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Twin comets are passing extremely close to the Earth. Here’s how to watch.

Flickr user akademy captured a shot of 252P in the Southern Hemisphere. It's the green glowing orb in the upper right.
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.

Two comets will make an incredibly close pass to Earth this week. And while NASA's official word is that they pose no threat to civilization (hurray!), their drop-by offers astronomers a rare chance to study the cosmic ice balls up close.

The closer (and smaller) of the two — a comet with the unsexy name BA14 — will pass 2.2 million miles from Earth on March 22.

The larger comet — with the equally unglamorous name of 252P — is reaching its closest distance today, March 21. It's 3.3 million miles away.

Those are near-historic, relatively close distances to Earth. NASA notes BA14 will mark the third-closest flyby of a comet on record. The second closest occurred in 1982, and the closest (aside from the one that killed all the dinosaurs) passed by in 1770, the Christian Science Monitor reports.

Though they will make a historically close pass, these ice balls will be many times further away than the moon. You can see that on this NASA illustration of the comets' orbits.

This is a rare opportunity for astronomers

BA14 was discovered just in January of this year. Astronomers originally thought it was an asteroid, until they photographed it and noticed a comet's tail. Quickly after it was confirmed as a comet, scientists noticed something strange. Its orbit was nearly identical to 252P, a comet that had been discovered in 2000, NASA reports. The two comets have such a similar orbit that it led astronomers to quickly hypothesize that they were originally one object. During the flyby, astronomers will use the Hubble Space Telescope in an attempt to confirm their suspicions.

"We know comets are relatively fragile things," NASA's Paul Chodas said in a press release. "Perhaps during a previous pass through the inner-solar system, or during a distant flyby of Jupiter, a chunk that we now know of as BA14 might have broken off of 252P."

The Hubble Space telescope will be aimed at the comets. Astronomers are hoping it will reveal details of their origins and compositions, and whether the pair are indeed twins.

Astronomers get excited about comets because they’re believed to be leftovers from the formation of the solar system. It's even possible that the chemicals and water trapped in comets are what seeded our water-and-life-filled world.

"When a comet does come this close to Earth it is something to get excited about, and take advantage of to learn whatever we can," University of Maryland astronomer Michael Kelley told the Los Angeles Times about this week's events. He added that there are more asteroids that come close to Earth than there are comets. This is a rare observational treat.

In this animation below, you can see the orbits of the two comets from last September to the next. The small blue dot is us, the planet Earth, and the green and white dots are the two comets.

How to spot the comets

While astronomers will aim their telescopes at these comets for observation, they'll be hard for the casual observer to spot.

While 252P is 100 times brighter than originally expected, it's still just barely visible to the unaided eye. (The light of a full moon this week won't help either.) Another confounding factor: It's only viewable in the Southern Hemisphere. (Universe Today notes it may be possible to see 252P in the Northern Hemisphere around March 28, if it continues growing brighter as it travels northward across the sky.)

BA14 is a lot smaller than its twin, so it will be much harder to view even with a telescope.

But this being the 21st century, you can get a glance of the comets by tuning in to this live stream. It starts up again at 5 pm Eastern Monday. If you're lucky, you'll catch 252P's spectacular green tail.

How we landed on a comet 300 million miles away

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