The Ultimate Guide to Observing Comets

 

Comets are among the most spectacular and mysterious objects to appear in our skies. These visitors from the far reaches of the Solar System have intrigued humans for thousands of years. Comets were once thought to be omens of disaster or messengers from the gods bearing bad news. It’s easy to see why ancient peoples feared them. A comet could appear suddenly without warning, grow in size, eventually form a glowing nucleus and a formidable tail, and remain visible for several weeks before fading and heading back out into the abyss. In those days, the sky was dark and pristine without manmade light pollution. So, comets—especially the brighter ones—lit up the sky in all their wondrous glory, making them even more intimidating.

Today, astronomers have a better understanding of comets. Comets no longer instill fear, but instead generate excitement and opportunities for people to observe, photograph, and to study these beautiful celestial visitors. Comets are now one of the most sought-after objects for sky-watchers whenever they appear—especially when they are bright enough to be seen with the naked eye from the city. With the next “ghostly wanderer” on its way for a possible end-of-year encounter with the Sun, now is a good time to learn more about comets and how to observe them.

 

What are comets?

Comets are a mixture of frozen gases, dust, rock, and water ice. Some planetary scientists believe they may be leftovers from materials that initially formed the Solar System about 4.6 billion years ago.

They are relatively small objects that travel through space in highly elliptical orbits that resemble a long, fat cigar. Comets are pulled in faster as they move closer to the Sun, then slingshot around its backside before heading back out into space. Some comets plunge into Sun while others break up entirely during this close encounter.

Scientists often describes comets as “dirty snowballs” due to their composition. As comets begin to heat up from the Sun’s energy, their nucleus spews volatile materials such as dust and gas, which sublimate and escape (outgassing). This gives comets their signature appearance of a glowing coma with dust and gas (ion) tails pointing away from the Sun. These two tails can stretch for over 620,000 miles (1 million km) out in space.

Because comets are known to contain water ice, some believe comets may have originally brought some water and organic molecules to Earth, but not enough of it to fill Earth’s oceans. Asteroids, which are composed of rock and metal, are much larger than comets and are also known to suppress water ice, so perhaps water-rich asteroids and comets both contributed to making regular water deliveries when they impacted Earth during its infancy.

 

How do comets originate?

Comets spend most of their orbital journeys at great distances from the Sun in the far reaches of the Solar System. They are believed to primarily originate from two regions: the Kuiper Belt, and the theorized Oort Cloud. The Kuiper Belt, named after Gerard Kuiper, a Dutch-born astronomer, is a ring-shaped region beyond Neptune’s orbit where many asteroids, comets, and other smaller bodies of icy material reside. It’s believed that short-period comets that orbit the Sun at least every 200 years are Kuiper Belt objects.

The second region, called the Oort Cloud, is located farther out from the Kuiper Belt (up to 100,000 astronomical units from the Sun). It’s thought to be a spherical shell of icy matter, rock, and dust. The Oort Cloud is named after Danish astronomer Jan Oort, who proposed that comets reside in a huge cloud far beyond the orbit of Pluto. Long-period comets that take more than 200 years to orbit the Sun are believed to come from the Oort Cloud, which may contain trillions of comets. The Oort Cloud’s existence has not yet been proven.  

 

 

Parts of a Comet

Nucleus

A comet’s nucleus is its solid head. It is composed of frozen gases, dust, and rocky materials, ranging from about 6 to 62 miles in diameter. Scientists call it a “dirty snowball” due to its composition. When it is heated by the Sun, the gases sublimate and produce an atmosphere surrounding the nucleus.

Coma The atmosphere of vaporized gases that forms around the nucleus is called the coma. These gases can consist of a mixture of ammonia, carbon dioxide, and water vapor. The coma gives comets a fuzzy appearance when viewed through a telescope, easily distinguishable from stars. If a comet’s coma contains carbon-nitrogen and carbon-carbon bonds, the Sun's ultraviolet light will energize the electrons inside of it, causing a green glow.

Dust Tail A comet’s dust tail is the most visible part of the comet. It’s composed of gases and tiny dust particles, which are blown back by solar radiation pressure. The dust tail can be long and curving, pointing away from the Sun due to its motion and gravity. After a comet travels around the Sun and moves away, its tails diminish and fade.
Ion Tail An ion tail is laced with rays and streamers of ionized gases that are blown off comets due to direct contact with the solar wind. This tail usually glows blue and is much thinner than the dust tail. Because it accelerates much faster, the ion tail looks like a straight line extending away from the comet and can be nearly 100 million miles long.

