by Duncan Lunan

First published in different form as ‘The Solar System’s First Interstellar Visitor ’Oumumua Revisited’, Concatenation, September 14^th 2021;  revised and updated, Analog, May-June 2023;  follow-up letter, ‘Brass Tacks’, Analog, September 2023.

The object we now call `Oumuamua traversed the inner Solar System in September and October 2017, leaving at 27 kilometres per second, well over Solar System escape velocity, in the direction of Pegasus [1].   First thought to be a comet [2], it was redesignated as an asteroid when it was estimated to be 400 metres long and only 40 metres across.  

The light-curves made clear that ‘Oumuamua was oddly shaped and rotating, possibly tumbling.  The European Southern Observatory issued an artist’s impression, depicting it as dark and spindle-shaped  (Fig. 1).  The shape recalled various fictional spaceships, including the one attached to Halley’s Comet in the 1985 film Lifeforce, based on the novel The Space Vampires by Colin Wilson, and the hollow ‘prison-world’ of Rhaam in Harry Harrison’s story ‘Out of Touch’ for Jeff Hawke, the world’s longest-running SF comic strip, drawn by Sydney Jordan [5]  (Fig. 2].  My own most popular SF story, ‘The Comet, the Cairn and the Capsule’, envisaged an interstellar comet passing through the inner Solar System [6], and my first commercial sale, ‘Derelict’, was about an unmanned starship entering the Solar System [7].  (‘My SF’, ON, 10^th September 2023.)  There was a certain irony in that both of them became topical, 50 years later, at the same time.

That image has dominated media coverage since, although it may be seriously misleading.  To begin with, its albedo was roughly 70%, as bright as the clouds of Venus, or polished metal.  In 2018 a team led by Dr. Michael Belton, of Belton Space Exploration Initiatives, Tucson, found that `Oumuamua could have biaxial or even triaxial rotation, with periods ranging between 6.58 and 54.48 hours [8].  8 hours was the best fit, at least for the primary rotation.  While the spindle shape was likely to be correct if the periods were long, the best fits to the light-curve were ‘an extremely oblate spheroid’ if they were shorter.  At Belton’s request, Dr. William K. Hartmann of the Planetary Science Institute, Tucson, painted `Oumuamua as a thick disc  (Fig. 3), as the ‘saucer section’ of the Kuiper Belt object ‘Ultima Thule’  (‘Arrokoth’) was found to be in the New Horizons flyby of January 2019.

As `Oumuamua left the Solar System it was slowly accelerating, perhaps because of outgassing due to solar heat belatedly penetrating to the interior  [9] (Fig. 4).  That heat should have been detected by the Spitzer infrared space telescope, but wasn’t, confirming that ’Oumuamua is highly reflective.  Furthermore no emissions were observed, not even by the SOHO or STEREO solar-orbiting spacecraft, both of which which would have detected dust or water vapour, nor by Spitzer, which would have detected carbon dioxide.  For that acceleration to be the rocket effect of expelled material, 10-40% of the total mass would be lost, and the spin rate would inevitably be altered.  No such alteration was observed.  If instead the acceleration was due to radiation pressure, then ’Oumuamua would be much less massive than believed.  The acceleration was inversely proportional to the square of the distance from the Sun, and remarkably, it was even and unbroken despite the evidence of rotation and possible tumbling [10].  If the acceleration was due to any form of outgassing, it could not remain steady if the body tumbled, especially with a period as long as 8 hours. 

Sergey Mashchenko’s study of the light-curves in 2019 produced a still more remarkable result.  While the spindle shape wasn’t entirely ruled out, the best fit appeared to be an extremely thin sheet  (less than 1 mm thick)  about 40 metres in diameter.  Mashchenko gave that a 91% probability of being correct, with the spindle’s probability at only 9% [11].   Shmuel Bialy and Abraham (Avi)  Loeb of Harvard had already argued that `Oumuamua could be an extraterrestrial artefact, most likely a solar sail  [12], and in his book Extraterrestrial, Loeb is more than a little annoyed that the cigar-shape still dominates almost all the published artwork [13].  David A. Hardy’s illustration contrasts the two extremes  (Fig. 5). 

