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

If you missed the recent passage of the green-hued comet C/2022 E3 (ZTF), or only saw a blurry smudge through binoculars, take heart!

Comets visible to the human eye attract a lot of attention from stargazers, and some of them are equipped with cameras, telescopes and a desire to share the experience. We’ve collected some of those here.

Discovered in March 2022 by astronomers using the Zwicky Transient Facility at the Palomar Observatory, the comet made its closest approach to the sun on Jan. 12, 2023. Almost three weeks later, on Feb. 1, it cruised within 26 million miles of Earth, moving across the northern sky as it began its voyage back toward deep space.

In that brief flash of days, eyes turned skyward, cameras snapped image after image, and telescopes bent a collective stare at the fleeting visitor. And though the comet remained elusive to the naked eye except under dark, moonless conditions, the photographs are stunning.

The color of ZTF — called the “green comet” by some — is not that unusual. Green can be seen in the gaseous haloes of many comets. The color is believed to come from a photochemical reaction between sunlight and certain molecules (like diatomic carbon) exuded by the comet when it gets close to the sun. Some comets even have a bluish hue.

Comet apparitions

Naked-eye comets can be one of the most thrilling sky shows we can behold, right up there with eclipses.

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.

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?

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.

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.

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.

Bad omens or good sightseeing opportunities, comets are rare celestial gems, sights to cherish when they come our way. C/2022 E3 (ZTF) won’t catch our eye again, but there are plenty more comets out there to anticipate.