Our Newest Celestial Visitor Is a Truly Gigantic Comet

There is a truly enormous new comet sailing in from the Oort Cloud. It’s always exciting to entertain a new celestial visitor, and this massive Trans-Neptunian Object (TNO) is causing a stir among astronomers worldwide.

First of all, it’s huge. There have been several big, bright comets in the last few decades: Shoemaker-Levy 9 was 1.8km long, Halley’s comet was 15km or so, Hale-Bopp was 40-80km. This one tops all of them by a long shot. Comet Bernardinelli-Bernstein is much larger than Mars’ largest moon. In fact, you could lay out both of Mars’ moons, Phobos and Deimos, end-to-end long-wise and the comet would still be twice the size of the two put together. It’s so big we originally tagged it as a minor planet. A preprint study on arXiv gives a low-end estimate that’s still 10 times the mass of Hale-Bopp, and Hale-Bopp became known as the Great Comet of 1997.

NASA is monitoring the new comet, so there will definitely be better and better portraits of this celestial visitor as time goes by. It is expected that this comet will be in the sky for some years, and we will have ample time to observe it. Scientists currently project that Comet Bernardinelli-Bernstein will make its closest approach to the sun in 2031.

The new comet is also really far away. When it was first discovered, it was the most distant comet we’d ever seen. Its orbit is incredibly elongated: from perihelion to aphelion, Bad Astronomy calculated that it goes from “about 1.6 billion km from the Sun (just outside Saturn’s orbit*) to a mind-numbing 2 trillion km out. That’s a fifth of a light-year!” (emphasis original)

Despite its distance from Earth, we can already see that the comet has begun to put on a show. Recently, the Las Cumbres Observatory reported an increase in the comet’s brightness. Data from TESS confirms it. Bernardinelli-Bernstein is getting brighter, month by month. Scientists monitoring the comet suspect that it has begun to develop a halo or tail. It’s still far enough away that our images are blurry. Based on measurements of its path, though, we’re certain of a few things.

This comet has such a wildly distorted elliptical path that it can’t pose any threat to Earth. It’s 90 degrees opposed to the orbital plane in our solar system, and the closest it’s expected to get to the Sun is still about as far out as Saturn’s orbit.

We can also tell something about the comet’s nature, even through these early observations. Based on the way the comet’s coma is expanding relative to how far it is from the Sun, we can make educated guesses about what this giant space snowball is made of. It would be an unpleasant snowball, though: the data suggests that the comet is made of ammonia ice and frozen CO2. The scientists who submitted the arXiv report even suggested that it might have a “dirtmosphere,” because the amount of dust we see in its halo is growing exponentially. Yuck.

Unfortunately, because it will still be quite a ways out from the sun when it reaches its perihelion, it probably won’t be visible to the naked eye. But as we watch it from earthbound and orbiting telescopes, we’ll begin to see it more clearly and understand it better. I can’t wait to see beauty shots of this thing. Stay tuned for updates!

Feature image credit: NOIRLab/NSF/AURA/J. da Silva (Spaceengine)

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