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‘Oumuamua: A Hydrogen Iceberg?

Studies of interstellar interloper ‘Oumuamua move at lightning pace, to judge from a recent exchange on hydrogen ice. A study by Greg Laughlin and Darryl Seligman (both at Yale) just published in June, has now met a response from Thiem Hoang (Korea University of Science and Technology, Daejeon) and Harvard’s Avi Loeb. The issue is significant because if, as Laughlin and Seligman argued, ‘Oumuamua were made of hydrogen ice, then the outgassing that drove its slight acceleration would not have been detectable. At least one mystery solved.

Or was it? One reason the 0.2km radius object didn’t fit the description of a comet was that there was no explanation for its tiny change in velocity. Hoang and Loeb have examined the hydrogen ice concept and found it wanting. Says Hoang:

“The proposal by Seligman and Laughlin appeared promising because it might explain the extreme elongated shape of ‘Oumuamua as well as the non-gravitational acceleration. However, their theory is based on an assumption that H2 ice could form in dense molecular clouds. If this is true, H2 ice objects could be abundant in the universe, and thus would have far-reaching implications. H2 ice was also proposed to explain dark matter, a mystery of modern astrophysics.”

Which sounds interesting in itself because anything bearing on dark matter is worth a look, given our frustration at understanding the matter behind the hypothesis. Laughlin and Seligman had suggested a giant molecular cloud [GMC] as the origin for ‘Oumuamua, but Hoang and Loeb argue that the earlier paper, while considering the destruction of H2 ice in the interstellar medium through evaporation, did not take into account the improbability of such ices forming within a GMC, or the effects of that environment upon their later growth. From the paper:

Assuming that H2 objects could be formed in GMCs by some mechanisms (Füglistaler & Pfenniger 2016; Füglistaler & Pfenniger 2018; Seligman & Laughlin 2020), we quantify their destruction and determine the minimum size of an H2 object that can reach the solar system. We assume that the H2 objects are ejected from GMCs into the ISM by some dynamical mechanism such as tidal disruption of bigger objects or collisions (see Raymond et al. 2018; Rice & Laughlin 2019).

Image: An artist’s rendering of ‘Oumuamua, a visitor from outside the Solar System. Credit: The international Gemini Observatory/NOIRLab/NSF/AURA artwork by J. Pollard.

Hoang and Loeb’s calculations show that H2 icebergs are unlikely to grow to large size because of collisional heating not just from dust but from gas within the birth cloud, meaning that ‘Oumuamua likely wasn’t a hydrogen iceberg (and, along the way, taking out the ancillary proposition that dark matter may be accounted for by H2 snowballs). Micron sized grains in regions where the density of gas is high will cause the hydrogen on the grains to sublimate.

Assuming that H2 objects could somehow form in the densest regions of GMCs, we found that sublimation by collisional heating inside the GMC would destroy the objects before their escape into the ISM [interstellar medium]. We also studied various destruction mechanisms of H2 ice in the ISM. In particular, we found that H2 objects are heated by the average interstellar radiation, so that they cannot survive beyond a sublimation time of tsub ~ 10 Myr for R = 300 m (see Figure 1). Only H2 objects larger than 5 km could survive.

While giant molecular clouds like (GMC) W51, one of the closest to Earth at roughly 17,000 light years, could be a point of origin for the object, the authors argue that even this close GMC is simply too far away. Moreover, it might be hard for a hydrogen iceberg even to exit the giant molecular cloud in the first place. Collisional heating within a GMC would destroy objects like this through thermal sublimation long before they reached a distant stellar system. The paper finds that objects below 200 meters in radius would be destroyed within the parent GMC.

We also have to find a way to get ‘Oumuamua all the way from its birthplace to our Solar System. What Hoang and Loeb point out is that an iceberg made of hydrogen would be unlikely to survive an interstellar journey that would probably take hundreds of millions of years. An object like this is going to begin to evaporate. Their paper goes to work out the survivability of H2 ice from interstellar radiation given thermal sublimation and photodesorption along the way.

