Explainer: What to expect during the green comet’s encounter with Earth

Explainer: What to expect during the green comet’s encounter with Earth

Jan 30 (Reuters) – A green-hued comet that has been lurking in the night sky for months is expected to be at its most visible to stargazers this week as it slowly passes Earth for the first time in about 50,000 years.

The cosmic visitor will swing by our planet at a distance of about 26.4 million miles (42.5 million km).

Here is an explanation of comets in general and this one in particular.


Comets, nicknamed “dirty snowballs” by astronomers, are balls of ice, dust and rock that typically originate from the ring of icy material called the Oort Cloud at the outer edge of our solar system. One known comet actually originated outside the solar system – 2I/Borisov.

Comets consist of a solid core of rock, ice and dust and are covered by a thin and gaseous atmosphere of more ice and dust, called a coma. They melt as they approach the sun, releasing a stream of gas and dust that is blown from their surface by solar radiation and plasma, forming a cloudy and outward tail.

Comets wander into the inner solar system when various gravitational forces expel them from the Oort cloud, and become more visible as they venture closer to the heat given off by the sun. Fewer than a dozen comets are discovered each year by observatories around the world.

This comet last passed Earth at a time when Neanderthals still inhabited Eurasia, our species extended its reach beyond Africa, large Ice Age mammals including mammoths and saber-toothed cats roamed the landscape and North Africa was a wet, fertile and rainy place.

The comet may provide clues about the primordial solar system because it formed during the solar system’s early stages, according to California Institute of Technology physics professor Thomas Prince.


The green comet, whose formal name is C/2022 E3 (ZTF), was discovered on March 2, 2022 by astronomers using the Zwicky Transient Facility telescope at Caltech’s Palomar Observatory in San Diego. Its greenish, emerald hue reflects the comet’s chemical composition – the result of a collision between sunlight and carbon-based molecules in the comet’s coma.

NASA plans to observe the comet with its James Webb Space Telescope (JWST), which could provide clues about the solar system’s formation.

“We’re going to look for the fingerprints of given molecules that we can’t get from the ground,” said planetary scientist Stefanie Milam of NASA’s Goddard Space Flight Center in Maryland. “Because JWST is so sensitive, we expect new discoveries.”


Using binoculars on a clear night, the comet can be seen in the northern sky. On Monday it appeared between the Big Dipper and Polaris, the North Star. And on Wednesday it was positioned to appear near the constellation Camelopardalis, bounded by Ursa Major, the Big Dipper and the Little Dipper.

Finding a remote location to avoid light pollution in populated areas is key to catching a good view of the comet as it travels past our planet on its way away from the sun and back into the solar system’s outer reaches.

Reporting by Joey Roulette in Washington; Editing by Will Dunham

Our Standards: The Thomson Reuters Trust Principles.

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