Astronomy Essentials

Comet C/2017 K2 is closest to the sun Dec 19

Starfield with small white tailed dot, and same image with black and white reversed.
View at EarthSky Community Photos. | Legendary astronomer Stephen James O’Meara in Maun, Botswana, took these images on September 21, 2022. Stephen said: “For those at mid-northern latitudes who might be interested in seeing how Comet 2017 K2 (PanSTARRS) is holding up in the southern skies, the image here shows the comet on September 21, holding steady at magnitude 8.7. The tail should appear longer as the comet nears the sun in the coming months, and the Earth-comet geometry gradually becomes more broadside; but anything can happen with comets. The bright star at top is 4th-magnitude Rho Scorpii.” Thank you, Stephen!

Comet C/2017 K2 (PanSTARRS)

Hey, Southern Hemisphere observers, this one’s for you! Comet C/2017 K2 (PanSTARRS) is making its closest approach to the sun – called perihelion – on December 19, 2022. And you have to be south on the globe to spot it. The comet is currently near the border of the constellation Ara the Altar and heading into Pavo the Peacock. Reports show the comet around magnitude 8.5, which means you’ll want good binoculars and dark skies to see it.

The comet’s close approach to Earth was back on July 14, 2022, in northern skies. But the comet continues to head deeper into southern skies, making this observing target the domain of the 12% of Earth’s population that lives in the Southern Hemisphere.

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How to see C/2017 K2

The website The Sky Live will help you spot the comet. You can find star maps there from the viewpoint of your location. Then you can locate the comet as it shifts among the constellations.

So, how do you go about seeing it? First, you’ll want to be under southern skies. Next, find a dark-sky site. Then, use binoculars or a small telescope to track down the comet.

You should 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 (129,900 km), which is 10 times the Earth’s diameter! And as the comet sweeps closest to the sun, the tail should appear longer than it did when it was farther away. 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

Green glowing comet head with streaking green tail in very dense star field with 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 with a background of interstellar gas, providing variations of color to frame the comet. Hopefully the best is yet to come.” Thank you, Eliot!

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 located between the orbits of Saturn and Uranus when they first saw it.

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 path.
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.

And 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.

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.

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.

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.

Comet C/2017 K2 images since closest approach to Earth

Comet with greenish head and long, faint tail surrounded by scattered stars and nebulous 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, bright star to one side, and very thin straight line across corner.
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!

Bottom line: Comet C/2017 K2 (PanSTARRS) reaches its closest point to the sun – perihelion – on December 19, 2022. It’s now a target for southern skies.

December 18, 2022
Astronomy Essentials

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