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C/2019 Q4 (Borisov): A Likely Interstellar Comet

What appears to be an interstellar comet is heading into the Solar System, with perihelion likely on December 10 of this year, a date that could change as orbital parameters continue to be firmed up. The natural comparison is with ‘Oumuamua, first discovered two years ago and now well on its way out of the system. But the object first labeled gb00234 and now carrying the provisional name C/2019 Q4 (Borisov), while clearly on a hyberbolic orbit, has been found before perihelion and should be visible for a much a longer period of observation and orbital calculation.

Image: Observations suggest that comet C/2019 Q4 (Borisov) may be from outside the Solar System. A hyperbolic solution for the object first labeled gb00234 passes between Mars and Jupiter. (Green=gb00234; Blue=Neptune). Credit: Tony873004 – Own work, CC BY-SA 4.0.

A professional optician and astronomer named Gennady Borisov at the Crimean Astrophysical Observatory (near the Crimean city of Bakhchysarai, on the Crimean peninsula) discovered the object on August 30, 2019 when it was at a distance of approximately 3 AU. This gentleman builds his own instruments, as physicist and radio astronomer Marshall Eubanks (VLBI) noted in a tweet this morning, replying to writer and journalist Corey Powell.

The Minor Planet Center (Smithsonian Astrophysical Observatory) now offers a Minor Planet Electronic Circular (MPEC) on C/2019 Q4 (Borisov) that notes the cometary nature of the object has been confirmed by ‘numerous observers.’ It also includes the news that the object will be visible for a good while. The appearance of the MPEC indicates sufficient data have been accumulated to make the call on the hyperbolic nature of the orbit.

Based on the available observations, the orbit solution for this object has converged to the hyperbolic elements shown below, which would indicate an interstellar origin. A number of other orbit computers have reached similar conclusions, initially D. Farnocchia (JPL), W. Gray, and D. Tholen (UoH). Further observations are clearly very desirable, as all currently-available observations have been obtained at small solar elongations and low elevations. Absent an unexpected fading or disintegration, this object should be observable for at least a year.

Image: C/2019 Q4 (Borisov), in the center of the image. Note what appears to be a short tail extending from the coma. Credit: Gennady Borisov.

What is striking here is the eccentricity, which at 3.08 (a figure worked out from 145 observations to this point) is clearly hyperbolic. Unlike ‘Oumuamua, we have a clear tail and a distinct coma, strengthening the object’s identification as a comet. Like ‘Oumuamua, its hyperbolic orbit is one that is unbound to the Sun, thus making this an interstellar object whose spectra will be examined with great interest. The object’s provisional designation will likely change, as did ‘Oumuamua’s, to something like object 2I/2019, making it the second in the category of interstellar objects, but clearly not the last as we tune up our methods.

Based on the orbital solutions now in play, C/2019 Q4 (Borisov) should close to within about 1.8 AU of the Sun before beginning its exit from the Solar System, allowing observatories all over the world to study it. While ‘Oumuamua presented puzzles that are still being resolved, the cometary nature of this object is becoming established and we will have plenty of time to study it. Remember, too, that the Large Synoptic Survey Telescope (LSST) team anticipates first light in 2020. Its planned surveys are likely to turn up interstellar objects in large numbers.

Particular thanks to Jean Schneider (Observatoire de Paris) for early thoughts on C/2019 Q4 (Borisov), as well as numerous readers who checked in via email.


Comments on this entry are closed.

  • Hamilton1 September 12, 2019, 9:52

    I remember in the early days of ‘Oumuamua, there were immediate attempts to try and trace its point of origin by backtracking the orbit. I haven’t seen any speculation as to where this object may have come from however.

    • Bruce D. Mayfield September 12, 2019, 13:36

      From Bob King’s report in the Sky and Telescope link shared by ljk below:
      “Assuming the comet’s hyperbolic orbital solution holds, the comet appears to be coming from the direction of the galactic plane in Cassiopeia. Tonight (September 11-12), the object is located about 1° northeast of Iota (ι) Cancri and moving slowly to the southeast.”

