How to Watch the ‘Green Comet’ in Night Skies (NYT)

How to Watch the ‘Green Comet’ in Night Skies

By Shannon Hall

January 20, 2023

A green-hued comet from the outer solar system is set to swing through Earth’s

neighborhood in the coming days for the first time in 50,000 years.

The comet has been steadily gaining brightness and will make its closest

approach on Feb. 2, when it comes within 26.4 million miles of the planet — 110

times the distance to the moon. From the Northern Hemisphere, the comet is

likely to be faintly visible to the naked eye.

But you don’t have to wait until February to spot this visitor. The coming

weekend may offer favorable viewing opportunities with a pair of binoculars

when the new moon creates darker skies.

What is the comet’s name?

The comet is known as C/2022 E3 (Z.T.F.) because astronomers discovered it in

March 2022 using a telescope on Palomar Mountain in California called the

Zwicky Transient Facility (or Z.T.F.).

At the time, the cosmic interloper was just inside the orbit of Jupiter and roughly

25,000 times dimmer than the faintest star visible to the naked eye. But Z.T.F.,

with a camera that has a wide field of view, scans the entire visible sky each

night and is well-suited to discover such objects.

What are comets, and why is this one green?

Comets are clumps of dust and frozen gases, sometimes described by

astronomers as “dirty snowballs.” Most are believed to originate from the

distant, icy reaches of the solar system where gravitational agitations

sometimes push them toward the sun — an interaction that transforms them into

gorgeous cosmic objects.

When they leave their deep freeze, the heat from the sun erodes their surfaces,

and they start spewing gases and dust until they host a glowing core, known as

the coma, and a flamelike tail that can stretch for millions of miles.

“They’re alive,” Laurence O’Rourke, an astronomer with the European Space

Agency, said. “When they’re far from the sun, they’re sleeping, and when they

get close to the sun, they wake up.”

C/2022 E3 (Z.T.F.), for example, is now glowing green because ultraviolet

radiation from the sun is absorbed by a molecule in the comet called diatomic

carbon — that is, two carbon atoms fused together. The reaction emits green


How bright will this comet be?

The brightness of comets can be unpredictable. When scientists first discovered

the object last year, they knew only that it had potential to be visible from Earth.

“Because each comet is its own living being, you don’t know how it’s going to

react until it passes the sun,” Dr. O’Rourke said.

Comet C/2022 E3 (Z.T.F.) made its closest approach to the sun on Jan. 12, and the

comet is now steadily brightening as it swings toward the Earth. While the

comet won’t pass us until Feb. 2, it is already nearly visible to the naked eye —

an encouraging sign for viewing opportunities, said Mike Kelley, an astronomer

at the University of Maryland and the co-lead of the solar system working group

at the Zwicky Transient Facility.

Still, seeing the comet could “require dark skies and an experienced observer,”

Dr. Kelly said.

In addition, comets can always surprise us. Sometimes there can be a big

explosion of gas and dust, and the comet might get suddenly brighter even after

it has left the sun behind.

How do I spot the green comet?

To catch the comet, look north.

On Jan. 21, the night of the new moon and thus the darkest skies, the comet will

be close to Draco — the dragon-shaped constellation that runs between the Big

Dipper and the Little Dipper.

Over the following nights, the comet will creep along the dragon’s tail. And on

Jan. 30, the comet will reside directly between the Big Dipper’s “cup” and

Polaris, the North Star. If you’re accustomed to finding the North Star by

following the two stars on the end of the Big Dipper’s cup, then you should be

able to spot the comet. Simply scan that imaginary line until you see a faint


If you’re struggling, the comet might still be too faint or there might be too much

light pollution. Try with a pair of binoculars.

“Even with relatively modest binoculars, the powdery, fuzzy or smoky character

of the ‘star’ ought to make it clear it’s a comet,” said E. C. Krupp, the director at

Griffith Observatory in Los Angeles.

A telescope will help you spot the colors and finer details, including the comet’s

glowing coma and lengthy tail.

For anyone living above the 35th parallel — imagine a curving East-West line

running from North Carolina through the Texas Panhandle out to Southern

California — the comet will be visible all night starting Jan. 22. But it is relatively

low on the horizon in the early evening, and it might be better to look for the

comet later in the evening or even early in the morning when the comet swings

higher in the sky.

Dr. Krupp recommends looking this weekend when the phase of the moon is new,

and it therefore won’t cast a glow over the sky. But the comet will become

brighter as it gets closer to Earth and will be easier to spot toward the end of the

month. If you wait until then, you might want to try early in the morning after

the moon has set.

Either way, the hunt will be fun.

“It’s sort of like searching for some endangered species, and then it pops into

view,” Dr. Krupp said. “That really is a charmer of an experience.”

Why are astronomers excited for this green comet?

Comets are relics of the early solar system and may have been responsible for

seeding early Earth with the building blocks for life.

“It really is a situation where we most likely would not exist without their

existence,” Dr. O’Rourke said.

And yet we don’t get many opportunities to study these objects, given that only a

few each year are bright enough to be seen with the naked eye. As such,

cometary astronomers across the globe will observe C/2022 E3 (Z.T.F.) over the

coming months.

“We’re looking for our solar system’s place in the universe,” said Dr. Kelley, who

will use the James Webb Space Telescope to observe the comet at the end of

February. He wants to better understand how our planet formed in order to note

the conditions that gave rise to life on Earth.

But Dr. Kelley and others have to work quickly. After a brief appearance in the

night sky, it’s unclear where C/2022 E3 (Z.T.F.) may go. Because these objects

are so loosely bound to our solar system, the sun’s gravitational influence might

force the comet to take another trip around our star — perhaps not returning for

another 50,000 years. Or the sun might fling the comet from the solar system


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