EarthSky predict that Comet K2 will likely only be as bright as magnitude 7, which is simply beyond the reach of the naked eye. This is confusing given the comet’s large size. The nucleus could be between 30 and 160 km in size, from initial observations by the Canada-France-Hawaii Telescope.
The estimate is controversial, with observations from the Hubble Space Telescope suggesting it may be 18 km at most.
However, skywatchers’ problem with comets is not that their size is indeterminate. Professional comet observer John Noonan told Space.com that a comet’s brightness (or luminosity) is determined by its distance from our planet, as well as the amount of sunlight hitting its surface.
Noonan, a researcher at Auburn University in Alabama, had studied comet explosions before using the Hubble Space Telescope. Comet activity is related to surface temperature, he said, and beyond, how quickly surface ice sublimates (or changes directly from solid to gas) and produces dust.
When K2 was discovered in 2017 on its inward journey from the cold and distant Oort Cloud, it spewed out carbon monoxide that picked up a lot of dust as it sublimated. However, comets appear calmer as they swing closer to our planet.
“The activity of comets from the Oort Cloud is difficult to predict, and it is rare that the increase in activity levels of these comets is accurately measured as they enter the inner solar system,” Noonan said. SpaceTuesday (12/7/2022)
At its closest approach on July 13 to the lita planet, Noonan said, the comet will be nearly two Earth-sun distances (1.8 astronomical units). That’s too far from the sun to produce many bright sublimation and dust-lifting events.
Even when K2 reaches its closest approach to the sun in December, it will remain outside the orbit of Mars at 1.8 astronomical units. The long distance and dust production together, he said, made the K2’s performances likely to be bad.
“None of the (factors) matched the extraordinary naked eye sighting,” he said.
Quanzhi Ye, an astronomer at the University of Maryland who specializes in comets, told Space.com that K2’s closest approach to the sun in December will also be dim because the comet will be on the other side of the sun relative to our planet and approximately 2.5 astronomical units from Earth.
“It would easily be a comet with the naked eye if it arrived half a year earlier or later,” Ye said.
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