      • Hamilton1 September 12, 2019, 17:33


        I find the different values for eccentricity quite surprising, even after 13 days of observations. Yesterday’s MPEC gave a value for e of 3.07 but the most recent calculation from JPL has raised this to 3.64. ‘Oumuamua’s degree of eccentricity was pinned down much more quickly than this, despite it being several times fainter.

        • Michael C. Fidler September 13, 2019, 14:29

          One other point about ‘Oumuamua that was suprising was that it was just sitting in space in galactic coordinates and the solar system flew by it. C/2019 Q4 (Borisov) seems to be moving thru space and that may give a clue as to where it came from. Being a fuzz ball and much further away then ‘Oumuamua is the reason the eccentricity is harder to pin down.

  • ljk September 12, 2019, 10:46

    Some useful links on our newest visitor, Interstellar “Comet” Borisov:

    MPECS page:


    Twitter updates:


    There is already a Wikipedia page for our visitor:


    Sky & Telescope’s take on our new visitor. It seems to have a tail:


  • Alex Tolley September 12, 2019, 14:24

    While only the 2nd interstellar object, we have a start on the Bayesian priors we need. When we have a few more, we will have a better idea of their frequency and likely nature. Rare or not. Comets rather than alien craft.

    Comets might also be a possible target to look for bacterial spores. Picking them up in the host system, releasing then in their tails in the target system and picking up new spores. Sampling these comets to determine their composition and any possible life would be our best chance to sample material for another system within the near future.

    • Robin Datta September 12, 2019, 21:48

      Looking for spores in the comet’s tail would be an effort with (presumably) a low-probability but ENORMOUS, world-view shaping payoff, immortalizing the name Borisov in human history.

      Some of the asinine government spending and misguided corporate efforts should perhaps (but almost certainly won’t) be redirected towards this.

    • ljk September 13, 2019, 9:48

      Borisov may be moving too fast for us to get a probe to it directly any time soon, but if it is a comet and it has a tail shedding particles into our neighborhood, we could send a probe to examine, collect, and return some of those tail particles it leaves behind!

      • Alex Tolley September 13, 2019, 11:28

        Exactly! We don’t need to chase the comet with a craft that has teh same velocity and trajectory. We can instead fly a craft through the tail. The craft might be something like the Stardust mission that collected dust from the coma of Comet Wild 2. The tail material is obviously less dense than the coma, so perhaps the aerogel sampler might have to be somewhat larger to compensate.

        The capture of rock and volatile (if possible) material would be the first, invaluable samples from another star system.

        Probes like this, on rockets ready to launch, would be the way to go to get at these, and other, transient objects.

        • Michael C. Fidler September 13, 2019, 17:15

          Just a thought, would there be any way to vacuum up the dust and bugs? Vacuuming in a vacuum? ;-)

  • William Woelke September 12, 2019, 15:11

    Will this be bright enough to be interesting or another disappointment?

    • ljk September 13, 2019, 9:26

      How will only the second interstellar object moving through our star system be a disappointment?

  • ljk September 12, 2019, 16:21

    NASA report on Borisov:


    The object will pass about 10 million kilometers from Mars. Perhaps one of our orbiters there (MRO) could examine it?

    • Michael C. Fidler September 12, 2019, 17:28

      I was going to ask if they knew how close it was going to come near Mars, 6.2 million miles! It is now going 93,000 mph, how fast when passing Mars? If this was heading for an Earth impact, we would have a four month warning, but being near the sun and unobservable now til mid October only 2.5 months! 10 miles across and going well over a 100,000 mph would wipe out everything and make the impact that destroyed the dinasours look like a small nuclear bomb. This is what is called a real planet killer, people of earth be scared, no oxygen left, no atmosphere left, no humans left.

