Comets are among the most captivating of sights for skywatchers. Their glowing green heads and glorious tails, powered by the Sun’s warmth and buffeted by its solar wind, sweep across the sky.
The brightest ones, easily visible with unaided eyes, are legendary. But those are extremely rare, often only visible from limited regions of Earth for only a brief time. The arrival of new, bright comets is completely unpredictable, adding to their mystique. But one or two dimmer comets are usually observable in binoculars or small telescopes every month, if you know where to find them.
Where do comets come from?
Comets can be one-time visitors that are flung out of the Solar System by the Sun or destroyed in its heat. Others traverse the Solar System like interplanetary shuttlecrafts.
Comets, named for the Greek phrase for “having long hair,” come from the Kuiper Belt and Oort Cloud, cold regions far beyond Neptune’s orbit. Here, primordial ice and other volatile elements survived after our newly-formed Sun cleared that material from between the planets. You can think of them as “dusty icebergs,” so low in density that you could crush a piece in your hand. Astronomers believe that the gravitational tug of a passing star or an outer planet can cause one of those icy bodies to fall towards the Sun, a journey that can take thousands of years. Once they venture near enough to it, the Sun’s heat and radiation releases gas and dust trapped in the ice, producing the characteristic tail and coma around the comet’s nucleus.
Some of the coma is ionized by the Sun’s radiation, causing it to glow with blue-green light. The ions align with the solar magnetic and electric fields around the comet, forming a straight ion tail pointed directly away from the Sun. Meanwhile, the disintegration of the ice releases trapped dust and larger particles that are dropped in the comet’s wake as a dust tail, like a dirt-filled truck on a bumpy road. When the comet gets close enough to Earth, we can see the glowing ions and the dusty tail lit by reflected sunlight.
Some of the lighter dust particles are pushed away by solar wind pressure. If the comet is travelling laterally as it approaches the Sun, the difference between the travel directions of the comet and the solar wind cause the dust tail to curve in a spectacular arc. In fact, many comets sport a curved yellowish dust tail in one direction and a straight bluish ion tail in another direction.
As a comet drops towards the Sun, it brightens and accelerates towards perihelion, or closest approach to the Sun. Many comets fail to miss the Sun and are destroyed during perihelion. This was the case for Comet ISON in late November 2013. After perihelion, it was predicted to re-enter our skies in spectacular fashion. But it disintegrated during its passage.
Comets that survive perihelion can be flung out of the Solar System forever, making them one-time visitors only. The rest enter stable orbits that return them to the Sun on periods of years to millennia.
A brief history
In ancient times, no one knew that some comets were returning periodically. In 1705, while researching historical records from previous comet appearances, English astronomer Edmund Halley noted that three comets shared the same orbit, and were likely a single periodic comet. He calculated a return date, but died before it re-appeared as predicted in 1759. It was named Halley’s Comet (despite the fact that it had been observed by many individuals since 240 BCE). The competition to first see Halley’s Comet launched the career of Charles Messier, and led to his famous list of “not-comets.”
For the next 200 years or so, periodic comets were named for the discoverers who calculated their orbits (although sometimes, prior observers were added to the name), but this became confusing when the same people discovered several comets. The confusion was alleviated by numbering all periodic comets in order of discovery, starting with “1P/Halley”. The list has reached “436P/Garradd”, discovered in Fall, 2021.
Since 1995, new comets have been given designations that indicate their type, the date of discovery, and the discoverer. The prefixes “P/” for periodic, “C/” for non-periodic, “X/” for uncertain, and “D/” for destroyed or lost, give the type of comet. Appended to it are the year of discovery and a letter code indicating which of the 24 half-months it was discovered in, where “A” represents January 1-15 and “Y” is December 16-31. Next, a number indicates whether it was the first, second, etc. comet discovered in that half-month. Finally, the name(s) of the discover(s) and/or the name of the robotic camera system used in the discovery, are appended. This system has been applied retroactively to all periodic comets, so many individual comets have multiple designations. That duplication can sometimes mess up amateurs who are using apps to search for observable comets.
When to hunt for comets
The best times to hunt comets are during dark moonless nights. Expect the comet to appear as a faint greenish blob (quite different from a star, but resembling a galaxy). If it has a tail, it will be much fainter, and pointing away from the Sun. If you use a telescope, be sure to search at low power (50×) initially, and then magnify once you have it in view, to 150× or more. Don’t be afraid to try long exposure photographs, either through the eyepiece, or using a tripod-mounted camera.
What to use
For celestial objects, the visual magnitude number increases as the brightness decreases. If a comet brightens to magnitude 4 or 5, you should be able to see it with your unaided eyes from an area free of artificial light or moonlight.
A small pair of binoculars should show comets as faint as visual magnitude of 8 or 9 under very dark skies. Large binoculars and small telescopes will capture comets down to about magnitude 11. An eight-inch (203mm) reflector or Schmidt–Cassegrain telescope can see comets beyond magnitude 12. A larger aperture telescope will work even better. But for visual observing, I wouldn’t go out of my way to see a comet dimmer than magnitude 8 or 9.
There are good websites out there to tell you what comets are currently observable. Here’s a shortlist.
cometchasing.skyhound.com: Current and upcoming comets, ranked in order of visibility, with notes about when and where to look, and what instrument is required (naked eyes, binoculars, small telescope, etc.). Links provide a printable finder chart showing each comet’s path over the month.
COBS: the Comet Observation Database records observers’ data and plots brightness graphs for current comets.
Weekly Information about Bright Comets: Seiichi Yoshida’s webpage provides observing notes, finder charts, recent photographs, and a light curve showing the predicted brightness profile over time as well as brightness estimates contributed by observers. His Visual Comets in the Future page lists the comets observable during evening, midnight, and the pre-dawn, month by month into the future.
Chris Vaughan is a science writer, geophysicist, astronomer, planetary scientist and an “outreach RASCal.” He writes Astronomy Skylights, and you can follow him on Twitter at @astrogeoguy. He can also bring his Digital Starlab portable inflatable planetarium to your school or other daytime or evening event. Contact him through AstroGeo.ca to tour the Universe together.