Because of his participation in Breakthrough Starshot, which proposes to send micro-lightsails to the nearer stars, Loeb is often dismissed with the German proverb, “To him who has a hammer, everything looks like a nail”.  He replies, “Not only do skilled carpenters most definitely not see nails everywhere, but they are trained to differentiate among those they do observe” [13].  Differentiate is the key word:  his point is that `Oumuamua is not like anything we have seen before, and needs a different kind of thinking, rather than trying to force it into familiar categories.  It has no similarity to the two other interstellar objects detected since:  one is an asteroid captured by the Solar System in its early history[14], and the other is Comet Borisov, whose composition resembles comets of our Solar System, particularly Comet Hale-Bopp  (1996) [15].   Both Borisov and Hale-Bopp appear to have formed in the outer reaches of their planetary systems, but incorporating material originating from much further in, and have seldom if ever passed close to a star before.

Two ‘natural explanations’ have tried to explain the non-detection of outgassing by STEREO, SOHO and Spitzer.  The first was that `Oumuamua was composed of solid hydrogen, which would have been undetectable by the three spacecraft as it boiled off.  Prof. Loeb and his colleagues showed that such material would have been sputtered away by high-energy cosmic radiation during the timespan of an interstellar journey[16].  The second suggestion was that instead it was composed primarily of solid nitrogen, like parts of the crust of Pluto, and it had been expelled from a similar but much thicker surface, common in the formation of planetary systems [17].  On those suppositions, nitrogen icebergs would outnumber comets by 200 to 1 in interstellar space;  yet only the second interstellar object was a conventional comet.  Prof. Loeb quickly showed that if nitrogen icebergs were so common, the amount of nitrogen involved would be greater than is known to exist within stars [18].   Dr. Hartmann’s painting of ’Oumuamua as a disc has been used in the media to illustrate the hypothesis, but when I asked him for permission to reproduce it, it turned out that he was unaware of that [19].  A nitrogen iceberg ’Oumuamua would be much thinner and shinier.

In any case. how likely it is that a crustal fragment of solid nitrogen would remain intact, while being blasted off a planetary surface at more than escape velocity?  Water has been found in Martian meteorites – but not in solid form.  Instead it’s chemically bound into the rock, which nitrogen is not likely to be.  Authors Jackson and Desch aren’t proposing that the fragments are rock interspersed with solid nitrogen, but are actual nitrogen icebergs which survive ejection intact.  They dismiss the extraterrestrial hypothesis because we’ve never seen an ET solar sail, but we’ve never seen a nitrogen iceberg either, although there’s enough nitrogen ice on Pluto and Triton for them to be generated by impacts today – if they can be.  The nitrogen hypothesis requires ’Oumuamua to have shed 92% of its mass while passing through the Solar System, which makes the steady rotation still harder to explain.  Jackson and Desch do recognise that in the configuration they envisage, ’Oumuamua’s acceleration would have to be variable, but in another of their many assumptions they suppose that the effect would ‘average out’ [17].  Obviously, it could, if the emission was from all over the sunlit surface (and only from there), or from small matching areas on both sides – but that doesn’t mean that it would even out, giving a constant acceleration despite a rotation or tumbling period as long as eight hours.

Astronautical Explanations

To explain the observed effects, ’Oumuamua has to maintain a constant surface area facing the Sun while also rotating as seen from Earth.  Addressing that issue, Prof. Loeb has recently noted that this requires ‘extreme geometry’, suggesting that that the part visible from here may have been a solar collector for a solar-electric drive  [10].  When I was asked to review the issue for the online journal Concatenation, a first idea that occurred to me was that ’Oumuamua might be a discarded hexagonal plate from a flexible Dyson sphere, an idea suggested by G. David Nordley as an alternative to breaking up the planets of a solar system, to build a shell around their star, or a sphere of asteroids.   (See ‘Unbuild Your Own Solar System’. ON. 13^th August 2023).  In his novella ‘Empress of Starlight’  (Analog, Nov/Dec 2018), he proposed a sphere of flexible, interlinked hexagons  (Fig. 6), sustained by sunlight pressure and therefore self-correcting if disturbed [20].   Since the sphere isn’t rotating, a hexagon which was cast loose for any reason would travel radially away from the star until it reached escape velocity, or until the hole it left was plugged.  Such a stray plate could be considered as a solar sail, once it was adrift, although it wasn’t intended to be one.  Mashchenko’s paper had suggested that a plate with different reflectivity on its two sides might provide a better fit to the light-curves –  but it would not explain the even acceleration, which Prof. Loeb stresses eight times in his book.