      • Thomas Hair September 12, 2019, 22:59

        The chance of Earth being hit on human time scales from an interstellar object are infinitesimal. Whole galaxies collide, as will the Milky Way and Andromeda, and nothing much happens but coalescence and assimilation. Space is very big and matter is but a cosmic afterthought. No worries about these interlopers.

        • Michael C. Fidler September 13, 2019, 7:10

          Don’t be so sure on that, 12,900 years ago this planet was hit with a number of impacts that flooded the world. The scientist assumed the the great flood was a myth, but now the evidence is overwhelming that it actual happened. The point being that these types of interstellar objects may be much more common then thought and MUCH MORE dangerous. A very big and bright comet zooming in at 100,000 mph as our second interloper??? 50 years ago geologist thought that the craters in the moon where volcanic. Nothing new under the Sun, till Jupiter was slammed by a giant comet. You live in an idyllic world, but space is filled with comets and asteroid’s, it is not a vacuum. Yes these objects from interstellar distances are rare but Venus may have been an ocean world till recently, now it’s a living hell.

          • ljk September 13, 2019, 9:56

            There are two recent examples alone where an impact happened that could have been a lot worse: Tunguska in 1908 and Chelyabinsk in 2013.


            Plus we see reports all the time of smaller bodies hurtling past Earth. Had they been closer their impacts might not have been dinosaur-killer level, but they would have caused a lot of destruction and deaths.

  • Juraj September 12, 2019, 19:53

    Congratulations to Gennadii Borisov!

    • Thomas Hair September 12, 2019, 22:46

      I second your comment Juraj. Nothing beats determination and perseverance. To think a single person working on their own with equipment they built themselves, probably with their own money alone, could pull this off is nothing short of amazing. Gennadii Borisov…winner of the (you pick your accolade) prize.

      • ljk September 13, 2019, 9:46

        So-called amateurs have played and quite obviously continue to play an invaluable role in the field of astronomy. With ever-improving telescope and computer technology and only a limited number of official professionals, their services are needed more than ever. Encouraging folks to become astronomers is more than just giving someone a hobby. This is real science.

    • Paul Gilster September 13, 2019, 5:06


  • jonW September 12, 2019, 22:06

    After ‘Oumuamua, it was suggested that we set up a small number of ready-to-go-at-short-notice interceptor crafts for subsequent discoveries of interstellar passers-by. I can’t remember the details–would they be parked in orbit, or what? Anyway obviously there’s nothing in place now, but had the plan been enacted, would this new visitor been a viable candidate for chasing down, given the lead time the earlier discovery gave us?

    • ljk September 13, 2019, 9:50

      Had we been able to get a probe to Oumuamua, we might have been able to determine with certainty if its light curve does indeed indicate that the object’s shape is not a cigar or a shard or a pancake, but a thin, flat disc.

      See here:


  • Jean Schneider September 13, 2019, 4:19
    • Harry R Ray September 13, 2019, 9:46

      If accepted by a scientific journal, this would be the “official” CONFIRMATION paper! This thing is REAL, folks, so now for the nitpicking details. I hope that there is available time on the HST for a dedicated campaign to determine whether there is going to be ANY non-gravitational acceleration(or deceleration) on the INWARD LEG of its journey! My take: Probably not, because the size of this object is not going to be measured in meters, but instead in KILOMETERS! That means that it is very massive, most likely TOO massive to be moved by ANY KIND OF OUTGASSING, but I hope that I am wrong and will wind up being pleasantly surprised! Finally, for all of you irony buffs out there, an Oort Cloud comet(Siding Spring)passes ~81,000 kilometers from Mars, AND an interstellar comet(Borisov)passes ~6,000,000 kilometers from Mars IN THE SAME DECADE! How ironic is that!

      • ljk September 13, 2019, 10:59

        Harry R Ray said above:

        “Finally, for all of you irony buffs out there, an Oort Cloud comet(Siding Spring)passes ~81,000 kilometers from Mars, AND an interstellar comet(Borisov)passes ~6,000,000 kilometers from Mars IN THE SAME DECADE! How ironic is that!”

