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The Story Behind a Once-in-a-Lifetime Green Comet That’s About to Fly Past Earth

There was no Palomar Observatory when the object now known as comet C/2022 E3 (ZTF) made its last close approach to Earth 50,000 years ago. There were mastodons and woolly mammoths, and great swaths of glaciers covering portions of North America and northern Europe, as the planet went through its last ice age. But there were no trained skygazers. Now, the ghostly green comet, spotted last year by two modern-day astronomers working at Palomar, in southern California, is swinging back our way. This week, the comet is heading for a close approach to Earth on Feb. 1 and 2, when it will be just 43 million km (27 million mi.) from our planet and be visible to the naked eye as a bright green smudge in the northern hemisphere’s skies.

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The researchers credited with discovering C/2022 E3 (ZTF) on March 2, 2022 are Bryce Bolin, a NASA postdoctoral fellow at the Goddard Space Flight Center in Greenbelt, Md., and Frank Masci, a senior scientist at Caltech, in Pasadena, Calif., the first humans or human-like beings (after the homo sapiens and Neanderthals of 50,000 years ago) to have seen the object—and initially they weren’t looking for comets at all.

The ZTF in the comet’s name stands for Zwicky Transit Facility—essentially a very sophisticated camera and computer attached to one of the telescopes at Palomar. The system is programmed to look for exotic objects outside the solar system and even the galaxy—like supernovas, emerging black holes, and pulsating stars, all of which appear as discrete points of light.

Comets, however, have a different optical profile: by the time they’re close enough to see by telescope, they appear as a larger, fuzzier blob, not the tiny dot the ZTF is trained to identify, so the facility was just discarding those images. To Bolin, who has discovered 50 comets during his time observing the skies, that would not do.

“This was a problem, where photons were falling to the ground,” he says. “We were wasting precious potential comet photons from reaching us through our data system.”

Palomar astronomers thus trained the ZTF’s deep learning network to recognize comet profiles too and not simply throw them away. Not long afterwards, the greenish face of C/2022 E3 (ZTF) showed itself—and straightaway, Bolin knew the team had bagged something noteworthy.

For one thing there was its direction: The comet was approaching the solar system from above, at a 90-degree angle, as if it were falling down from the sky. This indicated it was not coming from the Kuiper Belt—a band of icy, rocky, comet-like bodies that circles the solar system outside the orbit of Neptune—which sends us comets that fly flat in the plane of the solar system. Rather, it was coming from the Oort cloud, a sphere of similar objects that surrounds the solar system much more distantly, with its outer fringes reaching up to two light years away. (It was the trajectory of the comet that also allowed Bolin and Masci to determine its 50,000-year orbital period.)

“Okay,” Bolin recalls saying when he first saw the object, “this is something special.”

Then too there is the comet’s striking brightness and color. Comets flare green when they carry diatomic carbon—two-atom carbon molecules—which reacts with the sun’s outgassing particles, the solar wind. Many comets possess diatomic carbon, but few also approach the sun as closely as C/2022 E3 (ZTF), meaning they show their color less vividly. “I would estimate that the combination of the green color and brightness is maybe a once every few decades or so event,” says Bolin.

What’s more, C/2022 E3 (ZTF), has two tails, rather than the single tail characteristic of most comets. The more common tail is made of dust and ice interacting with the solar wind. The less common second tail is called the ion tail and is made of ionized carbon monoxide, carbon dioxide, and nitrogen molecules that some comet bodies carry. “The ion tail interacts with charged particles in the solar wind and is pushed in the opposite direction from the sun, along the flow of the wind,” says Bolin.

C/2022 E3 (ZTF) is special, but it’s not alone in taking such a long time to return to us. Its 50,000-year orbit places it in the category of so-called long-period comets, which typically come from the Oort cloud and take anywhere from 200 years to 1 million years to complete a single transit of the solar system. The sheer number of bodies in the Oort Cloud means that long-period comets are relatively common—even if the sheer number of years it takes for them to pass our way means that we don’t get a second look at any particular one of them for a very long time, making their appearance an occasion.

Just finding the comet, however, did not mean that it would be officially recognized. Bolin and Masci had to submit their discovery to the International Astronomical Union (IAU) headquartered in Paris, so that other astronomers could examine the object too and confirm that it was indeed a comet. The hive mind did agree, and C/2022 E3 (ZTF) got its listing. Alas, for Bolin and Masci, the comet will never bear their names. The collective nature of comet-spotting and confirmation—plus the group effort required to take whole-sky surveys like those the ZTF conducts—has made the process too much of a collaborative exercise for that.

“There’s a lot of moving parts that have to come together to make it all happen,” Bolin says. “Not only do you have the data system we worked on, you also have the people who set the telescope, the people who designed the telescope and built it, and so on. It’s like 100 people are involved.”

The precise spot to look in the northern sky as the comet approaches is very much a moving target, visible only at night. On Jan. 23, as reports, C/2022 E3 (ZTF) was near the constellation Draco below the Little Dipper; on January 26, it was just beside the Little Dipper itself. On Jan 30, through its closest approach to Earth on Feb. 1 and 2, it will be visible near the constellation Camelopardalis, just past the point of the Little Dipper’s handle. (See for the exact visibility from your location.) Then, the comet will begin making its outward-bound journey, resuming its long, looping transit back through the solar system, not to return to us for another 50 millennia.