Anyway, if discarded or shocked loose from its Dyson sphere, Nordley’s plate would probably not remain perpendicular to the light-beam from the hole, unless it had attitude control, which it would not have needed when locked into place.  Travelling around our Sun at close perihelion and then outwards on a hyperbolic trajectory, it could not remain perpendicular to the incident sunlight.  Its reflective area with respect to the Sun would keep changing and the acceleration would vary in consequence.  

To maintain a constant attitude to the Sun, a flat plate solar sail would need adjustable vanes at the corners, like those on the solar panels of the Mars probe Mariner 4  (Fig. 7).  They proved effective, but unnecessary, so subsequent probes have relied on attitude control jets, not an option for `Oumuamua.  Although Mashchenko considered that better fits to the light-curves might be found if the sail wasn’t flat, tip-vanes alone are not enough to explain the variations in `Oumuamua’s brightness;  likewise a parabolic sail, like the Solaris ‘Comet-Chaser’ designed by Gordon Ross [21] (Fig. 8), could change its orientation by adjusting the tension on shroud lines, or by varying the pressure in gas-filled ribs, but again could not explain the observed light-curves unless it was tumbling, in which case the acceleration would not be constant.

The cancelled US mission to Halley’s Comet might have been a flat sail with tip-vanes  (Fig. 9), but a more advanced design called the ‘Heliogyro’ would have had individually adjustable vanes and be spun to keep them under tension  (Fig. 10).  As it did so the Heliogyro could turn most of them to the Sun for continuous propulsion, and angle others for navigation, ringing the changes as it rotated – just what `Oumuamua would need to keep thrusting as it moved outward on its hyperbolic path, even though it was rotating and spin-stabilised.  But the Heliogyro components would have to be active and mobile;  at a pinch they might be self-correcting, but inevitably one starts to think of artificial intelligence.

Perhaps ’Oumuamua could be a faceted spheroid of hexagons, like a much smaller version of Gerry Nordley’s Dyson Sphere.  If the facets could both absorb and emit radiation, as the spheroid rotated one or more of them could be absorbing solar power while the reflected sunlight provided propulsion, and others could be tracking and transmitting data to any distant target on the celestial sphere, undetectable from here unless the beam happened to sweep across the Earth.  To explain the light-curve fully the spheroid might have to be patterned, as the Apollo spacecraft were for temperature control.  Perhaps the hexagons could be individually articulated, with a fixed number facing the Sun at all times as the sphere rotated.  Again Mashchenko was ahead of me, finding that a sphere with black and white hemispheres was a possible fit, which could be enhanced by more complex markings, but he didn’t investigate further because ’Oumuamua’s surface shows no sign of volatiles  [11].   

Another proposed natural explanation, early on, was that `Oumuamua could have disintegrated during perihelion passage, becoming a cloud of small particles.  But when that happened with Comet ISON in 2013, the cloud dispersed and became invisible very quickly.  If `Oumuamua was a cloud, there has to be an explanation for its remaining together and remaining so bright.  One possibility  (ironic, in view of the ‘hammer’ gibe above)  is a cloud of Starshot-type mini-sails  (Fig. 11), under central control, and manoeuvring like a flock of starlings  (Fig. 12), a shoal of fish, or bats rising from a roost.  At any given time, some could be under solar propulsion and others sending data, changing places as they overtake one another, looking like a single body when actually they’re all in motion with respect to one another.  Given the small size of the mini-sails, 8 hours seems a reasonable turnover time for the continual front-to-back rearrangement, especially if the swarm’s overall appearance from a distance is a thin disc.  It is possible to imagine the individual processors acting to produce an apparently concerted effect, but central processing seems more likely.  As with the spheroid or the heliogyro, we would have to ask, was that control purposeful?  If the spacecraft or swarm was transmitting data to some distant collecting point, then it would have to be so, or intended to be.