        Ironic enough that we should have had manned bases on Mars long before then as we originally wanted to. Or how about a small fleet of probes orbiting the Sun for just such situations? Yes they can be monitoring interplanetary space in the meantime, like we did with Pioneer 6 through 9.

        • Hamilton1 September 13, 2019, 11:43

          There is disagreement over whether it comes close to Mars or not –


          ‘This is now called C/2019 Q4 (Borisov) and the Minor Planet Center has verified its interstellar orbit. It alas will not come much closer to Mars than to Earth :(

          • Harry R Ray September 15, 2019, 14:42

            More irony. On October 22, 2018 `Oumuamua and Borisov made their closest approach to EACH OTHER at 8.5 AU, closer than Earth ever comes to Jupiter. HST stopped observing `Oumuamua in May(last image MADE PUBLIC in January), 2018, so I believe(correct me if I am wrong)that if HST imaged `Oumuamua on October 22, 2018 it still would have been BARELY VISABLE, and maybe Borisov would have been, too. If the alignment were fairly close to line of sight instead of spread out close to their MAXIMUM separation, BOTH OBJECTS could have shown up in the SAME PHOTOGRAPH! WHAT A MISSED OPORTUNITY! It could be the only image containing TWO INTERSTELLAR OBJECTS ever attainable.

    • ljk September 13, 2019, 10:05

      This is the correct link for Borisov on Exoplanet.eu:


      • jonW September 14, 2019, 12:25

        I see at the bottom of that webpage they refer to a paper by “HIBBERD A., PERAKIS N. & HEIN A.” titled “Sending a Spacecraft to Interstellar Comet C/2019 Q4(Borisov)”. These are some of the authors of the excellent paper about catching up with ‘Oumuamua. But there is no link to a paper, and I haven’t found it anywhere online. I would like to read it, if it’s real!

        • Paul Gilster September 14, 2019, 12:29

          I plan to write about this early next week, jonW. Andreas Hein just forwarded a copy of the paper.

        • Andreas Hein September 16, 2019, 4:18

          Dear jonW, thank you very much for your kind remark! The paper is indeed real and is now online on arXiv:


  • Enzo September 13, 2019, 6:08

    Probably this was mentioned before but I found this ~1 year old interview as podcast with Avi Loeb on Oumuamua :
    You can safely skip to 4:25

  • Michael C. Fidler September 13, 2019, 7:21

    Borisov 26 inch f1.5 is quite a feat, wondering what optical design and number of elements used for such a fantastic reflector?

    • Harald Ringsevjen September 13, 2019, 8:27

      The same here. I just wonder how did he design the mirror with this low f-ratio?

  • ljk September 13, 2019, 9:23

    MPECS realized they need to make a page on Borisov that is a bit more user friendly to the media and general public, so here it is:


    About sending a probe to Borisov, here is their reply:

    Can we send a spacecraft to it?

    I only wish. As described above, when this thing leaves the solar system, it’ll be moving at about 32.6 km/s. The fastest of our interstellar probes, Voyager 1, is going at about 16 km/s. At least with what we’ve got now, we can’t do it.

    However, when ʻOumuamua went past us, it was suggested that we keep careful track of it while we could so we’d have as accurate a trajectory as possible, in case future generations come up with something faster than our chemical rockets and ion propulsion; if we do our jobs right, they’ll know where to go find it. (Not to mention that all that careful tracking told us that ʻOumuamua was being unexpectedly influenced by the pressure of sunlight.)

    Keep in mind that this is a for-real comet from another star system. There would be a lot to learn by going out to it and maybe even bringing a bit back for analysis… but barring incredible breakthroughs, it’s not apt to happen in our lifetimes.

    It’s also been suggested that, while we may not be able to make something fast enough to catch up to an object like this, it’d be nice to have a “comet chaser” on standby. Then, if we saw an incoming interstellar object, we’d launch at it and could fly past it.

    To all space agencies and really rich guys with rockets: You now have two real examples of interstellar visitors in a two-year time period. So start building some dedicated probes and have a rocket or two ready exclusively to encounter these objects the next time we get one, because we will, and soon.

