Don't miss this once-in-a-lifetime green comet

With their unique shapes and long, brush-stroke-like trails in the sky, comets have always been a source of wonder. It’s always a special event when they appear — maybe you saw NEOWISE back in 2020 or even Halley's in 1986. 

This week, the spotlight is on comet C/2022 E3 (ZTF) — isn't that a name to reckon with? But don't let the dull name fool you, there's a reason behind it. We'll get to that soon. 

On Wednesday, the green comet flew by Earth, getting as close as 42 million kilometers (26 million miles) to the surface. Now it's slowly moving away from us. 

If you're in a place with clear skies and no light pollution, you might still be able to catch a glimpse of it via the naked eye, although binoculars are recommended. 

If you are in the Northern Hemisphere, look north, close to the Big and Little Dipper constellations. For those in the Southern Hemisphere, you can try looking north, but it might be too close to the horizon for you to catch it.

Last chance

This could be the last time C/2022 E3 (ZTF) makes itself visible, ever.  

Unlike Halley's Comet, which returns every 75 years, C/2022 E3 (ZTF) might not return. It was last seen over 50,000 years ago.

NASA Jet Propulsion Laboratory calculations point to the comet being on a different path around the sun than periodic comets like Halley. Instead of following a predictable, oval-shaped orbit, C/2022 E3 (ZTF) seems to be on a more open, no-return parabolic route. 

These calculations are based on rather limited data as we have only been tracking the comet since it was first discovered in March 2022.

Whether the comet leaves the solar system altogether or returns in thousands of years depends on how it will be impacted by other objects in space. 

Comet names 

Comet names sound like a jargony mess of letters and numbers, but they reveal a lot of information. 

The first letter tells us if it's a periodic comet that'll come back within 200 years (P), or if it’s more than 200 or just a one-time visitor (C) like C/2022 E3 ZTF. 

"C" comets, like the one hovering over us, can have orbits around the sun lasting thousands or even millions of years. Sometimes there's no guarantee they'll ever return.

The following number (2022) is the year of discovery. E refers to when in that year the comet was first spotted — scientists divide every month of the year in two and assign a letter to each of the 24 halves. "A" is reserved for the first half of January, then "B" for the second half, "C" for the first half of February, and so on.

So "E" refers to the first half of March. The "3" tells us it was the third comet discovered within that specific time frame. Finally, the "ZTF" refers to what scientists used to discover the comet, the Zwicky Transient Facility (ZTF), which scans the entire northern sky every two days. 

Comet composition 

Comets like C/2022 E3 (ZTF) "originate from a region of the outer solar system called the Kuiper belt or from a region called the Oort cloud," Charlotte Götz, who studies comets in Northumbria University in Newcastle, England, told DW.

Comets are massive chunks of gas, dust and tiny rocks. When they get close to the sun, they heat up and start evaporating, forming a glowing halo and sometimes a mesmerizing tail of gas and dust.

Comets usually have two tails: One that consists of gas and points away from the sun, and another that consists of dust and bends slightly in the direction opposite the comet's motion. 

Depending on its components, a comet can have different colors in its bright halo and tails, as they get excited and produce light when exposed to sunlight. If it weren't for its glowing gas and dust, comets would be as dark as coal, because their icy cores barely reflect any light.

C/2022 E3 (ZTF)'s halo might be green because it’s emitting diatomic carbon (C2), a gas made up of two carbon atoms bound together.

Edited by: Clare Roth