Many other factors come into play that cause problems for a hydrogen iceberg. It has to stand up to cosmic rays as well as impacts with matter in the interstellar medium. And we have to throw in what can happen when an object like ‘Oumuamua enters the Solar System, where solar radiation becomes an issue. The calculations presented here show the significance of thermal sublimation due to starlight, while going beyond this to reveal the effects of cosmic rays and impacts with interstellar matter, which turn out to be less significant.

Image: This is Figure 1 from the paper. Caption: Comparison of various destruction timescales (slanted colored lines) as a function of the object radius (in meters) to the travel time from a GMC at a distance of 5.2 kpc, assuming a characteristic speed of 30 km s−1 (horizontal black line). Credit: Hoang & Loeb.

The authors calculate a minimum radius of H2 objects in the range of 5 kilometers for survival in a journey that would have to take in formation in giant molecular clouds and movement through the interstellar medium. 10 million years would wreak havoc on an object the size of ‘Oumuamua.

We’re in a period of energetic debate, a time when the unresolved questions about ‘Oumuamua remain in play. It seems clear that we need a larger population of interstellar objects to place the current work in context, and Loeb has pointed out that we won’t have long to wait:

“If ‘Oumuamua is a member of a population of similar objects on random trajectories, then the Vera C. Rubin Observatory (VRO), which is scheduled to have its first light next year, should detect roughly one ‘Oumuamua-like object per month. We will all wait with anticipation to see what it will find.”

The paper is Hoang & Loeb, “Destruction of Molecular Hydrogen Ice and Implications for 1I/2017 U1 (‘Oumuamua),” Astrophysical Journal Letters Vol. 899, No. 2 (17 August 2020). (Abstract). The Seligman & Laughlin study that argued for “‘Oumuamua as a hydrogen iceberg is “Evidence that 1I/2017 U1 (‘Oumuamua) was Composed of Molecular Hydrogen Ice,” Astrophysical Journal Letters Vol. 896, No. 1 (9 June 2020). Abstract.

{ 33 comments… add one }
  • Robin Datta August 18, 2020, 17:44

    Could microcollisions coat a hydrogen iceberg with a protective layer of matter?

  • kzb August 18, 2020, 19:33

    This paper maintains that hydrogen snow balls are indeed possible:


    Not H2 icebergs however. On the other hand, clouds capable of forming H2 snow could be very close by.

  • Law Wong August 18, 2020, 22:08

    I’m quite annoyed that the full PAN-STARRS array didn’t get funded.
    We were promised one interstellar object a year and a few hundred Neptune trojans!

    • John walker August 19, 2020, 14:19

      I sympathize. But, Vera Rubin will have first light next year. And their discussions with SpaceX visavis Starlink trails have led to a good result. Amateur observations in the last week have indicated that SpaceX’s solution in the form of Visorsat has resulted in the achievement of the goal of +7.0mag at operational orbit(550km). A manageable luminosity.

      • Michael Fidler August 19, 2020, 21:55

        Is that a + or -7.0 magnitude???

  • Michael Fidler August 19, 2020, 3:17

    I hope the Vera C. Rubin Observatory (VRO) first light for November, 2021 has not been delayed by the Covid-19 pandemic. You can see the dome is being covered in the EarthCam; (works best during daytime)

    See what’s happening on Cerro Pachon!

    What is interesting about ‘Oumuamua is that it came almost directly from the Solar apex which is the direction that the Sun travels with respect to the local standard of rest. See the image below:



    This is the direction that we should be looking at: Right ascension (RA) 18h 28m 0s, Declination (dec) 30° N. This is where will will see advanced images of any large objects coming from the local standard of rest in interstellar space.


    Civilizations may be common thru out our galaxy and many may have explored and expanded thru the galaxy billion of years of ago. These objects could be relics from an earlier expansion. We could be like children walking along a road and seeing ancient structures that we have no comprehension of what they are. Like the Mayan mounds that laid buried for centuries until someone uncovered them and found ancient great temples…

  • andy August 19, 2020, 4:33

    “The authors calculate a minimum radius of H2 objects in the range of 5 kilometers for survival in a journey that would have to take in formation in giant molecular clouds and movement through the interstellar medium. 10 million years would wreak havoc on an object the size of ‘Oumuamua.”