  • Patient Observer September 13, 2019, 9:52

    (a modified repost)
    This shot across the bow is your final warning.
    Stop broadcasting day time Soap Operas now!

    • ljk September 13, 2019, 12:44

      Are soap operas still a thing? I would think they would be far more annoyed with reality programs like The Bachelor(ette) and just watching our news for two minutes.

  • ljk September 13, 2019, 10:01

    The ESA came out with a plan in June for intercepting an interstellar comet:


    The site:


  • ljk September 13, 2019, 11:04

    Astronomers spy a 2nd interstellar visitor

    Posted by Deborah Byrd in Space | September 12, 2019

    Astronomers think the object they’ve labeled C/2019 Q4 – discovered August 30 – came from a place far, far away. If confirmed, it’s only the 2nd interstellar object ever detected, after ‘Oumuamua in 2017.


  • Madhvesh K. Upadhya September 13, 2019, 11:24

    It would have been awesome, if we are able to send a satellite to land on this object and thus hitch a free ride to outer space. From there, it could send data just like how the two Voyagers do even now.

  • wdk September 13, 2019, 12:01

    Back about 1930 or so when Clyde Tombaugh discovered Pluto, he did so by comparing the photo image positions of faint objects of unknown nature against the celestial sphere background. If they moved, they were suspect, asteroids, comets or planets. And the technique has not changed very much for the last 90 years.

    And has been gone over in the past, many of us who inquired of, researched or discussed likelihood of interstellar wanderers, we came back with an astronomical community consensus that they were unlikely.

    I see no direct connection between the two objects just detected over the past two years. Because their directions, eccentricities, orbital planes with respect to the solar system and their sizes are very distinct from each other. But why should why should we have two of them in the space of two years? Even a couple of the recent ‘Oumuamua related reports, acknowledging that the object’s passage can happen, did not give a likelihood of occurrence or detection as often as that.

    Sounds scary if there is no obvious answer. Somebody just decided to start hurling things at us from all sides?

    • ljk September 13, 2019, 16:25

      My guess this appearance of two interstellar objects in two years has more to do with our being able to detect them now than a sudden actual influx.

      At one point shortly after Oumuamua arrived, astronomers were saying we should see about 10 such object each year. Not quite sure how they came to that number. Then it was reduced to one per year, also without a detailed explanation. I await better and more dedicated telescopes to give us a true estimate.

      • wdk September 14, 2019, 12:30

        In all likelihood (?) you are probably right with future occurrences bearing this out. I do remember a study last winter from the UK with estimates similar to what you quoted. Whether ten a year or every ten years, I couldn’t recall. But I still thought it worth posing the question as to whether our detection capabilities for such objects was really increasing exponentially. It seems like a ten object per year threshold should have matched up with state of the art earlier,

        But should something of this value of solar system entries adhere ( 1 to ten per year) for comets with tails, then comets become a medium after solar system formation for organic material transfer from one part of the galaxy to another. And not necessarily just from local neighbors. Meteoritics would be influenced too.
        Arguments posited by Fred Hoyle and others were based on such
        interactions. I wouldn’t want to say their existence verify their theories. At the very least they would have to be dusted off or reviewed. But to my mind, the cosmos is a little more linked (demonstrably) now than it was a decade or two ago.

  • ljk September 13, 2019, 13:09
  • ljk September 13, 2019, 14:34

    Why Borisov matters – even though I am pretty sure it is not terribly concerned about the opinions of the talking primates with digital watches on Sol 3:


  • ljk September 13, 2019, 16:26

    The first color image of Borisov via the giant Gemini telescope:


  • Michael Fidler September 14, 2019, 2:13

    You mean they are talking now, the last time we came by they were still throwing rocks at each other! (2001) What is important is the size and brightness, could entering the galactic arms or dense clouds cause storms of such objects? The solar system orbits around the Galaxy at close to a 90 degree angle, the beutiful images of the Sun and the planets corkscrew trail may indicate that impacts from these objects would be more common near the poles of the Suns planets. ;-/

  • Antonio September 14, 2019, 5:23

    GTC took an spectrum of the comet:


  • Hamilton1 September 14, 2019, 6:15

    The first spectrum of interstellar comet C/2019 Q4 (Borisov) reveals that this object has a surface composition not unlike that found in Solar System comets.


