Grab your binoculars: A comet that has fascinated scientists for five years approaches its closest distance from Earth this week — and you might be able to catch a glimpse. 
There’s a chance of spotting the C/2017 K2 PANSTARRS comet, also called K2, on Wednesday or Thursday as it makes it final pass through the solar system, said David Jewitt, an Earth, planetary and space sciences professor at University of California, Los Angeles.    
But not with the naked eye: Experts say people will need at least a small telescope or binoculars to see it. 
At a distance of about 170 million miles from Earth, Jewitt warned stargazers that the K2 comet will still be quite far away. For reference, the sun is about 93 million miles away, he said.
“That’s one heck of a long way,” Jewitt, who has studied the comet since 2017, told USA TODAY.
Here’s what to know about K2 and how you can view it.
The C/2017 K2 PANSTARRS comet caught the attention of experts at the Hawaii-based Panoramic Survey Telescope and Rapid Response System on May 21, 2017. Experts said pre-discovery images of the comet from 2013 were later found.
It had been traveling for millions of years from the frigid depths of the solar system, according to NASA, when it was discovered between the orbits of Saturn and Uranus about 1.5 billion miles from the sun. 
K2 was the farthest active inbound comet ever seen when NASA’s Hubble Space Telescope captured it. It was observed at 17 times the Earth-sun distance, Jewitt said. Scientists announced in June 2021 that C/2014 UN271, or the Bernardinelli-Bernstein comet, surpassed it as the farthest-observed comet on record.
K2, a frozen “city-sized snowball of ice and dust,” as NASA calls it, is thought to have come from the solar system’s most distant region where many comets are believed to have originated: the Oort Cloud. NASA experts said the cloud is a giant spherical shell made of icy pieces of space debris the size of mountains or larger.
Astronomers located K2 in a part of the solar system where sunlight is only 1/225th its brightness as we see from Earth and where temperatures are minus 440 degrees Fahrenheit, according to NASA.
The comet “is packed with materials that have been frozen since the beginning of solar system time,” Jewitt said. “When we study these comets, we’re trying to look at material has been preserved from the beginning of the solar system.”
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K2 will reach its minimum distance from our planet, about 170 million miles away, Wednesday night, said Italy-based astrophysicist Gianluca Masi, director of the the Virtual Telescope Project.
K2 is the brightest comet in the sky right now, he said. 
In July, the overall speed of the comet relative to the Earth is an average of 21 miles per second, Masi said. When K2 reaches its minimum distance around 11 p.m. Eastern time Wednesday, he explained the comet’s speed relative to Earth will be 0 miles per second.
What makes K2 intriguing to scientists is that it was coming in from the Oort Cloud at “an unusually large distance,” Jewitt said. Telescope data shows that K2 became active at an “unprecedented” 35 astronomical units, which represent the average distance from Earth to the Sun. 
K2’s close-approach distance of approximately 170 million miles Wednesday night is equivalent to 1.8 au, according to data from the Center for Near Earth Object Studies, part of NASA’s Jet Propulsion Laboratory.
“Neptune is 30 times the Earth-sun distance,” Jewitt said. “It’s really active really far out, and that’s why it’s scientifically interesting, because that allows us to study whatever process it is that drives the activity at very large distances and very low temperatures.” 
Experts believe frozen carbon monoxide has kept K2 active at extremely large distances from the sun, Jewitt said.
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On Thursday, the Italy-based Virtual Telescope Project plans to host a live feed starting at 6:15 p.m. ET for spectators who don’t have a telescope.
Jewitt noted there are “probably hundreds” of comets that have come closer to Earth than K2 and that it won’t be a “spectacular” viewing for the general public. Masi also noted that the full moon will be in the sky on K2’s flyby date, which could make it “significantly” harder to view, he explained.
But both experts agreed that you’ll be able to see K2 with binoculars or a small telescope.
The comet will be visible in the Ophiuchus constellation from the Northern and Southern hemispheres, Masi said. 
“A dark sky would offer the best sight,” Masi said. He recommends observing K2 over the next few nights as the moon leaves the evening sky. Looking up earlier in the evening before moonrise will afford the best view, he added. 
Small telescopes will show the comet for several months to come, he said.

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