 

How to Observe Comets

  • Check the Internet, an astronomy magazine, sky reports, or astronomy apps, like Celestron’s SkyPortal (included with any Celestron telescope purchase), to learn where and when you can see the next bright comet. Don’t forget to share the information with your friends and family!
  • When the time is right, travel to a remote location and let your eyes adapt to the darkness for at least an hour. With your eyes fully adjusted, fainter objects like comets and their tails will be easier to locate. This technique is called dark adaptation.
  • Find a clear view of the horizon where the comet is predicted to be. Avoid areas obstructed by buildings, trees, or hills. An open field or even a rooftop will give you a nice view.
  • Because some comets are faint, fuzzy blobs, try observing them using the averted vision technique. Instead of looking directly at the comet, look off to the side about 20 degrees to expose your eye’s most sensitive parts, the rods and cones, to the light coming from the comet. You will have a better chance of “seeing” the comet than you would using direct vision.
  • A binocular like Celestron’s Cometron 7x50 is perfect for scanning the sky with its large light gathering 50mm objective lenses. Binoculars can help you locate a comet in the sky and provide a wide-angle view of its impressive tail.
    • Telescopes offer even more light gathering ability and increased power to help magnify a comet’s coma and pull in additional detail. Medium to high magnification will provide great views of the coma as long as conditions are favorable.

      On the other hand, if you’d like to view more of the comet at once, a wide-field refractor telescope, like the Inspire 100AZ #22403, is a great choice. This type of telescope paired with lower to medium magnification eyepieces can reveal more of the comet’s tail without needing to move the telescope.

    To see the full lineup of Celestron telescope models, click on this link.

     

    Notable Comets

    Halley’s Comet (1P/Halley)
    Perhaps the most famous comet of all time, Halley’s Comet is a periodic comet that returns to Earth’s vicinity about every 75 to 76 years. Due to its relatively brief orbital period, it’s possible for some to view the comet twice in a lifetime.

    English astronomer, Edmond Halley, successfully concluded that the comet observed in 1531, 1607 and 1682 was the same comet. He predicted the comet would appear again in 1758. Although he didn’t live long enough to see it, the comet appeared just as he predicted. Because of his discovery, the comet was named after him.

    After a dazzling performance in 1910, expectations were high for Halley’s 1986 appearance. Unfortunately, on this pass, the comet was much farther away from the Sun and appeared fainter, much to the disappointment of the public. However, it was during this approach that the European robotic spacecraft, Giotto, flew by Halley’s Comet (within 370 miles of its nucleus) to study it up close and returned some amazing images. Halley’s Comet is predicted to return in 2061.

    Comet Hale-Bopp (C/1995 O1)

    Comet Hale-Bopp is the best known and most widely seen bright comet of the latter half of the 20th century. Named after its co-discoverers, Alan Hale and Thomas Bopp, who observed the comet separately around the same time in the summer of 1995, Comet Hale-Bopp put on a spectacular show in early 1997 as it was bright enough to be seen from the city. Multiple jets spewed dust and gas from its nucleus which appeared to be “rotating” when seen through a telescope. Millions upon millions of people around the world witnessed the Great Comet of 1997. It will return to Earth’s vicinity again in the year 4385, so mark your calendars!

    Comet Hyakutake (C/1996 B2)

    The “opening act” for Hale-Bopp, the Great Comet of 1996 was discovered by amateur astronomer Yuji Hyakutake, in the darker skies of southern Japan. He used a powerful set of binoculars with 6” objective lenses to scan the skies on the night of his discovery. This comet was visible to the naked eye by late February 1996 and remained visible to the naked eye for three months. Comet Hyakutake’s tail became spectacular later in March when it developed a long blue ion tail that stretched out more than 100 degrees across the sky and a white dust tail that was shorter but a bit wider. The comet became six times as bright as a first-magnitude star when it passed Earth at a mere 0.1 astronomical unit. It is not predicted to return to the inner Solar System again for approximately 70,000 years.

    Comet Shoemaker-Levy 9 (D/1993 F2)

    Discovered by Carolyn and Gene Shoemaker and David Levy in a photograph taken in 1993 with the 0.4-meter Schmidt telescope at Mt. Palomar, Comet Shoemaker-Levy 9 was a comet captured by Jupiter’s gravity. The gas giant’s gravitational pull broke the comet apart into at least 20 pieces that subsequently slammed into Jupiter like a freight train. In July 1994, a string of fragments labeled A through W impacted Jupiter's cloud tops with the force of 300 million atomic bombs, causing Jupiter to appear as having “black eyes.” Earth-based observatories and spacecraft including Hubble Space Telescope, Ulysses, and Voyager 2 studied the historic impact and its aftermath.

