A 1-kilometre-wide comet will make its closest pass to Earth early on Thursday morning, February 2, giving Australians a small chance of catching a glimpse of what’s being referred to as the “green comet”.
Here’s what you need to know.
What is the ‘green comet’?
The green comet, or Comet C/2022 E3 (ZTF), was discovered by astronomers using the wide-field survey camera at the Zwicky Transient Facility in California in early March 2022.
The comet was closest to the Sun on January 12 and it will be the closest to Earth – 41.8 million km — on February 2.
Back in December, NASA said that, while the brightness of comets is notoriously unpredictable, C/2022 E3 (ZTF) could become only just visible to the eye in dark night skies at its closest point to earth.
According to The Planetary Society, the comet has a solar orbit of roughly 50,000 years, which means it hasn’t been seen in the night sky since Neanderthal times.
How can I see the ‘green comet’ in Australia?
It will be visible from Australia from February 5.
But the best times to see it will be between February 9-11.
However, while NASA has said the comet might be slightly visible to the naked eye, amateur star searchers should not expect a bright-green light show.
“It won’t be green to the naked eye, maybe with a substantial telescope, but it will mostly be through astrophotography,” UQ astrophysicist Dr Ben Pope told ABC News.
“Nearly everything in astronomy is basically white to the naked eye except, like, red giant stars and Mars appears a little red, Jupiter is quite noticeably yellow.”
Thanks to light pollution, even seeing the comet with the naked eye could be a problem, unless you’re in a very dark part of the country.
“Basically, they’re very faint, you get a lot of people who go outside and wait and wait, and say, ‘I didn’t see anything’, but that’s because you’re in the inner suburbs, even outer suburbs it’ll be hard,” Dr Pope said.
“It’s very hard to see unless you’re in a very dark space.”
Where C/2022 E3 (ZTF) will shine, is through astrophotography. Some US experts have already been able to capture its brilliant, green glow as it passed by the Northern Hemisphere earlier this month.
So keep your eyes peeled for some beautiful pictures in the days after the comet’s passing.
Why is the comet green?
Comets are made of a mixture of rocky materials, similar to what’s in the Earth’s mantle: dust and ices, not just water-ice but also components such as dry ice, methane, ammonia and carbon monoxide.
According to a report in the Proceedings of the National Academy of Sciences, the green glow from some comets comes from the breakdown of a reactive molecule called dicarbon (C2).
To prove their theory, researchers isolated the C2 molecule and blasted it with high-intensity light. What they found was that two light photons push the C2 molecule into an energy-rich, unstable configuration.
From there, the molecule decays and radiates a green light photon — just like what we see with some comets.
- January 30, 2023: Originally, this story said the comet would make its closest pass to Earth on Monday night, January 30. This has been amended to February 2 at 4:54am AEDT.
- January 30, 2023: Originally, this story said the comet would be visible in Australia from February 1 to 2. This has been amended to February 5. A line adding that the best time to see it will be February 9 to 11 has also been added.