The Green Parabola



If any early humans or Neanderthals had turned their gaze to the cosmos 50,000 years ago, they might have been treated to a rarity.

If, however, no one happened to catch the streaking sky-body 50 millennia ago, it’s possible that today’s humans will be the first to ever spy a comet that will visit our neck of the stars during the next month, designated C/2022 E3 (ZTF).

And not just any ho-hum ice ball, but a green comet!

The Palomar Observatory at Zwicky Transient Facility

On 2 March 2022, astronomers Bryce Bolin and Frank Masci discovered what they thought was an asteroid at the Zwicky Transient Facility in California (the ZTF portion of the comet’s name). Further observation revealed the body had a coma, a fuzzy envelope around the head; a coma is a telltale sign that something is a comet and not an asteroid.

At discovery, the comet had an apparent magnitude of 17.3. The scale for apparent magnitude is reverse logarithmic, which means that lower numbers are brighter. We use the star Vega as the baseline for this system; the sun clocks in at -26. By November, the comet had brightened to an apparent magnitude of 10, which is still pretty dim. At 10, a body is just 0.010% the brightness of Vega, meaning that initial discovery took some serious technology and some serious searching.

As the comet neared the sun and Earth, an interesting attribute emerged. C/2022 E3 (ZTF) sported a bright green coma.

Comet C/2022 E3 (ZTF) - photo by Dan Bartlett/NASA

This emerald glow likely rises from molecules of dicarbon in the head of the comet. Two carbon atoms bonded together throw green hues when they are excited by ultraviolet radiation from the sun. Interestingly, as you can see in the photo above, the head is green but the tail is closer to a pale yellow. This effect occurs because the dicarbon undergoes photodissociation, meaning it breaks down thanks to photons. The lifetime of dicarbon, when exposed to radiation, is about two days, meaning the green disappears before any material reaches the tail sections.

The comet just achieved perihelion – its closest approach to the sun – on 12 January 2023. Between now and 1 February 2033, C/2022 E3 (ZTF) will get closer and closer to Earth. On that date, it will be about 0.28 Astronomical Units away from us, which is approximately 26 million miles. When it gets that close, it will reach an apparent magnitude of 6, which will just put it into the spectrum that is visible to the human eye (and on par with Uranus)!

Image by Chris Schur
Photo by Jose Francisco Hernández
The track of the comet through January - graphic by NASA/Caltech

Though this comet has the potential to be a treat for skygazers, the prospect is not a certainty.

It will be visible to the naked eye, but a few caveats exist. Its brightness is just low enough that sighting it will be extremely sensitive to light pollution. Going to areas with true darkness will enable a much better experience. Further, the closeness of this comet might counterintuitively aid its difficulty to see. Because it will not be a distinct point like a star, its light will diffuse across a greater area. You might look right at the comet and see just a bit of fuzziness if conditions aren’t ideal. Lastly, this comet will not put on the show that NEOWISE gave us in 2020, which hit a magnitude of 3.

Still, with some persistence and a little fortune, you could be one of the first humans to see this comet in 50,000 years. This long-period comet puts the “long” in “long-period.” Halley’s comet appears to earthlings every 76 years; no one alive will see NEOWISE again, though its period is a paltry 6,700 years. Obviously, at this point in aging technology, no one will be around to see the green comet again in 50,000 years.

Then again, no human might ever see this one again, even if we attain immortality. Its orbital path is parabolic, unlike the closed paths of Halley and NEOWISE. When C/2022 E3 (ZTF) heads off into deep space this time, current calculations show it won’t ever visit Earth anew. Take the time to appreciate this emerald marvel; no Neanderthal or human will have the opportunity again.

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