  • Patient Observer September 15, 2019, 11:31

    At the risk of creating an interstellar “incident”, perhaps an impactor similar to the one used in the “Deep Impact” mission to comet Tempel 1 can stir up Comet Borisov sufficiently to learn its composition from Earth and spaced based telescopes. The main advantage is that the probe itself would merely (said with deliberate understatement) need to collide with the comet with the sophisticated instrumentation already available albeit some distance away doing the data collection. Obviously, there would be no need to match velocities (in fact, the more mismatch the better) – just sort of get in its way. Perhaps a shot gun approach with numerous small impactors scattered along its trajectory would be needed to increase the chance of a hit. Given the high velocity of any impact, even a small impactor should yield something noticeable. In case it is an alien artifact, lets say it is easier to ask for forgiveness than to obtain permissions.

    The foregoing assumes such a mission can be launched and reach the intercept area before the comet begins its retreat from the sun otherwise its bye bye Borisov.

  • ljk September 16, 2019, 10:01

    Might as well put this here:


    So is Tabby’s Star and our two interstellar visitors common(er)? And what does this mean, besides the fact that we are just barely beginning to get our toes wet in the Cosmos?

    And this is why I laugh when I hear people say there are no aliens in the galaxy because none of them are trying to signal or visit us. We do not have a clue in that area yet.

  • Harry R Ray September 16, 2019, 10:03

    Michele Bannister tweeted this 17 hours ago. “Orbit of C/2019 Q4(Borisov) is cheerfully hyperbolic at e = 3.70 +/- 0.12 from a 14 day arc…” Before we allgo off the wall and start speculating that it is CHANGING DIRECTION ON ITS OWN, keep in mind that it is still so close to the Sun(line of sight, NOT actual)from Earth’s vantage point that it will be at least 2 months before the heavy artillery(like HST and SST)can be unleashed on this object and a really precise orbit can be pinned down. So, every CURRENT piece of data is just an ESTIMATE!

  • Hamilton1 September 16, 2019, 13:18

    We already have 305 observations for C/2019 Q4 –


    compared to 222 for the entire ‘Oumuamua apparition –


    Although, as Harry says above, the real big boys have yet to get involved, we still have data from some heavy-hitters like Gemini and Palomar. So you would think an estimate of non-gravitational acceleration might be possible even with the present figures.

    • Harry R Ray September 16, 2019, 16:32

      If this is non-gravitational acceleration(or deceleration)it would have to be a REAL WHOPPER(unlike `Oumuamua’s, which was extremely MINESCULE)and thus, ALSO effect the VELOCITY of the object as well in a really easily measurable way. Since I have heard ABSOLUTELY NO REPORTS of this happening, I summarily reject any hypothesis put forward that the massive discrepencies in e are due to actions of the object itself. Be patient, folks

  • Charley September 16, 2019, 22:19

    What an absolutely tremendous discovery! Not only can this fella grind his own lenses and make his own telescopes, but he’s obviously a patient and thorough observer. Sometimes you lose a lot of hope in the abilities of people to accomplish anything. And then you hear such a story is this man here and a restore some of your faith.
    All I can say is that they damn well better name the comet after him because to him is the glory of discovery and it would be wrong in the extreme to not give him credit where credit is due. It must’ve been quite exciting for him when he first realized he had come across this object. In what amazes me even more is that I would’ve thought it would’ve been beyond such relatively small instruments to resolve an object like this, especially since it’s still so far from the main plane of our solar system.
    So I say : Hats off to Borisov !!

  • Triffin November 4, 2019, 22:48