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A Closer Look at an Interstellar Comet

The interest in ‘Oumuamua and comet 2I/Borisov makes it clear that interstellar neighbors wandering into our system generate loads of media coverage. And why not: Here is a way to study material from another stellar system while remaining within our own. 2I/Borisov, for example, reaches its closest approach to Earth in early December, closing to within roughly 300 million kilometers. Whatever pushed an object like this out of the parent system cannot be known, but we’re likely dealing with gravitational disruption related to planets in the birth system. But more about that in a moment.

For thanks to Yale University astronomers Pieter van Dokkum, Cheng-Han Hsieh, Shany Danieli, and Gregory Laughlin, we have a fine new image of 2I/Borisov. This was taken on November 24 using the W.M. Keck Observatory’s Low-Resolution Imaging Spectrometer in Hawaii. The tail of the comet, according to van Dokkum, is about 160,000 kilometers long. Note the size comparison below to be reminded, as always, of the immensity of the objects we routinely study in the sky. For those of us who occasionally get jaded, here is another corrective.

Image: Left: A new image of the interstellar comet 2l/Borisov. Right: A composite image of the comet with a photo of the Earth to show scale. (Pieter van Dokkum, Cheng-Han Hsieh, Shany Danieli, Gregory Laughlin).

Gregory Laughlin points out that 2I/Borisov is evaporating as it moves through the Solar System, releasing the gas and dust so visible in its tail. Says Laughlin: “Astronomers are taking advantage of Borisov’s visit, using telescopes such as Keck to obtain information about the building blocks of planets in systems other than our own.” So true, and what an opportunity we’re opening up as we begin what will become the more routine study of such objects.

Bear in mind that according to a current Laughlin paper, written with Yale graduate student Malena Rice, we can expect a few such objects showing up every year, with much smaller (and therefore all but impossible to detect) objects coming into the Solar System in the hundreds per year. The duo studied three protoplanetary disks from the Disk Substructures at High Angular Resolution Project (DSHARP). Says Rice:

“We were looking for disks in which it was pretty clear a planet was there. If a disk has clear gaps in it, like several of the DSHARP disks do, it’s possible to extrapolate what type of planet would be there. Then, we can simulate the systems to see how much material should be ejected over time…This is actual material that makes up planets in other solar systems, being flung at us. It’s a completely unprecedented way to study extrasolar systems up close — and this field is going to start exploding with data, very soon.”

The paper on the frequency of interstellar objects is Laughlin & Rice, “Hidden Planets: Implications from ‘Oumuamua and DSHARP,” Astrophysical Journal Letters Vol. 884, No. 1 (10 October 2019). Abstract / Preprint.


Comments on this entry are closed.

  • Charley November 26, 2019, 16:46

    So I might to understand this clearly that this object will pass from the northern portion of the plane of Earth’s ecliptic, through the ecliptic and then proceed on a southerly outward leg away from the ecliptic ?? Do I have that story correct ?

    Additionally, is it correct that the closest of point of approach to the earth will occur when it is below (southerly) the ecliptic portion of its path ? Will that also mean that the point of closest approach to the sun will not be visible from the northern hemisphere ?

    • Vareck November 30, 2019, 0:03

      Yes, it is already south of the ecliptic (-15.5º declination as of November 29) and perihelion is not until December 8 and perigee on December 27. Declination at closest approach to the sun will be -20º, so unless you are > 70º North it will be above the technical horizon at that point, and probably reasonably observable before the sun rises. Closest approach to the Earth is a few weeks later, on December 28, and declination will be -30º or more, so you would need to be south of 60ºN for it to be above the horizon.

      • Charley December 3, 2019, 20:32

        Thanks Mr. Vareck for the extremely detailed reply; very, very much appreciated.
        My only lament is the fact that this particular (Comet ?) can’t appear in the northern skies at a reasonable time of day (say, 9 – 10 p.m., at least from my perspective), instead of having to be at the crack of dawn. I understand the geometry of the situation and why it has to be at that particular time of day – but still ! It’d be nice just once to have a natural phenomena be convenient for the Observer – just once in a while !

  • Robin Datta November 26, 2019, 19:56

    Perhaps the astronomical community should start contemplating how to accomplish future sample returns.

    • Al Jackson November 27, 2019, 9:23

      When I was at the International Astronautical Congress last October… was a paper on that topic (some earlier ones have been about also)… it it very expensive energy-wise, one would just about have to have a space based station ready to send a probe.

