Watch: Once-in-a-lifetime green comet approaching closest point to Earth
A comet that was last seen around 50,000 years could become so bright it will be visible to the naked eye as it makes its closest pass to Earth on 1 February.
The glowing green comet C/2022 E3 (ZTF) was last visible from Earth during the Stone Age, and will make its closest approach to Earth this evening, scientists said.
The comet is already visible using a telescope or binoculars.
In a statement, Nasa said: "Observers in the Northern Hemisphere will find the comet in the morning sky, as it moves swiftly toward the northwest during January. It'll become visible in the Southern Hemisphere in early February.
"This comet isn't expected to be quite the spectacle that Comet NEOWISE was back in 2020. But it's still an awesome opportunity to make a personal connection with an icy visitor from the distant outer solar system."
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How to see the rare green comet from the UK
The best way to get a good glimpse of the comet is by allowing at least 15 minutes for your eyes to adjust to the dark, the Royal Observatory suggests.
That means avoiding looking at devices with bright screens, such as mobile phones, laptops and tablets.
Checking what time the moon will rise over your area online, and looking at the sky before it rises, will help prevent the moon's light from obstructing your view of the comet.
Many astronomy and sky tracking apps will allow you to track Comet C/2022 E3 (ZTF), and University of Toronto astrophysicist Hanno Rein has created one specifically for it – available for free on Apple's app store.
Those unable to watch the comet soaring through the sky can catch it online via The Virtual Telescope Project, which will stream the event on its YouTube channel from 4am GMT on 1 February.
What is the green comet?
The comet is thought to have come from the Oort Cloud at the edge of the solar system and was first spotted in March last year.
Astronomers believe that its orbit means it will go into deep space after its visit to Earth and the sun, marking the last opportunity to see it.
The comet was spotted by the Zwicky Transient Facility in California – hence ZTF in its name.
It made its closest approach to the Sun on 12 January and has been growing brighter since.
On 1 February, it will be closest to Earth.
Its green colour is caused by UV radiation from the sun illuminating gases emitting from its surface.
Nasa said: "Comets are notoriously unpredictable, but if this one continues its current trend in brightness, it'll be easy to spot with binoculars, and it's just possible it could become visible to the unaided eye under dark skies."