    Might it have started life much bigger?

  • ljk August 19, 2020, 9:01

    As professional astronomers and other scientists try desperately to have Oumuamua be anything but an alien vessel, they have even said it could be a “fractal dust aggregate”…


    So absurdly improbable possibilities are okay, just so long as it isn’t the possibility that an ETI sent a probe flying through our Sol system to check us out.

    That’s what we planned to do with the BIS Daedalus star probe and now Breakthrough Starshot, so why not someone else, especially if they are more capable in this arena.

    • AlexTru August 20, 2020, 2:16

      As professional astronomers and other scientists try desperately…

      I suppose that astronomers and other scientists observed in reality lot of different space objects like asteroids and comets, that are formed naturally without any need to involve ETI to explain it’s existence.
      In same time ETI existence remains absolutely undetected, even fastest know to homo Sapiens media , like – electromagnetic waves , do not advertise any signs of ETI.

  • ljk August 19, 2020, 9:14

    Let us not forget that the shape of Oumuamua conforms to a thin, flat disk – because it seems to have been forgotten elsewhere as the professional community does its level best to make our first interstellar visitor au natural….


    • randomengineer August 19, 2020, 22:41

      Your comment and Mr Essig’s comment are complementary, and you could both be correct: suppose that the object is in fact a massive 10km hydrogen iceberg, but it is the fuel store. How big would the payload be? 300 meters x 300 meters? Could we even detect that? No, we can only see the fuel. (If I were designing a stealthy interstellar craft, I’d rather have fuel that doesn’t require a 10km container. Looks natural, too.)

    • Alex Tolley August 20, 2020, 13:05

      It is only postulated as thin because the acceleration was apparently not accompanied by outgassing. If the outgassing was H2, it has been suggested that this would not have been detected, thus invalidating the thin, flat dimensions argument.

      If anything, I would prefer @randomengineer’s suggestion if it was an alien starship. It is a stealthed H2 fuel for a an interstellar ship, rather like the Enzmann starship dimensions.

      However, these are all wild speculations based on very little data. In this day and age, such speculations in print are suspiciously clickbaity. If we were to spend teh resources to snd a probe, it is far more likely that the object would just be natural, with no hidden artificial components. Remember the speculation that Ceres had hints of occupation based on the bright spots? They turned out to be salts from upwelling in Occator crater. Or Hoagland’s face on Mars? Or a hollow Phobos back in the 1960s? IMO, ‘Oumuamua is just the latest example, and says more about us as humans than about the object itself.

      • randomengineer August 20, 2020, 22:02

        We could prove controlled or no visually, we don’t need a probe. The shifting brightness whilst close suggested moving axis orientation, i.e. tumbling. If we can see it somehow now, then if it is still doing the brightness changing tumbling thing, it’s probably natural; if it’s not doing that, then a payload end with a motor is pointing at us and it’s in a fixed orientation — controlled. Is this thing too far out to get snapshots?

        • Alex Tolley August 21, 2020, 11:37

          You may recall that the tumbling motion based on teh light curve was used to suggest that if it was an alien ship that it was dead and no longer controlled. IMO, a probe would be needed to “see” it from close up. “Seeing is believing” still holds and can eliminate much speculation. Now that we can see Phobos close up, the idea that it could be a hollow object dies. (Apart from the better calculations of its orbit).

          • ljk August 21, 2020, 14:52

            So how DO we know Phobos is not hollow inside, just by looking at it from the outside at a distance?

          • randomengineer August 22, 2020, 16:15

            Are we able to see this thing yet? If so, imaging has got to be less expensive and take less time than a probe. We don’t need to see the thing up close. If the behavior changed (i.e. not tumbling now) then we have our answer. If unchanged then it’s 99.999% chance of natural object.