      • James Jason Wentworth December 1, 2019, 15:14

        “Short-range starships” (those words kept sounding in my mind as I read this update–and especially, your comment about the great difficulty of even achieving fast, “particles-grabbing” [and/or quick-scan] flybys of these visitors). Chemical rockets are pitifully inadequate (for really worthwhile missions to these objects), and ion drives and HETs [Hall effect thrusters], while they have the ISP for the job–although not by large margins–accelerate so slowly that having such probes in the right place at the right time to intercept or rendezvous seems like a perversely frustrating game that a Pan or a Loki might devise… But:

        Looking around, there *are* existing propulsion technologies that could get worthwhile instrument packages out to these objects, even to “land” (rendezvous and dock) on them and return actual samples to Earth. But one is still at a rather low technological maturity level, although the “engine” it would use–our Sun–already exists and is ready to be engaged (for one-way missions), whenever we are. The other could be built today, but it would have to be built–and launched–in/from areas of the world where the “No nukes in space…!” (or anywhere else) luddites would be afraid to go (and wouldn’t be allowed in, in any event). It’s frustrating to know what we *could* do, and that it is stymied by ignorance-based fear. The engineering–and even actual designs–have long been ready, though; for example:

        A “small” Orion nuclear pulse spaceship, with a pusher plate “just” 33 feet (10 meters) in diameter, was designed for launch aboard a Saturn V (its first two stages, the S-IC and the S-II). As with Skylab in May of 1973, those two stages would have injected the Orion vehicle into low Earth orbit. Once there, its high-thrust ^and^ high-specific impulse (a rare combination, as one must usually choose one over the other) nuclear “shaped charge” pulse units–designed to ensure that fully half of the high-velocity bomb debris, including the forward-directed (by a nozzle in each pulse unit bomb assembly) vaporized polyethylene, intercepted the pusher plate–would start firing, and:

        While this Saturn V-lofted Orion spaceship was likely intended for interplanetary travel (I haven’t seen its specifications for some years), and thus utilized fission pulse units, the Orion team also designed starships (capable of reaching about 0.1 c, enabling a voyage to Alpha Centauri–as long as one didn’t need to slow down!–that could take less than fifty years), which used thermonuclear (fusion bomb) pulse units. Plus:

        With the advances in fusion bomb design (including weapon miniaturization) that have occurred since Project Orion was shelved about half a century ago, today we could build smaller, lighter, 0.5% – 10% of light velocity cruising speed (and maybe more) Orion starprobes, which could reach the interstellar interlopers *and* return after reasonable intervals of time (0.5% – 1% of c should be amply fast for such missions), carrying generous samples as well as enormous quantities of data. It might be cheaper–as Arthur C. Clarke speculated concerning automatic interstellar probes–for such interstellar interloper probes to come back than to transmit such prodigious quantities of data back to Earth by radio or even laser, at least until/unless we have huge radio and/or optical telescopes in space and/or on the Moon. As well:

        Elon Musk’s SpaceX is developing the Super Heavy (rocket)/Starship (a general-purpose upper stage/two-way shuttle/small-payload SSTO–Single-Stage-To-Orbit–interplanetary spaceship with in-space refueling capability) combination. With the Super Heavy first stage, this vehicle combination has an in-orbit payload mass capability, even initially, that is considerably greater than the Saturn V’s (220,000 pounds [100,000 kilograms] initially, with a target of 150,000 kilograms). The Super Heavy/Starship, once it is flying, will be able to loft actual starships of the Orion type (at least robotic ones, for now) into Earth orbit, from which they can begin their long ultraplanetary (to meet interstellar interlopers)–and ultimately, interstellar–journeys.

        • ljk December 3, 2019, 10:11

          Yes, Orion can come in various forms and do a lot all over the Sol system and beyond:


        • Charley December 3, 2019, 20:25

          I had a personal communication with one of the originators of project Orion himself recently, and he’s just as adamantly against the project, as he was nearly half a century ago. Here’s his take on the matter, and I ask you James Jason Wentworth your feelings about his reply:

          I would like to however give a slight push back on Project Orion. A website called Centauri dreams talks on interstellar flight and they tend to take the tact that laser sails could get us to alpha Centauri in approximately 20 years. Research on this technique indicates that it would require the full output of five large commercial nuclear power plants to provide the required power to get the sail to 20% lightspeed. Add onto that the fact that the laser would need to be fired from a vantage point of approximately 60 degrees South latitude and that entire area is nothing but Ocean region AND add on to the fact that you have a spacecraft of just one gram size – doesn’t that suggest that a larger, but somewhat slower craft would be more optimal?

          The following is an extract from a American nuclear Society website discussing project Orion:

          “Efficient directional explosives maximized the momentum transfer, leading to specific impulses in the range of 6,000 seconds, or about 12 times that of the Space Shuttle Main Engine. With refinements, a theoretical maximum of 100,000 seconds (1 MN·s/kg) might be possible. Thrusts were in the millions of tons, allowing spacecraft larger than eight million tons to be built with 1958 materials.

