Stargazers might remember Comet NEOWISE, which helped us survive the summer of our first COVID-19 pandemic year. It was faint enough that we required binoculars unless we viewed it from under very dark, un-light-polluted skies.
Now, nearly three years later, another comet is swinging past the Earth, and, while, at its brightest, it may appear some 40 times fainter than NEOWISE, it has become quite newsworthy because of the stunning green color we see in photos.
I’m referring to this visitor from deep space designated officially as C/2022 E3 (ZTF), named for the Zwicky Transient Facility at the Palomar Observatory near San Diego where it was discovered.
Astronomers just call it Comet ZTF.
ZTF, like all other comets, is one of countless icy remnants of the primordial solar system that tumble silently in a region known as the Oort Cloud, billions of miles from the sun.
Occasionally one of these cosmic nomads drifts inward toward the sun’s heat, and its ices disintegrate into a cloud of gas and dust around its nucleus (the “coma”).
Sunlight and the solar wind act as a fan and blow this material outward to create one or two tails that always point away from our star.
Occasionally a comet becomes noteworthy enough to make the news, and that’s just what’s happened with ZTF.
The big news is about its color. This comes from the diatomic carbon and cyanogen within its atmosphere; when this is struck by sunlight it glows green.
Of course, all comets have these chemicals in them — some more than others — and all show at least a wisp of greenish color.
Estimated to be about two-thirds of a mile across, the icy nucleus of ZTF is much too small to be visible with even a powerful telescope, but its coma and tail are.
As compact as a comet’s tail may appear to us from Earth, its material is spread over tens of millions of miles; in fact, to achieve the density of the air we breathe, a comet’s entire tail would need to be compressed to fit into the size of an average suitcase.
Comet ZTF is what’s known as a long-period comet, and it hasn’t journeyed our way for 500 centuries. On Feb. 2, however, it will pass only 26 million miles from our planet.
Just how bright ZTF will become is anyone’s guess.
Will it be visible to the unaided eye or fade as it recedes into the depths of space? No one can say for sure.
Comets are notoriously unpredictable. As noted comet-hunter David Levy likes to say: “Comets are like cats. They both have tails and they both do what they want.”
Nevertheless, you may be able to spot this interplanetary nomad in the evening sky if you’ve got dark skies far from the lights of big cities.
Watch for it in the northern sky after dark as it drifts from the vicinity of Polaris (the North Star) on Jan. 29 and 30 up toward the planet Mars, where it will appear on Feb. 10.
Be sure to check out the accompanying illustration and grab binoculars to locate and enjoy the amazing interplanetary iceball this week after dark!