to several tens of billions of miles from the Sun and is home to dwarf planets, like Pluto and Eris. Some comets, such as Comet 2022 E3 (ZTF), now passing near Earth, come to us from the distant Oort Cloud, hypothesized to exist between about 200 billion to 18 trillion miles from the Sun. Eighteen trillion miles is the limit of the Sun’s gravitational reach, about three quarters of the distance to the nearest stars!
When comets enter the inner solar system, sunlight begins to sublimate and ionize their ices, creating jets, which eject particles into space. Some of these particles form a spherical halo around a comet’s nucleus that we call a coma. The solar wind, a high velocity particle stream from the Sun, interacts with the coma, pushing comet particles outward, away from the Sun, forming a tail. Many comets have two tails, a relatively straight, blue-greenish tail, consisting of ionized gases derived from ices, and a more diffuse, yellowish- white “dust tail”, consisting of mineral and organic particles.
Bright comets are still objects of wonder. Use finder chartsand binoculars, and try spotting Comet 2022 E3 (ZTF), as it crosses our skies during early February.
You’ve been listening to “Western Slope Skies”, produced by the Black Canyon Astronomical Society and KVNF Community Radio. I’m Art Trevena.
Web links:
General
https://en.wikipedia.org/wiki/Comet
For Updates on Comet C/2022 E3 (ZTF)
http://www.aerith.net/comet/catalog/2022E3/2022E3.html
http://astro.vanbuitenen.nl/comet/2022E3
https://www.cobs.si/cobs/comet/2323/
https://skyandtelescope.org/astronomy-news/spot-circumpolar-comet-ztf-c-2022-e3-in-binoculars/