          The reference design was to be constructed of steel using submarine-style construction, with a crew of more than 200 and a vehicle takeoff weight of several thousand tons. This low-tech single-stage reference design would reach Mars and back in four weeks from the Earth’s surface (compared to ≈50 weeks for NASA’s current chemically powered reference mission). The same craft could visit Saturn’s moons in a seven-month mission (compared to chemically powered missions of about nine years).”
          you can see quite clearly based upon their analysis that thousands of tons of payload could be delivered to alpha Centauri (for example) within a 40 year time period. Hopefully if they could extract magnetic field energy from the plasma of the explosion, as well as solid particles, you might be able to get even more speed. The following website is below:


          Definitely no. Orion is much too slow for an interesting interstellar mission. Now we have much better technologies such as laser propulsion and microwave propulsion, ten times faster and much cheaper..

          • Paul Gilster December 3, 2019, 22:08

            Charley, re your statement “Add onto that the fact that the laser would need to be fired from a vantage point of approximately 60 degrees South latitude and that entire area is nothing but Ocean region…”

            Breakthrough Starshot is looking at Chile’s Atacama Desert for the beamer. It is not true that the only vantage point for it is in the ocean.

            • Charley December 4, 2019, 2:19

              Mister Gilster, I realized that they would need an latitude vantage point that would be as close to the -60 degrees South as possible. However, I do believe that while it is feasible, it would mean that the laser would of necessity have to blast through a greater amount of atmosphere to strike the probe and start it on its way.

              You may not be aware of the fact that a study was done at one of these conferences on what is necessary to reach the alpha Centauri system: it essentially amounts to accelerating the probe and then the probe moves in basically a straight line throughout its voyage. What that translates into is that the aiming of the laser must be quite precise and being offset at an angle different from the approximate -60 degree south latitude (even so slightly) means that any error would be greatly compounded when it gets near the star system. That’s why the Atacama Desert , while better, invariably by its very nature introduces unavoidable error.

              • Paul Gilster December 4, 2019, 7:38

                All of these issues are under discussion within Breakthrough Starshot. And yes, I am aware that there have been conferences where getting to Alpha Centauri was discussed, along with the problems involved in laser beaming and targeting in Starshot’s mode. I’ve reported on a number of them in these pages.

          • ljk December 5, 2019, 14:07

            Orion may be slow if you want to get to a nearby star system within a current human lifetime, but this assumes you want to place humans on board a starship for migration purposes. Otherwise machines can and will only do better if your purpose is the scientific exploration of interstellar space.

            Orion would be great for interplanetary travel and transportation, however. Correct me if I am wrong, but I get the feeling that some of these professionals who make the comments quoted above still seem stuck in an antiquated mindset about space travel, one that thinks a spaceship needs a human crew to function. Perhaps we need another generation or two to get out of this paradigm.

  • Neil Stahl November 26, 2019, 21:08

    Sounds innocent, but what if that tail, some of which will eventually reach earth, contains dried deadly bacteria, or dried green goo, or ?? We’ll hope we haven’t disturbed anyone who could arrange something like that.

    • Robin Datta November 26, 2019, 22:54

      In all likelihood it ain’t the first interstellar comet to skirt so close to the earth – close enough for panspermia. If the passengers had the same genetic code they could blend in with the natives.

      A different code might be overwhelmed by an established order, unless it was so robust that it might elbow out and replace the existing order. If such replacements had happened on earth, the last replacement would have to be complete before the beginning of the fossil record, and so thoroughly complete that no alternate genetic code seems to exist.

      • Al Jackson November 27, 2019, 9:29

        Robin, excellent statement , well put.
        Indeed over a few billion years the event should have happened many times. There have probably been a lot of interstellar comets and asteroids that passed even closer to the earth. Anyway a fraction of the extraterrestrial dust that hits the earth each day is interstellar , 5 to 300 metric tons a day! So one does not need an interstellar comet or asteroid.

      • Thomas Hair November 27, 2019, 13:42

        You nailed it Robin in only two very short paragraphs. Brevity truly is the soul of wit.

      • Alex Tolley November 27, 2019, 16:37

        Any life from another star would have to have the same DNA/RNA structure and genetic code for those organisms to bring viral or bacterial plagues to Earthlings as thought by medieval thinkers about the evils of comets.

        Meteors and comets from extra solar sources might be the best source of material to test whether panspermia is possible over interstellar distances. If we found bacterial spores that could be revived, their DNA and genetic code would offer important information about life elsewhere, whether it is the same/similar/dissimilar to terrestrial life, and if the same, then a very interesting set of questions emerge about our place in the universe and the possibilities of biological humans colonizing planets in the galaxy.

        One test I would like to see in our own system is whether Kuiper belt or Oort cloud objects have collected any bacterial life from Earth. Find a few bacterial spores in or on a large dirty snowball would be quite an undertaking but might be possible with a lander/rover. If such spores were to be found, then it would increase teh possibility that life could possibly spread thoughout the galaxy through perturbations from passing stars.

