Comet twice the size of Mount Everest to pass by Earth next month and you might be able to see it

A comet first spotted in the distance in 2017 might finally be within view soon of amateur astronomers.

Comet C/2017 K2 (PANSTARRS), called K2 for short, was then the farthest active comet ever spotted, a title it recently surrendered to megacomet Comet Bernardinelli-Bernstein, detected last year. But even down one superlative, K2 is remarkable for activity. The comet began to spew gas and dust in the far outer solar system, whereas it's more typical for comets to wake up around Jupiter's orbit, much closer in.

Five years later, the icy body is finally drawing within reach of Earth and its amateur astronomers. K2's closest approach to our planet will be on July 14, and it will get closest to the sun on Dec. 19.

Assuming K2 survives the heated journey and continues to brighten, EarthSky (opens in new tab) predicts people with small telescopes will be able to spot the sojourner soon.

"It should brighten to magnitude 8 or even 7, still too dim for the unaided eye," EarthSky wrote. 

Sharp-eyed viewers can usually spot stars of magnitude 6 in dark-sky conditions with no aid. In the case of this comet, you will also need areas away from light pollution to spot it with a telescope. 

"The darker the skies, the better the contrast will be," EarthSky advised.

As the comet approaches us, professional observatories may be able to figure out how big its nucleus is. Early observations by the Canada–France–Hawaii Telescope (CFHT) suggested K2's nucleus could be between 18 and 100 miles (30 to 160 kilometers) wide; Hubble Space Telescope observations suggested it might be only 11 miles (18 km) at most, EarthSky said.

In 2017, Hubble imagery determined that the comet's coma (or fuzzy atmosphere) likely includes oxygen, nitrogen, carbon dioxide and carbon monoxide, all turning from solid to gas as the comet warmed.

An archival search of CFHT imagery suggested K2 was active at least as far back as 2013, when it was between the orbits of Uranus and Neptune, NASA said at the time.

All predictions for comet activity are subject to change, however. Comets are prone to falling apart or brightening unpredictably when the draw close to the intense heat and gravity of our sun. That characteristic, however, makes them all the more interesting to astronomers who want to understand how comets are put together.

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