What Makes the Green Comet Green?

Professional and amateur skywatchers are tracking a "green" comet zooming through space that could soon be visible to the naked eye from Earth. But why does this comet and others like it often appear to be green?

Comet C/2022 E3 (ZTF) is currently located around 74 million miles from Earth and is set to reach its closest point to the sun, or perihelion, on January 12, 2023.

It will then make a close approach to our planet on February 1, coming within roughly 26 million miles of us. This is equivalent to more than 109 times the average distance between the Earth and the moon.

While the comet is not visible to the naked eye yet, it can already be seen using telescopes. Astronomers across the world have managed to capture images of the object, in which the comet appears to have a hazy, green aura around its nucleus. But why is this the case?

Comets are "cosmic snowballs" made up of frozen gases, dust and rock that orbit the sun. As they approach our star, these objects are blasted with increasing amounts of radiation, releasing gases and debris. This process forms a glowing atmosphere around the comet (known as a coma) and two vast tails of gas and dust.

The green color can be explained by the presence of certain compounds in the coma—primarily, diatomic carbon but also cyanogen.

"Under the effects of sunlight, both those substances glow green," Gianluca Masi, an astronomer with the Virtual Telescope Project—a service provided by the Bellatrix Astronomical Observatory in Ceccano, Italy that operates and provides access to robotic, remotely operated telescopes—told Newsweek.

Sunlight breaks down the diatomic carbon molecules into single carbon atoms before they can move into the comet's tail. This is why the green color is limited to the region surrounding the icy nucleus of the comet. The object's vast dust tail, in contrast, appears to be white.

Several recent comets also displayed such a green hue, including C/2007 N3 Lulin, C/2014 Q2 Lovejoy and C/2021 A1 Leonard, to mention just a few.

Excitement is growing around Comet C/2022 E3 (ZTF) given that it has the potential to be the first comet visible to the naked eye since NEOWISE flew past our planet in 2020.

Predicting the brightness of comets is notoriously hard but astronomers estimate that it could be faintly visible to the naked eye, under very good conditions, as soon as the second half of January.

It will reach peak brightness around February 1 and will then fade over the following days and weeks.

Some estimates indicate that the comet might have an orbital period—the time it takes for an object to orbit the sun—of at least 50,000 years. But the shape of its orbit means there is a possibility that it may never return to the inner solar system.