        A final thought. Hoyle and Wickramasinghe hypothesized that epidemics might be delivered by passing comets. While that would not likely work with life from from another star as you point out, it does make me wonder if comets within our system could deliver ancient bacterial life swept off from Earth and subsequently returned.

      • Mike Serfas November 27, 2019, 19:03

        The hypothesis is not quite that easy to falsify if the previous lifeforms came from the same source. It is difficult to make a genetic tree and molecular clock of all the life on Earth because past a certain point the homologies are hard to detect. However, there are efforts like https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5912509/ that do show a deep history of lateral gene transfer – which would to my first glance appear to rule out, for example, a notion such as that archaea arrived recently from whatever planet sent us the bacteria.

      • Patient Observer November 28, 2019, 14:12

        Probably not panspermia but still interesting:


      • ljk December 6, 2019, 10:31

        A suitably advanced ETI species exploring our Sol system and wanting to study Earth and its inhabitants in detail could do so easily by dropping off nanotech and picotech “probes” and we would be none the wiser.

        This brings into question even more strongly all those reports of huge alien ships full of humanoid crew abducting humans – yet another antiquated paradigm that we have yet to shake.

    • Bruce D. Mayfield November 27, 2019, 11:03

      Horrors! Yee olden ill omen comet terrors!! Really? Look for such stories in tabloid rags soon.

      • Al Jackson November 27, 2019, 18:51

        When I published the Black Hole of Tunguska paper in 1973 (in Nature) (while doing graduate work at U of Texas) … I noticed a few years later a Tabloid picked up on it and titled it HURTLING HORRORS FROM OUTER SPACE!
        Best headline I ever got.

        • Bruce D. Mayfield November 28, 2019, 11:27

          Very cool story Al.

        • James Jason Wentworth December 1, 2019, 15:29

          That was also the plot (or part of it) of the 2001 movie “Joe Dirt,” which Dennis Miller starred in. As the quotes on IMDb (see: https://www.imdb.com/title/tt0245686/characters/nm0131985 ) say:

          Meteor Bert : Well, it ain’t a meteor.

          Joe Dirt : Yeah, it is. It came out of the sky.

          Meteor Bert : Well I’m sure it did but it ain’t no meteor. It’s a big ol’ frozen chunk o’ s–t.

          Joe Dirt : What?

          Meteor Bert : Oh yeah, see them airplanes they dump their toilets 36,000 feet. The stuff freezes and falls to earth. We call ’em Boeing bombs.

          [chomps teeth]

          Joe Dirt : No, that can’t be. That’s not what it is.

          Meteor Bert : Oh, afraid so. See that peanut? Dead giveaway.

          Joe Dirt : Uhhh, no, that’s a space peanut.

          Meteor Bert : No, afraid not. That just a big ol’ frozen chunk of p–py.

          Studio manager : Dude, you were eating off it!

        • ljk December 3, 2019, 10:28

          Al, were you the author or at least involved/mentioned in the cover story I fondly recall from the 1974 New York Times Sunday Magazine supplement on this very subject, about the possibility of the Tunguska Event being caused by a black hole?! I recall this quite well and am sure I still have a copy of it somewhere.

  • Robert Flores November 27, 2019, 0:10

    For those not versed in Kinetic Energy Differences between
    Extra-Solar VS Intra-Solar comets/Objects.

    Assume for a moment that the Cretaceous Sized Dino Killer
    was extra solar, Similar in composition and equal in mass to the Borizov object, but moving faster.

    The extra Energy would be many times the power
    of that event depending on the relative speed of our planet
    and the object. Only in special cases would it be less than a comet flung from Our own kuiper belt.

    The Dino Killer is estimated to have a final speed of 20Km/Sec.
    Via Kinetic Energy Formula KE = 1/2 x M x V*2 we see even a extra solar object with a speed 50% faster at 30Km/sec will result in
    a more than doubling of the KE from the impact.

    More likely an extra solar object would be moving in the range of 40-60Km. With that much more energy you get something more closely resembling a Permian-Triassic Extinction level event.

    Incidentally, objects in our Oort cloud have a much higher probability
    of impacting the Earth as such speeds than extra solar objects, such comets however would probably be one time events as these type are usually flung out of our solar system. Rarely they turn into short period comet (Halley) via gravitation interaction with our Jovian planets.

    • Bruce D. Mayfield November 27, 2019, 11:23

      You’re points re the additional potential damage from impacting extra-solar system objects suggest this is yet another factor in long term habitability: Increasing distance between star systems lessons the odds of such devastating interstellar impacts. This would also tend to make natural interstellar panspermia less possible, I would think.

  • Ljk December 6, 2019, 0:19

    Might our alien comet visitor not survive its visit to our celestial neighborhood?