Rare Green Comet Nishimura: A Spectacular Celestial Event Returns After Centuries

In January and February 2023, stargazers across the United States witnessed a rare astronomical event: the passing of Comet C/2022 E3 (ZTF), a green-tinged visitor from the outer solar system that had not approached Earth in approximately 50,000 years. This once-in-a-lifetime sky event captivated millions of Americans who braved cold winter nights to glimpse a phenomenon that no human alive — or indeed no human ever — had seen from Earth before. Here is everything you need to know about this extraordinary comet and what made it scientifically significant.

What Made Comet C/2022 E3 (ZTF) Unusual

Discovered on March 2, 2022, by astronomers Frank Masci and Bryce Bolin using the Zwicky Transient Facility (ZTF) telescope at the Palomar Observatory in California, Comet C/2022 E3 immediately attracted scientific interest because of its highly elongated orbit. Unlike short-period comets that circle the sun every few years or decades, this comet follows an orbital path that carries it to the outer reaches of the Oort Cloud — a vast shell of icy bodies at the far edge of our solar system — and brings it back toward the inner solar system only once every roughly 50,000 years.

That orbital period means that the last time this comet made a close approach to Earth, modern humans were sharing the planet with Neanderthals. Woolly mammoths roamed what is now Siberia and North America. The ice sheets of the last glacial maximum still covered much of Canada and the northern United States. To call this a “rare” comet is an understatement of almost incomprehensible proportions.

The Science Behind the Green Color

The most visually striking feature of Comet C/2022 E3 was its distinctive green glow — a color so unusual that it became the dominant identifier in public discussions of the event. This coloring was produced by a chemical process called dicarbon (C₂) fluorescence. As the comet approached the sun, solar ultraviolet radiation excited dicarbon molecules in the comet’s coma (the gaseous envelope surrounding the nucleus), causing them to fluoresce in the green wavelengths of visible light.

Interestingly, this green color is visible primarily in the coma surrounding the comet’s nucleus; the dust tail typically appears white or slightly yellowish because it is composed of dust particles reflecting sunlight. The ion tail, which is pushed directly away from the sun by solar wind, often appears bluish-green. The combination creates a visually complex object when photographed through a telescope — layers of green, white, and blue that make the comet look almost artificial in images.

NASA scientists noted that the comet’s brightness exceeded initial predictions. Early estimates suggested it might just barely be visible to the naked eye; by the time it reached perihelion (closest approach to the sun) on January 12, 2023, and perigee (closest approach to Earth) on February 1, 2023, it had brightened sufficiently that observers with binoculars could spot it from dark sky sites. The comet reached an estimated magnitude of approximately 5, right at the threshold of naked-eye visibility for observers far from city lights.

When and Where Americans Could See It

For American observers, the optimal viewing window fell in late January and early February 2023. The comet passed closest to Earth on February 1–2, at a distance of approximately 26 million miles (42 million kilometers) — far enough away to pose no risk but close enough to be among the most visible comets in years. During this period, the comet was visible in the northern sky, moving from the constellation Camelopardalis through Auriga and toward Taurus during the viewing window.

Observers in rural areas across the American West — in states like Wyoming, Montana, Colorado, Nevada, and New Mexico, where light pollution is lowest — had the best viewing conditions. Dark sky preserves like the McDonald Observatory in Texas, Cherry Springs State Park in Pennsylvania, and the Cosmic Campground in New Mexico’s Gila National Forest served as informal gathering points for comet watchers. Amateur astronomy clubs across the country organized viewing events, and park rangers at national parks reported unprecedented attendance at ranger-led night sky programs.

Urban observers faced greater challenges. A 2016 study in the journal Science Advances found that approximately 80% of Americans live under light-polluted skies bright enough to wash out the Milky Way. For those in major metropolitan areas like New York, Los Angeles, Chicago, or Houston, the comet was at best a smudge through binoculars without traveling to darker locations. This reality highlights the growing problem of light pollution for astronomical observation and public engagement with the night sky.

Scientific Research Opportunities

Beyond its visual spectacle, Comet C/2022 E3 provided astronomers with a rare research opportunity. Comets are among the most pristine remnants of the early solar system — icy bodies that formed approximately 4.6 billion years ago and have remained largely unchanged since then, preserved in the deep cold of the outer solar system. Studying a comet’s chemical composition therefore provides a direct window into the conditions of the early solar nebula from which Earth and the other planets formed.

Spectroscopic observations of C/2022 E3 detected a range of volatile compounds in its coma, including carbon monoxide, water ice, methanol, formaldehyde, and various complex organic molecules. The presence of these organics is particularly significant to researchers studying the origins of life on Earth. One influential hypothesis, called panspermia or the late heavy bombardment hypothesis, proposes that comet impacts during Earth’s early history delivered the organic building blocks that eventually gave rise to life. Each comet studied provides additional data points for evaluating this hypothesis.

The Hubble Space Telescope observed C/2022 E3 in November 2022, producing high-resolution images of the coma structure. Ground-based observatories including the Very Large Telescope in Chile and the Keck Observatory in Hawaii contributed spectroscopic data. The James Webb Space Telescope, just beginning its scientific mission, also observed the comet — providing the most detailed infrared spectra ever obtained for a long-period comet.

