Astronomy Essentials

Look for comet C/2017 K2 now in dark skies

View at EarthSky Community Photos. | David Hoskin in Halifax, Nova Scotia, Canada, took this image on June 25, 2022. David wrote: “Last night I captured this image of comet C/2017 K2 (PANSTARRS) during the short period of time between darkness and the comet passing behind trees in my backyard. This large comet (located in the constellation Ophiuchus) is currently 174,058,942 miles (280,120,714 km) from Earth. It took over 15 minutes for the photons from the comet to reach my camera. The sphere of gas (coma) surrounding C/2017 K2 has an estimated diameter of 80,700 miles (130,000 km), which is 10 times the Earth’s diameter! The comet’s closest approach to Earth will be on July 14, 2022.” Thank you, David!

Comet C/2017 K2 (PanSTARRS) has entered the inner solar system, and now’s a great time to look for it. The comet’s closest approach to Earth will be on July 14, 2022, but a supermoon on July 13 will brighten the sky. So get out your telescope now while moonlight is negligible and skies are dark.

The comet’s backstory

Astronomers first spotted the comet in 2017 using the Pan-STARRS survey instrument in Hawaii. At the time, they said it was the farthest active inbound comet they’d yet seen. It was between the orbits of Saturn and Uranus when they first saw it. Now it’s in the inner solar system, with closest approach to Earth on July 14. The comet will be closest to the sun several months later, on December 19, 2022. With a small telescope, you should be able to spot the comet throughout the summer.

Comets are mostly rock and ice. They become active when warmed by the sun. However, this comet was already active in 2017. The Hubble Space Telescope took an image of the comet looking like a fuzzy snowball while it was still in the outer solar system. The comet appears to have a large nucleus, and it shows a huge cometary atmosphere or coma.

View larger. | Artist’s concept of the orbit of Comet C/2017 K2 (PanSTARRS), on its maiden voyage into the inner part of our solar system. Image via NASA/ ESA/ A. Feild/ STScI.

Comet C/2017 K2 (PanSTARRS)

If the name Comet PanSTARRS rings a bell, that’s because there are many of them. Pan-STARRS is a sky survey that is particularly good at spotting new asteroids, comets, supernovae and the like. This is Comet C/2017 K2 (PanSTARRS). At its discovery, K2 was 1.49 billion miles (2.4 billion km) from the sun, 16 times farther away than the Earth is from the sun.

By the way, the new title-holder for farthest active comet is Comet Bernardinelli-Bernstein. Astronomers spotted this behemoth comet, about 100 times the size of a normal comet, when it was more than 2.7 billion miles (4.4 billion km) from the sun. Comet “Bern-Bern” will have its closest approach on January 21, 2031. But you have nothing to fear from it, as it will be slightly farther away than Saturn’s orbit.

Below we have star charts through August to help you spot K2. Although it appears to be a large comet, it will probably remain a telescope object because it will pass some 168 million miles (270 million km) from Earth.

It should brighten to magnitude 8 or even 7, still too dim for the unaided eye. But you can easily spot an object of this magnitude in a small telescope. The darker the skies, the better the contrast will be.

How large is C/2017 K2?

Initial observations from the Canada–France–Hawaii Telescope (CFH) suggest a nucleus with a diameter of about 18 to 100 miles (30 to 160 km). But observations with the Hubble Space Telescope indicate the nucleus should be smaller, at some 11 miles (18 km) or less.

If the name K2 reminds you of the second largest mountain on Earth, also named K2, keep in mind that the mountain is 28,251 feet (8,611 meters) tall. If the comet K2 is around 11 miles or 18 km (a reasonable estimate), that translates to about 58,000 feet or 18,000 meters. In other words, the comet K2 dwarfs the mountain K2, which is about half its size.

View at EarthSky Community Photos. | Legendary skywatcher Stephen James O’Meara emailed from Botswana on June 21, 2022, with this image. He wrote: “Comet C/2017 K2 (PanSTARRS) is now passing by open cluster IC 4665 in Ophiuchus. The comet is barely visible now in handheld binoculars, I estimated its magnitude at magnitude 8.8 on June 19 and magnitude 8.6 on June 20 through a 3-inch (76 millimeters) telescope. The brightness of comets is unpredictable, however, as we know. The comet also has a short tail, but that is most apparent in images. Currently, it appears to be on a brightening trend. This shot is a composite of 2 nights, showing the comet’s movement near IC 4665, made with a 200-mm telephoto lens.” Thanks, Steve!

