That Bright Green Comet Looked Exactly as Cool as You’d Think


Unlike eclipses, their appearances are unpredictable. Forecasting how visible a comet becomes and how long a tail it grows — if it grows a tail at all — is often guesswork until weeks, or even days, before it passes by. Every near-Earth encounter with a comet raises hopes that it will bloom into a brilliant nocturnal spectacle, like Hale-Bopp in 1997 or the passage of Halley’s Comet in 1910.

Comet C/2022 E3 (ZTF) on Jan. 30, 2023. Picture taken from Saguaro National Park near Tucson, Arizona. (Courtesy of Robert Sparks)

But, more like comet ISON in 2013 and Kohoutek in 1973, both of which were predicted to be “comets of the century,” ZTF did not rise to the level of captivating naked-eye drama. It became much more famous for its alien green coloration and wispy multiple tails revealed in astrophotos.

Despite the naked-eye fizzle, the comet’s eerie hue and mysterious point of origin made it grow large in the eye of the public’s imagination.

When will it come back?

Going, going … gone? Comet C/2022 E3 (ZTF) will not return for a very long time, if ever. Since its discovery, astronomers have tracked its orbit to figure out where it came from, whether it originated in the Oort Cloud, the vast and diffuse cloud of comets, ice and dust surrounding the solar system, or somewhere farther out. Does it orbit our sun in a periodic cycle like most known comets, or did it wander in from interstellar space and is it now headed back there?

Even now the answer isn’t clear. If it is gravitationally bound to our solar system, then it follows a long, looping elliptical orbit that would carry it repeatedly past the sun — though the far end of that orbit is projected to stretch at least 4.3 light years into space, equivalent to the distance to the nearest star system, Alpha Centauri. At such great distance, who knows what the comet will do within the error margins of the orbital math. Will it come to a crawling halt and begin the slow fall back toward the sun, or break free of our solar system’s feeble gravity and drift away into the galaxy at large?

Comet C/2022 E3 (ZTF) seen near the planet Mars. The image was taken on Feb. 10 near Rush Valley, southwest of Salt Lake City, Utah. (Martin Ratcliffe/StarryNightSkyArt)

Even if it does swing back toward us, it won’t pass through our neighborhood again for millions of years. So, if you managed a glimpse of this ancient space traveler, count that as a blessing, a rare peek at something humans had never seen before, and probably never will again.

Anatomy of a comet

As enormous as a comet may appear, most of the visual spectacle is actually a cosmic form of “smoke and mirrors” deception — or, in the comet’s case, ionized gas and reflected sunlight.

At the heart of every comet is a nucleus of frozen materials: water, ammonia, methane and other compounds. This nucleus is typically no larger than half a mile to 6 miles across — so small that we’d never see it unless it whizzed very close to Earth. ZTF is on the lower end of that range, estimated to be a bit more than half a mile across.

Diagram of a typical comet and the relationship of its iconic tails. (NASA/JPL-Caltech)

When a comet swings by the sun, it is heated by sunlight, and some of its frozen material sublimates, spewing out into a cloud of gas and dust that engulfs the nucleus. This cloud, called the coma, can grow to become hundreds of thousands of miles across — so big that it could easily engulf the entire Earth and moon system.

The gasses of the coma may be blown off into space by the solar wind, forming the comet’s iconic tail. The solar wind is a stream of ionized particles flowing outward from the sun, so the comet’s gas tail always points away from the sun, like a giant wind sock. The gas tail may grow to millions, or hundreds of millions, of miles long, depending on the size of the nucleus and how much gas it dumps into space.

A comet often develops more than one tail. Dust embedded in the nucleus is carried into space by the spewing gasses and drifts along the comet’s path.

Backdrop of a stary night sky, a bright circle in the center surrounded by a white and green hue, along with several tails.
You can clearly see the details of the coma (bright spot), ion tail (right), dust tail (curving veil) and antitail (left) in this picture taken Jan. 20, 2023, from Yosemite National Park. (Tara Mostofi)

In pictures of C/2022 E3 (ZTF), three tails can be seen clearly: a long, thin gas-ion tail; a wide, curving dust tail; and a pale, spikey “antitail” pointing in the opposite direction from the other two.

The antitail, a fairly uncommon feature of comets, is formed of dust particles larger than the finer-grained dust tail. The larger grains are less affected by the blowing force of the solar wind, and tend to be left behind in the wake of the comet’s passage, like dust kicked up by a car moving down a dirt road.

Bad omens?

Being objects that can be sometimes seen with the naked eye, comets have been talked about by humans since prehistory. And because they occasionally come close to Earth and can grow to startling proportions, ancient cultures often interpreted them as portents of calamity, bad omens or heralds of impending disaster. And in a way they weren’t wrong, considering the catastrophe one would cause were it to actually collide with Earth. Fortunately, most comets only deliver a visual warning of that possibility.

In fact, the word “disaster,” used in the mid-16th century in connection with the passage of a comet, derives from the Italian “disastro,” meaning an “ill-starred” event.


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