A green comet is passing by Earth. Here's how to see it.

Later today, a visitor from the solar system’s frigid outermost regions will come within 26 million miles of Earth: a ball of ice and dust that will glow in our skies with an emerald gleam. 

For the past several months, the recently discovered comet C/2022 E3 (ZTF) has been dazzling stargazers and astrophotographers, with the excitement ratcheting up in recent weeks. Now the comet is making its closest approach to our region of the solar system in 50,000 years—providing a vibrant glimpse into one of the building blocks of our cosmic home.

Where did this comet come from, and how can you see it for yourself? We’ve got you covered.

Where did the comet come from?

C/2022 E3 (ZTF) hails from the Oort cloud: an icy hinterland on the outskirts of the solar system where hundreds of billions to trillions of comets lurk, frozen leftovers from the planets’ formation more than 4.5 billion years ago. According to In-the-sky.org, it made its closest approach to the sun on January 12, coming within 103 million miles (166 million km) of our home star. (Learn about the differences between asteroids and comets.)

Matthew Knight, a comet astronomer at the United States Naval Academy in Annapolis, Maryland, says the comet’s trajectory at its moment of discovery implied an orbital period of roughly 50,000 years. Rewinding the clock, the last time this comet passed this close to the sun, Earth was in an ice age, and Neanderthals and mammoths were still alive and kicking.

However, as comets heat up in the sun’s glare, they release gas and dust, which can change their trajectory through the solar system. For that reason, Knight warns that it’s impossible to say with certainty for how long C/2022 E3 (ZTF) had been orbiting the sun along its inbound path.

What we do know, though, is that the comet is white-knuckling its way through the inner solar system right now. Later today, when it comes within 26 million miles (42 million km) of Earth, it’ll whiz past us at a relative velocity of more than 128,000 miles per hour (207,000 km/h), according to NASA’s Jet Propulsion Laboratory.

I’ve heard that the comet’s green. What’s going on there?

This comet is green because its coma—the gassy shroud surrounding its nucleus—contains a reactive compound called diatomic carbon (C2), which is bright in green wavelengths of light. This green colour doesn’t carry into the comet’s tail because C2 breaks down in sunlight, with a single molecule surviving slightly less than two days on average.

The colour of C/2022 E3 (ZTF) isn’t unique: Most comets that have higher gas contents tend to yield C2, so they “are generally going to look green to our eye,” Knight says. That said, only a subset of comets happen to make it as close to Earth as C/2022 E3 (ZTF) will get, so it’ll provide an uncommonly good view of a comet’s emerald hue.

How can I see C/2022 E3 (ZTF)?

Knight recommends using a pair of binoculars or a small telescope to try and see the comet. It will be in the northern sky during the evenings, lingering around the constellation Camelopardalis on February 1.

To orient yourself, look to the north and find the Little Dipper (the constellation Ursa Minor). The tip of the Little Dipper’s handle (or Ursa Minor’s tail) is Polaris, the north star. In the early evening of February 1, the comet will be up and to the right of Polaris by roughly twice the width of your fist at arm’s length. As the evening progresses, the comet’s position will appear to swivel counterclockwise relative to Polaris, along with the rest of the night sky. (For more detailed instructions, Knight recommends consulting skywatching guides.)

Over the first week of February, the comet will appear to move to the east, coming close to Mars on February 10. Not every evening is going to be great for comet viewing, however. During the first week of February, the comet’s position in the night sky will come fairly close to the full moon, which may drown out the comet’s fainter glow for some observers, especially those in areas with high light pollution.

How was this comet found?

C/2022 E3 (ZTF) was found on March 2, 2022, by astronomers Frank Masci and Bryce Bolin. The pair was using the Zwicky Transient Facility (hence the “ZTF”), a wide-field camera at the Palomar Observatory in California that sweeps across the Northern Hemisphere’s night sky every two days, on the lookout for newly appearing objects such as comets and supernovae. When Masci and Bolin spotted C/2022 E3 (ZTF), it was about 399 million miles (643 million km) from the sun, according to Space.com.

The day after it was discovered, Japanese observer Hirohisa Sato saw that the object had a coma, confirming that it was a comet. In the months since, astronomers also have found hints of the comet hiding in archival data, with its first known observation dating to October 25, 2021, according to a database maintained by NASA’s Jet Propulsion Laboratory. (Learn more about the largest comet discovered in modern times.)

Wait, back up: we discovered this comet last year?

C/2022 E3 (ZTF) is part of an extraordinary trend over the past 25 years in astronomy. As telescopes have gotten bigger, data have gone digital, and computers have gotten better, astronomers have been able to spot objects in the night sky more easily, revolutionising the study of asteroids, comets, and other small bodies within the solar system. 

The solar system’s farthest reaches hold upwards of a trillion comets, but their orbits are so gigantic and take so long to complete, we can’t see them unless they venture into the inner solar system and reflect enough sunlight our way. Now we can see smaller, dimmer comets at greater distances from the sun than ever before—which means that discoveries are piling up. Only about 3,900 comets have ever been observed formally by astronomers, according to the Jet Propulsion Laboratory database. More than a fifth of them, including C/2022 E3 (ZTF), have been found since 2010. 

The Zwicky Transient Facility, which found the new green comet, provides a preview of what to expect from the Vera C. Rubin Observatory, a gigantic facility currently under construction in Chile. Once operational in 2024, it will be the biggest survey telescope ever built, opening up a whole new era of cosmic understanding. Projections estimate that the Rubin Observatory will discover about 10,000 more comets all by itself. “It’s really going to blow things out of the water,” Knight says.

What else does this comet tell us?

C/2022 E3 (ZTF) is also a good sign for what we might learn in future robotic space missions.

The European Space Agency is busy designing a new robotic mission called Comet Interceptor. Once launched in 2029, Comet Interceptor will park itself in the L2 Lagrange point, a gravitational sweet spot a million miles behind Earth from the sun’s point of view, and “hover” there for three years. Its target? Nobody knows yet. 

If astronomers discover an incoming object that Comet Interceptor can get to with enough notice—ideally a pristine comet that’s nearing the sun for the very first time—scientists will scramble the spacecraft and plot out a reconnaissance flyby. In doing so, they would get an unprecedented glimpse at the surface of a primordial leftover of the solar system, largely untouched by the sun’s heat and glare.

According to Knight, who is working on Comet Interceptor, Comet C/2022 E3 (ZTF) is the first comet found since June 2019—when Comet Interceptor was formally selected—that the spacecraft could have theoretically reached. That’s promising for Comet Interceptor, as well as for the future of robotic space exploration.