      • Michael Fidler August 20, 2020, 22:54

        Oh, but it is so nice to be a hopeless romantic. Where would we be if the only hope of change was the warring bone of the great ape in 2001 a space odyssey? If you notice we constantly try to put anything that may be alien in contact text of our recent society, such as canals on Mars or H2 gas stations. But and that is a big but something that may be in the local standard of rest may be like our G5 cell towers or Starlink in the sense of being on another plane from our tecno wizardly world. Just a little dreamland romantic.

        • Alex Tolley August 21, 2020, 11:47

          I personally find more than enough romance in nature without needing to resort to supernatural agents or ETs. While I would be very interested in contact with and ETI, I also think there is a danger (as commented on a recent CD post). That danger comes from us, not them. Unless the ETs understand our various cultures intimately, they could easily cause all sorts of problems for us. At this stage in our development, I would prefer observable life in the universe to be largely unintelligent – offering scope for research, but far less for internal strife, particularly during this immediate period. If I need ETs, I can retreat into SciFi stories.

          • ljk August 21, 2020, 15:07

            That is one of my main concerns. Astronomers are “tired” of the concept of aliens. Or they don’t want their existence messing with their wonderful, placid ideas of reality. Or that higher intelligences from other worlds conflict with their religious notions.

            That is not the way the Universe works. Astronomers may feel fortunate that aliens might be very far away from Earth, far enough not to “interfere” with our quaint and cozy little worldviews that do not involve or allow anyone else in.

            But the odds are they exist nevertheless regardless of where they may reside in the Universe – and what will happen if one or more end up being detected by humanity, or – gasp! – make direct contact with us? Shall we shoot them down? Cover up the news? That isn’t very scientific, for starters.

            I feel like I am just this side of an early Renaissance discussion of whether Sun goes around Earth or vice versa.

            The Universe will show up on our doorstep in all sorts of potential ways one day, whether we are ready or not. And no Prime Directive, either. We are part of a much larger and more complex Cosmos than our culture has allowed us to really grasp. It won’t be the Universe’s fault if we aren’t ready for it.

            • Alex Tolley August 21, 2020, 18:06

              I don’t know that astronomers are tired of aliens so mauch as wishing to avoid being associated with speculation and dealing with the crackpots.

              While I admire your open-mindedness on alien contact, I cannot help feeling that it could morph into an almost religious hope for contact. We might then be spending generations building space telescopes like cathedrals in the hope of making contact, but with as much result that the faithful praying in cathedrals have contacted God.

              If the contactable universe is devoid of ETI, then we will have spent a lot of resources on a fruitless quest. More likely we will find artifacts of extinct civilizations. May they not prove as dangerous as the Krell machines.

              OTOH, suppose ETI is plentiful. Then we have the problem that our factional species could become even more unstable, making the historic tribal, religious, and national conflicts seem like the proverbial kids’ tea party squabbles.

              The default null hypothesis is that terrestrial life is unique. Extra terrestrial life would be wonderful, but complex life rather than bacteria is rare. Intelligent life is very rare, and we may even be unique. Now it may turn out that ETI is very common, perhaps not even biological, and that it has exploited the resources of many systems in the galaxy. I doubt that is the case (Fermi again) but it is possible. Whether we could do anything useful with that knowledge is another issue. What I don’t expect is that we will receive some enclopedia galactica that will allow us to leap ahead in science and technology. I expect us to make a slow, more protracted than I like, exploration of our system, along with improving knowledge of the universe. No superior agency will emerge, although we might, if we are lucky, discover evidence of dead civilizations.

              To me this is pursuing a rational, maximum likelihood of success, science strategy, but not blinkering oneself to what unexpected things the universe throws at us.

  • James Essig August 19, 2020, 12:50

    Potentially great nuclear fusion fuel for nuclear fusion powered electrical propulsion systems and also relativistic rockets that directly exhaust fusion reaction products.

    If the object is made of hydrogen, I would like to know how pure it is.

    Hydrogen is perhaps the most prevalent accessible cosmic energy source so it makes sense for advocates and researchers in interstellar relativistic spaceflight to begin charting the sources of usable hydrogen.

