Astronomy Essentials

Comet C/2017 K2 still slowly brightening

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!

Comet C/2017 K2 (PanSTARRS)

If you have a small telescope, it’s not too late to catch a view of summer’s best comet, C/2017 K2 (PanSTARRS). The comet made its closest approach to Earth on July 14, 2022, but it’s still slowly brightening. It will reach its closest point to the sun – perihelion – on December 19, 2022. After that, it will remain a good observing object for another month before fading. But that’s if you live in the Southern Hemisphere. If you live in the Northern Hemisphere and haven’t seen it yet, you’ll want to get a look before about mid-September. From then on it will be cruising through southern constellations.

How to see C/2017 K2

First, find a dark-sky site. Then, using a small telescope, find the comet, which in August, is heading from Ophiuchus into Scorpius. Check out the finder maps below.

You should be able to see comet K2 as a fuzzy patch of light. The fuzziness is mostly the comet’s gigantic outer atmosphere, or coma. That coma is about 80,700 miles (129,874 km), which is 10 times the Earth’s diameter! You might also see the comet’s tail, 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

It first became visible in small telescopes from the Northern Hemisphere in May and will remain visible until September. After mid-September, it’ll be close to the southwestern horizon as seen from the Northern Hemisphere. Afterwards, it’ll be best for observers in the Southern Hemisphere.

The comet’s backstory

Astronomers first spotted this 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.

Diagram: Oblique view of solar system orbits with steep parabolic curved line of comet's orbit.
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.


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. That’s 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. That’s 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.

Star charts

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 possibly 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.

Comet K2 is big!

Initial observations from the Canada–France–Hawaii Telescope (CFH) suggest a nucleus for this comet 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.

Dense star field with 2 small fuzzy green ovals labeled with dates.
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-millimeter) 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. 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!

Almost as big as Jupiter

Another indication that suggests C/2017 K2 is large, or at least 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 would mean this comet’s coma is 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 early 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.

Finder maps for C/2017 K2

Star map showing Scorpius and red tick marks for comet at top.
Location of Comet C/2017 K2 on August 13, 2022, around 9:30 pm CDT. Illustration via Eddie Irizarry/ Stellarium.
Star map showing Scorpius and red tick marks for comet at top.
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.

Custom finder charts from Heavens-Above

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 since closest approach to Earth

Comet with bluish head and tail surrounded by stars and wisps.
View at EarthSky Community Photos. | Jelieta Walinski took this image of Comet PanSTARRS on July 28, 2022. Jelieta wrote: “I captured the comet at Medicine Rocks State Park in Montana. This particular state park is full of history, culture and unique topography. Listed on the National Register of Historic Places, Medicine Rocks was a place of ‘big medicine’ where Indian hunting parties conjured up magical spirits. It’s a certified International Dark Sky Sanctuary with Bortle 1 skies. I felt so privileged to witness the grandeur and beauty of the place and so connected with my ancestors, capturing the C/2017 K2 was just a bonus to me.” Thank you, Jelieta!

Images of Comet PanSTARRS before closest approach to Earth

Star field with slightly oval faintly fuzzy object and inset with closeup.
View at EarthSky Community Photos. | David Hoskin in Halifax, Nova Scotia, Canada, captured 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.” Thank you, David!
Starfield with fuzzy object in middle.
View at EarthSky Community Photos. | Grace Wheeler of Eureka, California, took this image of comet C/2017 K2 (PanStarrs) on June 21, 2022. The bright star in the right corner is Beta Ophiuchi, and in the left corner is the trail of a satellite. Thank you, Grace!
Dark night sky full of stars. The comet is a bigger fuzzy yellow dot with a short tail.
View at EarthSky Community Photos. | Steven Bellavia in Upton, New York, captured comet C/2017 K2 (PanSTARRS) on June 20, 2022. Thank you, Steven!
Dense starfield with fuzzy greenish spot with a short tail in the middle.
Raymond Negron from San German, Puerto Rico, photographed comet C/2017 K2 (PanSTARRS) on May 29, 2022. Raymond took the image using a 92-mm apochromatic telescope. Thank you, Raymond!

Bottom line: Comet C/2017 K2 (PanSTARRS) is a great telescopic object for the rest of summer, slowly brightening toward perihelion, its closest point to the sun.

August 2, 2022
Astronomy Essentials

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