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A comet is coming unusually close to Earth this weekend. Here’s how to watch.

How — and where — to spot comet Wirtanen as it comes within 7.1 million miles of the Earth.

Comet 46P Wirtanen seen through a telescope.
Brian Resnick is Vox’s science and health editor, and is the co-creator of Unexplainable, Vox's podcast about unanswered questions in science. Previously, Brian was a reporter at Vox and at National Journal.

This Sunday, December 16, a comet called 46P/Wirtanen is going to come within 7.1 million miles of the Earth (about 30 times the distance of the Earth to the moon). While that may sound extremely far away, it’s actually close enough for us to potentially see its green-hued tail with the naked eye.

According to NASA, Wirtanen’s flyby will be one of the top 10 closest flybys of a comet to the Earth in 70 years. The University of Maryland’s astronomy department — which is leading a scientific observation campaign of the comet — assures us there’s “no chance of the comet hitting Earth.” So look on in amazement, not horror.

It’s hard to predict the exact brightness, and thus visibility, of a comet. Comets are balls of ice and rock that form tails of debris as they approach the sun. (The sun melts some material off the comet.) But the quantity of material in the comet’s tail is not stable.

Sometimes a comet will have more material in its tail to reflect sunlight and appear brightly in our night sky. Sometimes it won’t. It can also be hard to predict the spread of a comet’s tail. The more spread out the tail, the more diffuse the comet becomes.

Regardless, “even if it does not reach naked eye brightness, it will still be a great object to view with binoculars or a small telescope,” the University of Maryland explains. Wirtanen, named after its discoverer, the astronomer Carl A. Wirtanen, is a “hyperactive” comet, meaning it produces more water in its tail than most comets its size. That makes it more likely to be bright. In any case, try to find the darkest spot you can with this dark sky finder app.

The path of comet 46P/Wirtanen

On the night of December 16, you can look for Wirtanen near the constellation Taurus (the bull) high up in the Southeastern sky. I recommend downloading a smartphone astronomy app like Sky Guide to know exactly where to look in your area. These apps use your phone’s GPS to show you where objects are in the night sky.

Sky Guide

And here’s how it will move across the sky over the next several nights.

The comet passes by Earth every 5.4 years. But this weekend might be the best time to view it in a lifetime. “This will be the closest comet Wirtanen has come to Earth for centuries and the closest it will come to Earth for centuries,” Paul Chodas, a scientist at NASA’s Center for Near-Earth Object Studies, said in a press statement.

Scientists are excited to observe the comet as it approaches. It’s a comet that regularly comes close enough to the Earth that one day we could send a spacecraft to rendezvous with it. (It was actually the original target of the Rosetta mission launch by the European Space Agency. Rosetta ended up orbiting the comet 67P/Churyumov–Gerasimenko.)

Comets are believed to be leftover material from the formation of our solar system. Understanding them better helps us better understand how star systems form.

And if you miss the comet on Sunday, don’t fear. It’s should also be visible for a few more nights.

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