A comet not seen since early Homo sapiens roamed the Earth is making its way across the sky - and humans have a once-in-a-lifetime chance to see it this week.
Comet C/2022 E3 (ZTF), also known as the 'Green Comet', will make its closest pass to our planet this Wednesday. And like other comets that have long-orbital periods, it doesn’t come around very often.
In fact, the ‘Green Comet’ last paid a visit to the inner Solar System 50,000 years ago, around the time Stone Age humans were thought to have first begun developing language.
And due to the nature of its orbit, it may never visit the inner Solar System again - meaning this could be humanity’s last chance to set eyes on Comet C/2022 E3 (ZTF).
What do we know about Comet C/2022 E3 (ZTF) - and why is it green?
The comet was discovered in March 2022 by astronomers conducting a wide-field sky astronomical survey at the Zwicky Transient Facility in California in the United States.
Since then it has brightened substantially and can be spotted in the sky from the northern hemisphere early in the morning.
It already made its closest approach to the Sun, known as its perihelion, on January 12, while its perigee (its closest pass of Earth) will occur on Wednesday (February 1).
It became brighter as it passed the Sun, as the star’s radiation heated it up, creating a green coma - the hazy envelope of gas from heated materials such as water, carbon dioxide and carbon monoxide.
It also has a short broad dust tail, and a longer, faint ion tail, created when the solar wind interacts with the comet’s coma.
The green glow is a result of UV radiation from the sun lighting up the gases that are streaming off the comet’s surface.
C/2022 E3 (ZTF) is thought to have come from the Oort Cloud, a region around the outer solar system that is thought to contain billions of comets that are mostly in a stable orbit around the Sun.
How to spot the ‘Green Comet’
It can currently be viewed in the night sky from the Northern Hemisphere with a telescope or binoculars.
However, as it gets closer, it's already reported to be visible to the naked eye, against very dark skies, according to the Royal Observatory in Greenwich, London.
When it makes its closest approach to Earth on Wednesday, the comet will be around 42 million km away from our planet, and also visible from the Southern Hemisphere.
But NASA cautions that comets - and their visibility - are notoriously unpredictable.
You can find out where in the sky the comet will appear by using a stargazing app.
On Wednesday, it's expected to appear near the constellation Camelopardalis, bordered by Ursa Major, the Big Dipper and the Little Dipper.
To avoid disappointment when trying to see it in the night sky, ensure there won’t be too much cloud cover, ensure you have a view of the correct part of the sky from your vantage point, and wrap up very warm before heading out, the observatory said.
It added that you should allow your eyes to adapt to the darkness for at least 15 minutes, and avoid looking at mobile phones or other bright devices so your eyes remain sensitive enough in the dark.