    Comet Ikeya-Seki (C/1965 S1, 1965 VIII, and 1965f)

    One of the brightest and most fabulous naked-eye comets to be discovered in the 20th century was found on the morning of September 19, 1965. Two amateur astronomers, Kaoru Ikeya and Tsutomu Seki of Japan separately discovered this very bright comet that managed to escape detection until it was less than 100 million miles from the Sun. Comet Ikeya-Seki will be remembered for coming out of nowhere and putting on a memorable show in the early morning twilight sky. Sky-watchers described the comet as being 10 times brighter than the full Moon!

    Comet West (C/1975 V1, 1976 VI, and 1975n)

    Discovered in late 1975 by Danish astronomer Richard West from photographs taken at the European Southern Observatory in Chile, Comet West was a great comet of the 1970s. It had a spectacular coma and structured tail that resembled "a fantastic fountain of light."  The comet developed into a beautiful object in the morning sky during March 1976 for Northern Hemisphere observers. Some observers reported the comet was bright enough to be seen in full daylight. Although Comet West put on a fine display, the mainstream media did not give the comet much publicity due to the hype of Comet Kohoutek two years before, which turned out to be a major “dud.” As result, most of the public was unaware of the comet and missed out on seeing it.

    Comet McNaught (C/2006 P1)

    Discovered by astronomer Robert McNaught at Australia's Siding Spring Observatory in 2006, this comet became known as the Great Comet of 2007. It became a spectacular comet to view as it traveled past the Sun in January 2007 at a distance of just 15.9 million miles. Comet McNaught developed a very large, curving, fan-shaped dust tail. According to new data, it’s the largest comet measured to date! It was also one of the few bright comets that was bright enough (magnitude -5.1) to be visible in broad daylight for favorably placed observers. Unfortunately, those in the Northern Hemisphere were out of luck as the best views of Comet McNaught were seen south of the equator.  

    Comet PanStarrs (C/2011 L4)   

    Discovered in June 2011, Comet PanStarrs was the fourth new comet detected during the first half of June and received the designation C/2011 L4. Researchers first spotted the comet on images taken through the 1.8-meter Panoramic Survey Telescope and Rapid Response System on Haleakala on the island of Maui, Hawaii. Because a large team of astronomers, observers, and scientists were involved, the comet was named after the telescope instead of the people. It received the instrument’s acronym, “PANSTARRS.” The comet peaked around magnitude –1, and put on a nice show after sunset. Comet PanStarrs (C/2011 L4) paired up nicely with a thin waxing crescent Moon in the darkening twilight sky on Mach 13, 2013 and made for a picturesque photo opportunity for astroimagers.

    Comet ISON (C/2012 S1)

    Comet ISON will be remembered for not being the spectacular Thanksgiving comet of 2013! In fact, it was quite a letdown for millions around the world. On the morning of November 28, expectations were at an all-time high as Comet ISON reach perihelion (closest approach to the Sun). As it rounded the Sun, observers held their breath, watching live images taken from several orbiting solar observatories. Hopes were high that an already good comet would morph into a great comet. ISON disappeared behind the Sun and finally re-emerged as a fan-shaped comet, giving everyone false hope. Then, ISON quickly began to fade and reality set in that the comet had been vaporized in a cloud of dust.

    Comet NEOWISE (C/2020 F3)

    Comet NEOWISE was first spotted by NASA’s Near-Earth Object Wide-field Infrared Survey Explorer (NEOWISE) mission in March 2020. It is one of the more recent comets that came about unexpectedly and put on a fine display for sky-watchers during the summer of 2020. While it wasn’t a great comet like Comet Hale-Bopp was 23 years earlier, it was still a binocular and naked-eye comet that came within 64 million miles from Earth. Comet NEOWISE made its closest approach to Earth on July 22 and images showed a classic white dust tail and thinner blue ion tail. As the days went by, the comet grew dimmer as it made its way back out into the Solar System. Due to the global pandemic, many sky-watchers who quarantined at home were able to get some reprieve from life’s unexpected challenges and enjoy observing a rare comet visitor. Comet NEOWISE (C/2020 F3) is predicted to return in about 6,800 years.