  • Tony D. August 19, 2020, 20:32

    An interstellar starship “traveling to a rocky water world, with a detected oxygen atmosphere in the habitable zone of a distant star (Sol).” Somewhere along the journey something catastrophic happened to the interstellar voyagers: disease, infighting/mutiny, major technical malfunction. However, the light sails used for breaking still automatically deployed upon entering the alien planetary system, which also provided an acceleration assist as it shot around the star, thus exiting the system without ever completing the original mission.

    • ljk August 21, 2020, 15:28

      Unless these ETI had to use a multigenerational starship to get from one star system to another for whatever reason, I would imagine an interstellar vessel – especially one on a mission of scientific exploration – would be fully artificial.

      So while there could still be a failure of the ship due to mechanics, a meteoroid strike, etc., I doubt there would be a problem due to the usual breakdowns by most organics – psychological in particular.

      In addition, presuming these hypothetical aliens wanted to hide their presence from humanity – although I cannot imagine why they might worry considering we were not even ready to send a simple probe to Oumuamua despite decades of being able to be ready, let alone any other kind of action – then looking and acting like a big ol’ space rock would do the trick.

      I can guarantee if we were able to get a probe to Oumuamua on a flyby route and it returned images of what looked like a standard planetoid on the outside, most of the professional community would consider that good enough, heave a collective sigh of relief, and move on. They wouldn’t even wait for any other kind of measurements to come in.

      I have evidence of this from when a few simple Radio SETI efforts were done on Oumuamua as it was passing through our planetary neighborhood. No signals were detected, so that was that.


      Comments I have seen in this very thread today only further confirm my view on the social behavior of scientists when it comes to encountering a Universe they are not truly ready for.

      • randomengineer August 22, 2020, 1:01

        The stealthy space rock to disguise things bit I proposed earlier isn’t the only explanation. For all we know, the most efficient means of flying about the cosmos with fuel is to use a huge rock as the fuel. (If so, that would be a parallel to the paleolithic.) Any disguise is coincidental.

        • randomengineer August 22, 2020, 9:22

          And if I’m unclear re parallel, imagine scientists etc in the future describing man’s technology phases as paleolithic, neolithic, and astralithic. We may not be as modern as we believe; we’re still the Flintstones.

  • laintal August 19, 2020, 23:27

    Hi Paul

    Vera C. Rubin Observatory (VRO) yes this is one mission I’m going to follow with interest and the results too.

    Cheers Laintal

  • ljk August 21, 2020, 14:04

    Mystery of Interstellar Visitor ‘Oumuamua Gets Trickier

    Aliens? Or a chunk of solid hydrogen? Which idea makes less sense?

    By Rafi Letzter, LiveScience on August 19, 2020

    Full article here:


    Asked if there is a clear leading candidate explanation for ‘Oumuamua’s acceleration, Loeb referred Live Science to a not-yet-released book he authored called “Extraterrestrial: The First Sign of Intelligent Life Beyond Earth,” due for publication in January.

  • ljk August 21, 2020, 14:55

    Interstellar Now! Missions to and Sample Returns from Nearby Interstellar Objects

    Press Release – Source: astro-ph.IM

    Posted August 19, 2020 2:54 AM


    Two extrasolar objects, 1I/’Oumuamua, 2I/Borisov, have passed through our home system in just the last three years. Such interstellar objects provide a previously unforeseen chance to directly sample physical material from other stellar systems.

    By analyzing these interlopers, we can acquire data and deduce information about their planetary system of origin, planetary formation, galactic evolution and possibly even molecular biosignatures or even clues about panspermia. In this paper, we show that missions to interstellar objects can be performed with existing or near-term technology, demonstrating that different categories of missions to different types of interstellar objects are feasible within the next decade.

    We present three categories of missions: fast flybys, ideally combined with an impactor to sample the surface, rendezvous missions with orbiter or lander, and a fast flyby returning samples generated by high-velocity impactor(s). We present exemplary scenarios for these mission categories.

