Interstellar Comet Borisov is no longer in one piece

Imagery of interstellar Comet Borisov captured by the Hubble Space Telescope suggests that a piece broke off the object’s nucleus between March 23 and March 28, 2020. (The middle photo was taken with a different filter than the two on the sides, explaining its different appearance.)
Imagery of interstellar Comet Borisov captured by the Hubble Space Telescope suggests that a piece broke off the object’s nucleus between March 23 and March 28, 2020. (The middle photo was taken with a different filter than the two on the sides, explaining its different appearance.) (Image credit: NASA/ESA/Hubble/STScI/Jewitt et. al)

Our solar system's second known interstellar visitor doesn't seem to be in one piece anymore.

Photos of the interstellar Comet Borisov taken by NASA's Hubble Space Telescope on March 28 and March 30 show an elongated nucleus that appears to have two distinct components, astronomers announced in a note Thursday (April 2). That's a very different situation than the one Hubble observed on March 23, when the nucleus was a single entity.

These two pieces are about equally bright, but that doesn't mean they're of equivalent size, said UCLA astronomer David Jewitt, who led the new observations. 

Related: Interstellar comet: Here's why it's got scientists so pumped up

"Actually, it turns out almost never to be that case," Jewitt told, referring to similar observations of native-born comets. 

"Usually, the main nucleus drops off a piece, and the piece is small compared to the main nucleus — it contains a tiny fraction of the total mass," he said. "But, because it was just plucked out of the nucleus, it's pretty icy. And the ice fizzes and sublimates like crazy, making it a good producer of dust." 

Dust is what Hubble is picking up in these photos. So, it's unlikely that Borisov is breaking apart in a significant way, Jewitt added. He estimated that Borisov may have just lost 0.1% to 1% of its total mass, comparing the probable scenario to a car that just dropped a side mirror. 

Still, the researchers don't know for sure that this is the case; it's possible that something more dramatic just happened. Jewitt and his colleagues hope to figure it out, and additional Hubble observations could do the trick.

For example, further Hubble imagery could show researchers how fast the shed fragment is accelerating away from the main nucleus and how long it remains observable. Both of these data points would provide clues about the size of the newly liberated shard, Jewitt said. 

NASA's Hubble Space Telescope captured this view of the interstellar object Comet 2I/Borisov  on Oct. 12, 2019. (Image credit: NASA/ESA/D. Jewitt (UCLA))

Comet Borisov was detected in August 2019 and made its closest approach to the sun in December. It's not necessarily surprising to see a chunk fly off the comet now, four months after this perihelion passage, Jewitt said; the timeline is consistent with two prominent models of comet shedding.

In one of those models, comets begin spinning faster after close solar approaches because the heating they experience causes significant, and asymmetrical, mass loss. It may take several months for the spin-up to get dramatic enough for pieces to go flying off into space, Jewitt said.

In the other model, this same heating causes pressure to build up inside comet nuclei. Eventually, the pressure reaches a pocket of supervolatile ice, such as carbon monoxide or carbon dioxide, resulting in a blowout. And, again, this may take some time. (But comets may be too leaky for this explanation to be viable in most circumstances, Jewitt said.) 

There's also the possibility that Comet Borisov got hit by something. But an impact is unlikely to be the explanation, Jewitt said, given how empty space is and how strange Borisov's orbit is compared to objects that were born in our solar system.

Borisov is the second known interstellar body ever spotted in our solar system. The first was the mysterious object 'Oumuamua, whose multilayered weirdness has prompted speculation that it may be an alien spacecraft of some sort.

'Oumuamua was already barreling toward the outer solar system when it was detected. Borisov had not yet rounded the sun in August 2019, meaning astronomers have been able to track it for longer stretches and in greater detail. 

This tracking, however, has been compromised recently by the coronavirus pandemic, which has shut down many ground-based telescopes

"It's entirely possible that the virus is going to kill the comet for most astronomers," Jewitt said.

But Hubble is still operating despite the outbreak, and this iconic eye in the sky is well suited to observe Borisov. Indeed, even the biggest ground-based scopes don't have the resolution required to detect the recent shedding event, Jewitt said.

"So, really, only [the Hubble] Space Telescope can see this sort of thing," he said.

Mike Wall is the author of "Out There" (Grand Central Publishing, 2018; illustrated by Karl Tate), a book about the search for alien life. Follow him on Twitter @michaeldwall. Follow us on Twitter @Spacedotcom or Facebook

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Michael Wall is a Senior Space Writer with and joined the team in 2010. He primarily covers exoplanets, spaceflight and military space, but has been known to dabble in the space art beat. His book about the search for alien life, "Out There," was published on Nov. 13, 2018. Before becoming a science writer, Michael worked as a herpetologist and wildlife biologist. He has a Ph.D. in evolutionary biology from the University of Sydney, Australia, a bachelor's degree from the University of Arizona, and a graduate certificate in science writing from the University of California, Santa Cruz. To find out what his latest project is, you can follow Michael on Twitter.

  • dfjchem721
    It has been suggested that these interlopers from beyond our solar system are actually aliens visiting our neighborhood. So this "break-up" may actually be two parts of an alien space ship separating and beginning their mission of conquest. Its appearance as a "comet" is stealth technology to lull us into a false sense of security.

