A highly anticipated comet, currently in its closest approach to Earth in 50,000 years, will finally be visible from Australian skies in the coming days. Here is a guide to spotting the rare celestial spectacle.
What is the comet?
C/2022 E3 (ZTF) – or the “green comet”, as it has been nicknamed – was only discovered by astronomers last March, at the Zwicky Transient Facility in California, and initially misidentified as an asteroid.
Comets are icy objects known as small solar system bodies, which orbit around the sun in elliptical trajectories. “We don’t see them that often,” said Dr Sara Webb, an astrophysicist at Swinburne University of Technology.
While Halley’s Comet is visible from Earth every 75 to 79 years, C/2022 E3 (ZTF) takes around 50,000 years to orbit the sun. “This is one of the ones that is literally a once-in-a-lifetime – once in almost a human civilisation – to look at,” Webb said.
The green comet’s closest approach to Earth was early in the morning of 2 February, Australia time. But the comet is crossing between the northern and southern sky, and will not be visible from most parts of Australia until 5 February, Webb said.
Comet orbits “are often off the plane,” she said. “What that means is that they come in on angles that don’t quite line up with where all the planets sit in the solar system.
“When they come into the inner solar system and work around the sun, they’re travelling incredibly fast. As they go back out towards the … colder, the more distant regions of our solar system, they tend to take a long time because that orbit is so elongated.”
Why is it called the ‘green comet’?
The comet has been nicknamed for its green glow – but its viridescent appearance is not unique. Webb says it results from different types of carbon being ejected from a comet as it warms.
“As they get close to the sun, [comets] tend to heat up,” Webb said. “They’ve got a lot of ice in them – the ice will heat up, it will boil and then the steam and bits and pieces will start falling off the comet. That’s what makes those beautiful long tails that are really unique to comets.”
When is the best time to view the comet?
Because of the direction C/2022 E3 (ZTF) is travelling, it should offer prime viewing in the southern hemisphere.
“It starts peeking above the horizon from about February 3, but it’s really low-lying,” Webb said. “On the 4th and 5th … it’s high enough that if you are living in suburbia you could probably see it in the sky.”
The best visibility will likely be from 7 February and 8, Webb says, with less light disruption from the moon, which will have started to set below the horizon. The comet will gradually dim as it gets farther away from Earth, but will appear very close to Mars on 11 February.
The comet should be visible between 10pm and midnight. “The best rule of thumb is to look towards the north,” Webb said. “If people look due north, and then once they spot it, it will be travelling in an upwards direction towards Mars, which looks like a big bright red spot on the sky.”
The comet should look like a “tiny little fuzzy blob”, with an appearance more elongated than a star.
Although the comet is travelling quickly, this movement won’t be noticeable to an observer unless you look at it over multiple nights. A comet’s appearance is distinct from the obvious motion of a meteor – a shooting star – entering Earth’s atmosphere, which is only visible for a few seconds.
Webb recommends binoculars or a telescope for clear viewing. “It’s not going to be glaringly obvious. It’s going to be about the same magnitude as some of the fainter stars that we see twinkling in the sky.”
If you’re trying to spot it with the naked eye, you can maximise your chances by going somewhere dark, away from light pollution.
Viewing information specific to different locations can be found at Stellarium Web, a free online planetarium.