Categories: AstronomyCometsHubble

Here’s the Picture We’ve Been Waiting for. Hubble’s Photo of Interstellar Comet 2I/Borisov

Leave it up to the good ole Hubble Space Telescope. The workhorse telescope has given us a photo of the new interstellar comet 2I/Borisov. Take that, fancy new telescopes.

2I/Borisov has wandered into our Solar System from the deep cold of interstellar space, but nobody knows from whence it came, or how long it’s been travelling. Boris only the second object we’ve observed that’s come into our Solar System from somewhere else in the galaxy, and the Hubble snapped photos of it speeding along at about 177,000 kph (110,000 mph.) So far, the Hubble images are the sharpest ones yet.

Annotated image of comet 2I/Borisov. Image Credit:

Our first interstellar visitor was Oumuamua, which sped through our Solar System in 2017. But that object didn’t give up its secrets easily. It came and went and generated a lot of clicking and conjecture, and panicky headlines in some quarters. But it had not coma, and no tail, meaning it had no ice. But Boris is clearly a comet.

Artist’s impression of Oumuamua. It wasn’t a comet; it had no coma and no tail. But it was from somewhere else in the galaxy. Credit: ESO/M. Kornmesser

Comets contain a lot of water ice and other volatiles. When they get close enough to the Sun, some of that ice sublimates into gas, creating the characteristic coma and tail that is clear in many comet images. A coma and a tail are clearly visible in these Hubble images of 2I/Borisov.

“Whereas ‘Oumuamua appeared to be a rock, Borisov is really active, more like a normal comet. It’s a puzzle why these two are so different,” said David Jewitt of the University of California, Los Angeles (UCLA), in a press release. Jewiit is the leader of the Hubble team who observed the comet.

A classic image of probably the best known comet, Halley’s comet. The coma and tail are clearly visible in the image. 2I/Borisov has the same characteristics, while interstellar object Oumuamua did not. Image Credit: By The Yerkes Observatory Public Domain,

Of course it’s more than just a transient piece of curious eye candy. Boris will contain the same building blocks as any other bodies, including planets, in its home solar system. The fact that it’s behaving like comets in our own Solar System is intriguing too, though it’s too soon to assume too much as a result of that.

“Though another star system could be quite different from our own, the fact that the comet’s properties appear to be very similar to those of the solar system’s building blocks is very remarkable,” said Amaya Moro-Martin of the Space Telescope Science Institute, Baltimore, Maryland.

Hubble captured these images of Boris when it was 418 million km (260 million miles) from Earth. At its closest approach it’ll be two astronomical units away from the Sun, on December 27th, 2019. As it approaches and recedes, other telescopes will observe it, and we’ll learn more about it.

But we won’t have long. At 177,000 kph (110,000 mph) it won’t stick around for a portrait session. It’s going to fall past the Sun and in mid-2020 it’ll be as far away as Jupiter. Then it’ll disappear into interstellar space again. “It’s traveling so fast it almost doesn’t care that the Sun is there,” said Jewitt.

Amateur astronomers have discovered a lot of comets, and this one is no exception. Amateur astronomer Gennady Borisov, from Crimea, discovered this one on August 30th, 2019. More observations from other amateur astronomers followed (they all talk to each other, you know.) Professional astronomers got involved too, and eventually the IAU’s Minor Planet Center and JPL’s Center for Near-Earth Object Studies calculated the trajectory. That confirmed the object’s interstellar origins.

We know where most comets come from. They have two sources: the Kuiper Belt and the Oort Cloud, which is still hypothetical at this point. So to have an interstellar comet come barging through our Solar System means we need to update our encyclopedias.

Astronomers think there’s probably many more interstellar objects that’ll come through our neighborhood. In fact, there may be more passing through right now, but we just can’t see them. But we may see more of them once the Large Synoptic Space Telescope gets up and running in 2020. (It may be renamed the Vera Rubin Space Telescope by then.)

We may never know exactly where Boris came from. But we do know that there is a circumstellar disk of icy debris around young stars in other solar systems. There’s a lot of gravitational shenanigans going on those situations, before a solar system settles down. It’s possible that the chaos in young solar systems ejects comets and sends them out into interstellar space.

Artist’s impression of circumstellar disk of debris around a distant star. These disk are common around younger stars, and could be the source of interstellar comets like 2I/Borisov. Credit: NASA/JPL

But for now, astronomers will use Hubble to keep an eye on this one as it gets closer. It’s likely that as it gets closer to the Sun, there’ll be more activity and we can find out what it’s made of.

“New comets are always unpredictable,” said Max Mutchler, another member of the observing team. “They sometimes brighten suddenly or even begin to fragment as they are exposed to the intense heat of the Sun for the first time. Hubble is poised to monitor whatever happens next with its superior sensitivity and resolution.”

Thanks Hubble. Keep us informed, please.


Evan Gough

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