    Let's hope the SOBs are sensitive to the coronavirus!
  • Lord Hung
    You might be on to something. It's a strange coincidence that the Corona virus started right when this came by, quite a coincidence...
  • Catastrophe
    Seems quite the fashion!


    Remember this one?
  • dfjchem721
    And this could have been another atmospheric probe from one of their scout ships:

  • rod
    Okay, here is another report on the breakup, with more astronomical detail :), 'Possible fragmentation of interstellar comet 2I/Borisov'

    Now let us explore the alien spaceship connection, a scout ship. How many confirmed exoplanets? 4241, The Extrasolar Planets Encyclopaedia Question: if this is an alien scout ship, we have 4241 exoplanets, which one did it come from?

    Question. How long did it take for the scout ship to travel just 4 light-years to reach our solar system? At a constant velocity of 30 km/s, the trip is 40,000 earth years.

    Question: is flat earth science and astronomy more secure than reports of 2I/Borisov as an alien spaceship?

    Hale-Bopp was a spectacular comet visible but some, claimed an alien space ship with that comet too with bad results.,
  • dfjchem721
    I understand your interest in their origins rod, but they seem academic if this is an invasion of the earth by what can only be a hostile force of advanced aliens. If they were going to be friendly, they would have stopped by and said howdy, and maybe try for some give and take. But no, they just sail through, scan our systems capabilities and then plan for the attack. Where they came from is academic.

    Seems we need to plan for this alien invasion. We need to assemble all the highest tech weapon systems, like scram-jets mounted with maximum yield nukes for an minimally effective response. It will at least show them we are not helpless. Kinetic high speed rail-guns and any lasers they have been working on need to be brought in and give them hell. We might be able to bluff our way out of this if they think we are too advanced for their risk/reward. Some of these aliens could be wimps. Look at the Ferengis. They wouldn't hang around after one nuke went off.........We can only hope they are of a similar character, or coronavirus will look like a hangnail.
  • COLGeek
    Maybe just a change in aspect angle or simply the movement of chunks of the comet.

    If aliens, we are in big trouble. Any species advanced enough to travel the stars is going to consider us a nuisance or something to poke at with a stick, at best.
  • dfjchem721
    Getting back to the more mundane but probable nature of this object, this "comet"* has an estimated size of ≤0.5km, so it is not very big. Still COLGeek's notion that this could simply represent a perspective issue is valid without further data. I would expect an increase in the brightness if this is disintegrating since that would expose more surface area to "ablation", and that was not reported.

    All prior comets are believe derived from the solar system. If this object is truly from inter-stellar space, it should be referred to as an interstellar object with cometary appearance. I don't think it should be called a comet. I see from Wiki that they agree. This object is now known as 2I/Borisov, indicating the second interstellar visitor.
  • rod
    FYI, glad to see the Wikipedia report cited dfjchem721 :). 2I/Borisov has spectral image too, similar to comets already observed and documented in our solar system. "A preliminary (low-resolution) visible spectrum of 2I/Borisov was similar to typical Oort Cloud comets. Its color indexes also resemble the Solar System's long period comets."

    FYI, 2I means the 2nd interstellar comet observed now, I for interstellar like 1I/2017_U1, 'Oumuamua.

    2I/Borisov has a fuzzy, coma shape too. Very common optical observation of comets using telescopes. My stargazing log records 52 comet sightings I made going back to April 1997 and April 1996, Hale-Bopp and Hyakutake. The most recent comet I observed is C/2019 Y4 (Atlas) on 30-March-2020 using my 10-inch Newtonian. From my stargazing log -

    "Observed 2000-2200 EDT. Sunset 1929 EDT. I was able to view the comet again tonight but this time using the XT10i telescope. Very good views using 35-mm PO, 32-mm plossl, and 14-mm Delos eyepieces. Early in the evening I used the 35-mm PO and viewed Venus. Brilliant nearly half-moon shape, poked me in the eye :) This was a lovely early spring evening outdoors. I observed 3 polar orbiting satellites while I viewed the comet. 2 with unaided eyes, one passing through Auriga about as bright as Capella, the other passed across the FOV while I viewed the fuzzy coma of the comet. Not very bright but more distinct brighter core at 86x using the 14-mm Delos. Plenty of stars visible in the FOV with the comet too and using the 35-mm PO, I could see HIP40215 star and the comet in same FOV near 34/35x and 1.8-degree true FOV. The comet and star a bit more than 1-degree apart in angular separation near 2030 EDT or 0030 UT. While I observed in the east pasture, the waxing crescent Moon lit up the fields, I was bathed in moonlight. Very ethereal observing like this. Venus below the Pleiades was quite a sight in western sky tonight. Orion out and Leo."

    I will stay the course charted by astronomy that 2I/Borisov is indeed a comet and not aliens visiting our solar system, plotting an invasion :)--Rod
  • Catastrophe
    IF there are aliens in the area (which I doubt) they are clearly technologically our superiors and have an ulterior motive for being here.
    They will totally ignore our presence, quickly and efficiently collecting from Antarctica the ice they need for their G and Ts, and departing happily to the next item in the Galactic Zoo.

    Cat :)