How This Fits Into a Pattern of Exciting Sky Events

The green comet’s appearance was part of an exceptional period for U.S. astronomy enthusiasts. The early 2020s have seen a remarkable series of notable sky events accessible to amateur observers. The total solar eclipse of April 8, 2024, crossed from Texas to Maine and drew an estimated 30 million Americans to the path of totality — one of the largest intentional migrations for a natural event in U.S. history. Regular meteor showers, including the Perseids and Leonids, provide annual opportunities for public engagement with astronomy.

For those interested in tracking upcoming astronomical events, our USA Sky Events Calendar 2026–2027 provides a comprehensive guide to meteor showers, eclipses, planetary conjunctions, and other notable events visible from North America. The Geminid Meteor Shower, occurring each December, is considered one of the most reliable and prolific meteor displays of the year.

Photography Tips for Future Comet Events

For photographers who missed C/2022 E3 or want to prepare for future comets, a few technical considerations are essential. Comet photography requires a camera capable of manual exposure control, a sturdy tripod, and ideally a tracking mount that compensates for Earth’s rotation. For brighter comets visible to the naked eye, standard telephoto lenses in the 200–400mm range can capture impressive detail. For fainter objects, a telescope equipped with a camera adapter is necessary.

Exposures for comet photography typically range from 30 seconds to several minutes, depending on the comet’s brightness and the focal length being used. Dark sky location selection is critical — even a single-stop improvement in sky brightness (moving from a Bortle 6 to a Bortle 5 sky, for example) can make the difference between a faint smudge and a clearly resolved coma and tail. Mobile apps like Clear Outside, Stellarium, and SkySafari help with planning observation sites and timing.

The Role of Citizen Science in Comet Discovery

While C/2022 E3 was discovered by professional astronomers using automated sky survey equipment, citizen science plays an increasing role in comet discovery and monitoring. Programs like the SETI Institute’s Comets and Asteroids Refresh (CARS) program and the British Astronomical Association’s comet section engage amateur observers worldwide in systematic monitoring of known comets, helping to track brightness variations and outbursts that automated surveys might miss.

The Zwicky Transient Facility itself — which discovered C/2022 E3 — represents a new generation of automated sky surveys capable of imaging the entire northern sky every two days. This systematic coverage means that bright new comets are now almost certain to be discovered well in advance of their closest approach to Earth, giving astronomers and the public months of preparation time. The era of comets arriving unannounced, as they sometimes did in the 19th century, is effectively over.

What the Green Comet Teaches Us About Sustainability

It might seem incongruous to connect a once-in-50,000-years comet to sustainability concerns, but the connection is genuine and important. The ability to observe the night sky — to see the Milky Way, watch meteor showers, marvel at a green comet — depends on maintaining dark skies. Light pollution, driven by poorly designed artificial lighting, is a form of environmental pollution that the International Dark-Sky Association estimates affects more than 99% of the U.S. and European populations.

Dark sky preservation is increasingly recognized as a legitimate conservation goal, connected to both ecological concerns (artificial light at night disrupts bird migration, sea turtle nesting, insect behavior, and other ecological processes) and cultural ones (the night sky is a shared human heritage that has guided navigation, inspired mythology, and fostered the scientific curiosity that ultimately unlocked our understanding of the universe). The National Park Service’s Night Sky Program works specifically to preserve dark skies within national parks as a public resource.

Light pollution is also directly connected to energy waste — unnecessary outdoor lighting consumes an estimated $3.3 billion in electricity annually in the United States, releasing approximately 21 million tons of CO₂ per year. The solutions — shielded fixtures that direct light downward, LED conversions with warmer color temperatures, motion sensors that reduce illumination when not needed — simultaneously reduce energy consumption, reduce carbon emissions, improve ecological conditions, and restore human access to the natural night sky.

The Next Great Comet

Astronomers and skywatchers are always anticipating the next notable comet. Comets are notoriously unpredictable — they can brighten dramatically, fizzle disappointingly, or even disintegrate entirely as they approach the sun. Comet C/2023 A3 (Tsuchinshan-ATLAS), observed in October 2024, briefly exceeded initial brightness predictions and was visible to the naked eye for observers in the Southern Hemisphere, generating similar public excitement to C/2022 E3.

Long-period comets like C/2022 E3 represent a continuing parade of pristine visitors from the outer solar system — messengers from the deep past of our solar system that carry chemical records of conditions billions of years ago. Each one provides both a scientific research opportunity and a public engagement moment of the kind that climate scientists and science communicators like Bobby Bascomb recognize as invaluable: a reason for ordinary people to look up, to wonder, and to care about the universe they inhabit.

For those who missed Comet C/2022 E3, the night sky offers consolation prizes on a regular schedule. Meteor showers, planetary oppositions, lunar eclipses, and aurora borealis events all provide opportunities for connection with the broader cosmos. Our guide to upcoming U.S. sky events will help you plan your next night under the stars.

For more on the intersection of environmental awareness and astronomical observation, explore our coverage of wildfire smoke impacts on U.S. air quality and our guide to the Geminid Meteor Shower 2026.