Astronomers describe the current celestial event as a "rare and thrilling opportunity" for sky-watchers to witness a comet with their naked eyes, characterizing it as a "once-in-a-lifetime" experience.

The comet Nishimura was discovered only in August but will be closest to Earth in a week's time – visible in Northern Europe just before dawn on Tuesday, Sept. 12.

The object, which is traveling through space at 386,000 kph (240,000) mph), is already visible to the naked eye, according to professor Brad Gibson, director of the E.A. Milne Center for Astrophysics at the University of Hull.

Gibson said Nishimura can be seen in the hour after sunset and the hour before dawn by looking east-northeast toward the crescent moon and Venus.

He said: "The comet takes 500 years to orbit the solar system, Earth takes one year and the outer planets can take many decades.

"Halley's Comet, which caused much interest during its last nearby visit to Earth in 1986, takes 76 years to orbit the solar system. So, to say this is a once-in-a-lifetime opportunity to see Nishimura isn't an exaggeration."

Gibson said: "It can already be seen, but it will be 78 million miles from Earth on Sept. 12 and that should be the best chance to see it with the naked eye.

"On average, people have the chance to see such a naked-eye comet once a decade – this is a rare and exciting opportunity."

Comet C/2023 PI is named after Japanese astrophotographer Hideo Nishimura, who recorded it when he was taking long-exposure photographs of the sky with a digital camera on Aug. 11.

Gibson said Nishimura will pass closest to the sun on Sept. 17, when it will be just 27 million miles away.

He said there is a real chance it may not survive this close fly-by.

Scientists are still trying to estimate Nishimura's size, but Gibson believes it could range from a few hundred meters to potentially several kilometers in diameter.

He said it is thought the comet could be responsible for an annual meteor shower named the Sigma-Hydrids, which takes place in December every year.

The professor said comets are "chunks of ice and rock" left over from the formation of the solar system nearly 5 billion years ago.

As they pass closer to the sun, it heats the comet, liberating an icy gas that gives them their distinctive tail.

He said tiny particles of dust and rock from comets are freed by the sun as a comet passes nearby and each year, the Earth passes through this debris, leading to meteor showers.

According to Gibson, there is no danger of Nishimura colliding with Earth as astronomers have carefully charted its orbit and speed of travel.

There is a debate between scientists over whether it was an asteroid or a comet that caused the extinction of dinosaurs 65 million years ago.

He said: "What happened to the dinosaurs is a once-in-a-100-million-year event.

"People have been watching comets since ancient times with their interpretation then spanning everything from portents of doom to simply being heralds of good news."