Exotic green comet not seen since stone age returns to skies above Earth

An exotic green comet that has not passed Earth since the time of the Neanderthals has reappeared in the sky ready for its closest approach to the planet next week.

Discovered last March by astronomers at the Zwicky Transient Facility at the Palomar Observatory in California, comet C/2022 E3 (ZTF) was calculated to orbit the sun every 50,000 years, meaning it last tore past our home planet in the stone age.

The comet, which comes from the Oort cloud at the edge of the solar system, will come closest to Earth on Wednesday and Thursday next week when it shoots past the planet at a distance of 2.5 light minutes – a mere 27m miles.

Comets are balls of primordial dust and ice that swing around the sun in giant elliptical orbits. As they approach the sun, the bodies warm up, turning surface ice into gas and dislodging dust. Together, this creates the cloud or coma which surrounds the comet’s hard nucleus and the dusty tail that stretches out behind.

Images already taken of comet C/2022 E3 reveal a subtle green glow that is thought to arise from the presence of diatomic carbon – pairs of carbon atoms that are bound together – in the head of the comet. The molecule emits green light when excited by the ultraviolet rays in solar radiation.

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Astronomers armed with telescopes have captured stunning pictures of the comet in the past month, showing the body’s head, dust tail and the longer, more tenuous ion tail.

But the cosmic ice ball has recently become bright enough to see with the naked eye, at least in very dark, rural areas with minimal light pollution.

Since mid-January, the comet has been easier to spot with a telescope or binoculars. It is visible in the northern hemisphere, clouds permitting, as the sky darkens in the evening, below and to the left of the handle of the Plough constellation.

It is heading for a fly-by of the pole star, the brightest star in Ursa Minor, next week.

The window for spotting the comet does not stay open long. While the best views may be had about 1 and 2 February, by the middle of the month the comet will have dimmed again and slipped from view as it hurtles back out into the solar system on its return trip to the Oort cloud.