     

     

    Comet Fast Facts

    •  Comets are significant to scientists because they are primitive bodies left over from the formation of the Solar System.
    • Scientists believe comets originate in two regions—the Kuiper Belt and the theorized Oort Cloud.
    • Comets are often referred to as “dirty snowballs” as they are composed of a mixture of frozen gases (water based, dry ice, carbon monoxide, methane, and ammonia), dust, rocks, and water ice.
    • Comets orbit the Sun in elliptical paths but are far more elliptical than the paths planets take.
    • A comet has four components: a nucleus, a coma, a dust tail, and an ion tail.
    • A comet’s dust tail appears white or yellowish in color and curves slightly due to the comet’s motion and gravity, while its ion tail is usually thinner and glows blue.
    • A comet’s nucleus contains the vast majority of its total mass.
    • If a comet’s coma contains carbon-nitrogen and carbon-carbon bonds, The Sun’s ultraviolet light will energize the electrons inside of it, causing it to glow green.
    • Because comets are known to contain water ice, it’s believed comets may have originally brought some water to Earth, but not enough to fill Earth’s oceans.
    • Comets are named after their discovere­­­­rs or investigators. Their names can be a person’s last name, such as Halley’s Comet and Comet Hale-Bopp. Or they can be named after an observatory/telescope/mission used in the discovery, like Comet NEOWISE.
    • A comet can be named after up to three discoverers. The discoverers may work together as a team or make the discovery independently at around the same time.
    • The most famous comet is Halley’s Comet. It has been observed since at least 240 B.C. Its orbit makes it visible from Earth every 75 to 76 years. It was named after the British astronomer Edmond Halley. Halley’s Comet was last seen in 1986 and is predicted to return in 2061.
    • One “great” comet (visible to the naked eye) occurs approximately every ten years.
    • It is notoriously difficult to predict how bright a comet will become. Comet Kohoutek (C/1973 E1) in 1973 and Comet ISON C/2012 S1 in 2013 were both touted as the “Comet of the Century” only to fade away and disappoint the public.
    • There are over 3,000 currently known comets (depending on who you ask), but scientists believe that may be up to one billion comets in our solar system.
    • Scientists believe a comet’s lifespan may only be a few thousands years due to the amount of “shedding” or vaporizing of materials each time a comet comes close to the Sun.
    • When Earth passes through the tail of a comets that has already passed by, the dust left behind can burn up in the sky, causing a meteor shower.
    • Comets travel about 2,000 miles per hour when far away from the Sun. However, as they move closer to the Sun, their speed increases and can reach over 100,000 miles per hour!
    • NASA's Stardust spacecraft encountered Comet 81P/Wild in January 2004 and collected dust grains that were sent through the comet’s coma. The collector looked like a tennis racket made of silica aerogel that was exposed to the dust. The samples were collected, sealed, and jettisoned back to Earth for analysis in January 2006.
    • Despite a comet’s incredible speed, it does not zip across the sky like a meteor or shooting star due to its extremely far distance away from Earth. You can tell that a comet has moved by comparing its position in the sky the following day/night.
    • Comets can strike Earth. The last time one did was approximately 28 million years ago.
    • Comets move in highly eccentric orbits around the Sun. This means they can appear from any direction of the sky and put on a short- or long-term display.

     

    Helpful Observing Hints

    Tip #1:
    Use an Astronomy App

    Use an Astronomy App or Star Chart

    The most modern and informative tools today can be found in astronomy apps such as Celestron’s SkyPortal mobile app. This full featured planetarium app is included with the purchase of any Celestron telescope, available from the Apple App Store or Google Play. SkyPortal instantly provides new telescope owners with a wealth of information at their fingertips, including audio and written descriptions about various objects, including comets! It also provides comets’ celestial coordinates, a real-time sky map, rise and set times, physical and orbital parameters.

    Tip #2:
    Telescope Acclimation

    Set up your telescope outside and let it acclimate to the outside temperature before attempting to observe.

    Tip #3:
    Collimation

    Collimation

    Collimate, collimate, collimate! If you own a Newtonian or Schmidt-Cassegrain telescope, make sure your telescope’s optics are collimated. It can make a difference when it comes to discerning fine detail in celestial and terrestrial targets. If the optics are slightly out of alignment, you may be cheating yourself out of seeing the clearest images.

     

    Final Thoughts

    Excitement is already beginning to build as news is circulating that a possible naked-eye comet C/2021 A1 (Leonard), named after astronomer Greg Leonard, may be arriving in our skies this coming December 2021 and January 2022. The comet was discovered on January 3, 2021 at the Mount Lemmon Observatory in Arizona. If it puts on a good show, it just may make for an unforgettable holiday season. While this is indeed very good news, comets are notorious for failing to live up to expectations. However, the odds are in our favor that we will see another spectacular comet again in our lifetime whether it’s Comet Leonard, or yet an undiscovered comet that is quietly making its way into our inner Solar System. Until then, let’s keep our fingers crossed and hope for the best.

     

    Clear skies!

    Other articles you might be interested in: Ultimate Guide to Observing the Universe