    A combination of Falcon Heavy or Space Launch System, chemical propulsion, and Parker Solar Probe-derived heatshield technology would be sufficient for fast flybys. For a rendezvous, solar electric propulsion would also be needed. For sample return, nuclear thermal propulsion would be required as well. The minimal suite of onboard instruments for answering questions about the origin of these objects is a camera and mass spectrometer.

    Andreas M. Hein, T. Marshall Eubanks, Adam Hibberd, Dan Fries, Jean Schneider, Manasvi Lingam, Robert Kennedy, Nikolaos Perakis, Bernd Dachwald, Pierre Kervella

    Comments: Mission Concept White Paper submitted to the 2023-2032 Planetary Science and Astrobiology Decadal Survey

    Subjects: Instrumentation and Methods for Astrophysics (astro-ph.IM); Earth and Planetary Astrophysics (astro-ph.EP); Space Physics (physics.space-ph)

    Cite as: arXiv:2008.07647 [astro-ph.IM] (or arXiv:2008.07647v1 [astro-ph.IM] for this version)

    Submission history

    From: Andreas M. Hein

    [v1] Mon, 17 Aug 2020 22:10:58 UTC (790 KB)



  • ljk August 21, 2020, 15:31

    Interstellar visitor ‘Oumuamua could still be alien technology, new study hints

    By Rafi Letzter – Staff Writer 2 days ago

    Aliens? Or a chunk of solid hydrogen? Which idea makes less sense?


  • Eniac August 21, 2020, 19:16

    I usually refrain from wild speculation, but wanted to note that a hypothetical craft that was aiming to gain speed from stellar flybys would best approach its next slingshot opportunity head on, to gain a maximum amount of acceleration. It appears that ‘Oumuamua approached our sun just that way.

    It would have to be a very patient and long-lived craft to plan flybys like that, but still….

    • Michael Fidler August 23, 2020, 9:22

      Or it’s seeding…

  • Marshall Eubanks August 25, 2020, 18:05

    Cosmic Microwave Background and Big Bang Nucleosynthesis data rule out any explanation for “the” dark matter involving sequestered hydrogen. Sure, you could possibly have _some_ dark hydrogen, but no way could Omega_Hydrogen be 32%.

  • ljk August 26, 2020, 13:35

    We Have the Technology to Retrieve a Sample From an Interstellar Object Like Oumuamua


    On October 19th, 2017, astronomers were astounded to learn that an interstellar object (named ‘Oumuamua) flew by Earth on its way out of the Solar System. Years later, astronomers are still debating what this object was – a comet fragment, a hydrogen iceberg, or an extraterrestrial solar sail? What’s more, the arrival of 2I/Borisov two years later showed how interstellar objects (ISOs) regularly enter our Solar System (some even stay!)

    It’s little wonder then why proposals are in place to design missions that could rendezvous with an interstellar object the next time one passes by. One such mission is Project Lyra, a concept proposed by researchers from the Initiative for Interstellar Studies (i4is). Recently, an international team led from the I4IS drafted a White Paper that was submitted to the 2023-2032 Planetary Science and Astrobiology Decadal Survey.

    The team behind the paper, titled “Interstellar Now! Missions to and Sample Returns from Nearby Interstellar Objects” was led by Andreas Hein and Marshall Eubanks of i4is and Space Initiatives Inc. They were joined by associates from the Paris Observatory, the Florida Institute of Technology (FIT), the Institute of Theory and Computation (ITC), the Technical University of Munich (TUM) and the FH Aachen University of Applied Sciences.

    Full article here:



    In their paper, the team explored three types of mission profiles that could be mounted using today’s technology. These included flybys, rendezvous, and sample return missions. In terms of flybys, which would be similar to what the New Horizon‘s spacecraft performed with Pluto and Arrokoth, the spacecraft wouldn’t decelerate and would gather images and data as it flew past the ISO.

    However, they also make allowances for an impactor, a small sub-probe that crashes into a body to create a plume that can then be collected and studied by the spacecraft. Examining these particles or light generated from the impact with a spectrometer would reveal secrets about its composition. This is similar to what NASA and India did with their Lunar Crater Observation and Sensing Satellite (LCROSS) and the Chandrayaan-1 probe.

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