Huge coma on C/2017 K2

Another indication that suggests C/2017 K2 is not small, or at least is very active, is that observations showed it developed a cometary atmosphere or coma with a diameter of about 81,000 miles (130,000 km). That’s a sphere of gases 10 times the diameter of planet Earth, or almost as big as the diameter of planet Jupiter. That’s huge!

Also, some observations detected an incredibly large tail, some 500,000 miles (800,000 km) long.

Scientists think that comets that are too far from the sun shouldn’t sublimate huge amounts of ice. So this comet’s activity is probably driven by a mix of ices with substances like nitrogen, carbon dioxide, carbon monoxide and molecular oxygen.

Most comets have a nucleus of about 0.5 to 2 miles (1 to 3 km) in diameter, while others may be up to 10 miles (16 km) wide. However, a few are really huge, including Hale-Bopp (37 miles or 60 km), Bernardinelli-Bernstein (93 miles or 150 km) and 95P/Chiron, also known as 2060 Chiron) at 124 miles or 200 km. In fact, 95P/Chiron may be a dwarf planet. However, it showed cometary behavior and thus got a comet designation.

What to expect?

Observers using a small telescope (preferably from a dark-sky site) should be able to see a diffuse or fuzzy patch of light (the coma) around the nucleus of comet C/2017 K2. Careful observations using averted vision at the telescope may reveal the coma is really big compared to other comets. A hint of the comet’s tail might be visible, while long-exposure images should reveal the comet and its tail in all its glory.

Astronomers estimate that Comet C/2017 K2 (PanSTARRS) has been traveling from the Oort cloud for some 3 million years in a hyperbolic orbit. By July 2022, the celestial visitor will be some 172 million miles (277 million km) from Earth and gradually getting closer. Comet K2 first became visible in small telescopes from the Northern Hemisphere in May and will remain visible until September. After mid-September, it becomes closer to the southwestern horizon as seen from the Northern Hemisphere. Thereafter it will be best for observers in the Southern Hemisphere.

Finder maps for C/2017 K2

On June 21, 2022, Comet C/2017 K2 will be very close to where we see Cebalrai, a star in Ophiuchus. It is at Right Ascension 17hr 43.5 minutes, declination +04.6°. Facing southeast at 11:00 pm CDT. Illustration via Eddie Irizarry/ Stellarium.
On July 2, 2022, observers using a “GoTo” or computerized backyard telescope can point the instrument at reference star HIP 84905 (RA: 17 hr 21.1 min, Dec: +01.4°) around 10:30 pm CDT to locate Comet C/2017 K2. Illustration via Eddie Irizarry/ Stellarium.
Comet C/2017 K2 passes very close to where we see the globular cluster Messier 10 on the night of July 14, 2022, around 10:00 pm CDT. The comet will be at its closest point to us on that date. Illustration via Eddie Irizarry/ Stellarium.
Location of Comet C/2017 K2 on August 13, 2022, around 9:30 pm CDT. Illustration via Eddie Irizarry/ Stellarium.
On August 21, 2022, Comet C/2017 K2 will be very easy to locate, close to the star Acrab in the constellation Scorpius. Illustration via Eddie Irizarry/ Stellarium.

When will we see the next bright comet?

Comet C/2022 E3 (ZTF) is expected to be as bright as magnitude 5 in January 2023. After that, the next possible good one appears to be comet 12P/Pons-Brooks, which might reach magnitude 5 or 4 in March 2024.

Meanwhile, let’s see what Comet C/2017 K2 has in store for us. Look up!

Comet c/2017 K2 images from our community

View at EarthSky Community Photos. | Steven Bellavia in Upton, New York, captured comet C/2017 K2 (PanSTARRS) on June 20, 2022. Thank you, Steven!
Comet C/2017 K2 (PanSTARRS) photographed on May 29, 2022, by Raymond Negron from San German, Puerto Rico. Raymond took the image using a 92-mm apochromatic telescope.

Bottom line: Comet C/2017 K2 (PanSTARRS) will brighten in our skies this summer. You can watch it with a small telescope.

July 1, 2022
Astronomy Essentials

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