References

1.  Deborah Byrd, ‘Small Object Visits from Beyond the Solar System’, EarthSky, online, October 29^th, 2017.

2.  Fraser Cain, ‘Has the First Interstellar Comet Been Discovered?’, Universe Today, online, October 26^th, 2017.

3.  Matt Williams, ‘Updates on `Oumuamua, Maybe It’s a Comet, Actually.  Oh, and No Word from Aliens’,  Universe Today, December 19^th, 2017.

4.  Matt Williams, ‘That Interstellar Object Is Probably Pretty Strange Looking’, Universe Today, November 20^th, 2017.

5.  Harry Harrison, Sydney Jordan, ‘Out of Touch’, Jeff Hawke strip, Daily Express, October 4^th 1957 – April 5^th 1958, reprinted in Jeff Hawke’s Cosmos, Vol. 5 No. 2, June 2009.

6.  Duncan Lunan, ‘The Comet, the Cairn and the Capsule’, If, July-August 1972;  seven times reprinted, including German and Polish translations, e.g. in Asimov, Greenberg and Waugh, eds, The Science Fictional Solar System, Harper & Row, 1979, most recently in From the Moon to the Stars, Space-travel Stories by Duncan Lunan, Other Side Books, 2019.

7.  Duncan Lunan, ‘Derelict’, Amazing, April 1974;  reprinted in From the Moon to the Stars.

8.  Michael Belton et al, ‘The Excited Spin State of 1I/2017 U1 ‘Oumuamua’, Astrophysical Journal Letters, April 1, 2018.

9.  Deborah Byrd, ‘Interstellar Asteroid Update: It’s a Comet!’, EarthSky, June 28^th, 2018.

10.  Todd F. Sheerin and Abraham Loeb, ‘Could the Interstellar Object ‘Oumuamua Be a Solar Thermal Propulsion Vehicle?’, Journal of the British Interplanetary Society, November 2021.

11.  Sergey Mashchenko, ‘Modelling the light curve of ‘Oumuamua:  evidence for torque and disc-like shape’  (Monthly Notices of the Royal Astronomical Society, August 29^th, 2019.

12.  Abraham Loeb, ‘How to Search for Dead Cosmic Civilisations’, Scientific American, September 27^th, 2018;  Shmuel Bialy, Abraham Loeb, ‘Could Solar Radiation Pressure Explain ‘Oumuamua’s Peculiar Acceleration?’, Astrophysics, October 30^th, 2018;  Matt Williams, ‘Could ’Oumuamua Be an Extra-Terrestrial Solar Sail?’, Universe Today, October 31^st, 2018;  Paul Scott Anderson, ‘Could ’Oumumua Be an Interstellar Light Sail?’, EarthSky, November 6^th, 2018.

13.  Avi Loeb, Extraterrestrial, The First Sign of Intelligent Life Beyond Earth, Houghton Mifflin Harcourt, and John Murray, 2021.

14.  Matt Williams, ‘`Oumuamua Was Just the Beginning:  Astronomers Find an Interstellar Asteroid Orbiting Retrograde Near Jupiter’;  ‘The Solar System Probably Has Thousands of Interstellar Asteroids’, Universe Today, May 23^rd, 2018 and February 7^th, 2018.

15.  Anon, ‘News:  Interstellar Comet Was Exceptionally Pristine’, Astronomy Now, May 2021.

16.  Thiem Hoang, A. Loeb, ‘Destruction of Molecular Hydrogen Ice and Implications for 1I/2017 U1 (’Oumuamua)’, Astrophysical Journal Letters, August 17^th, 2020.

17.  Alan P. Jackson and Steven J. Desch, ‘1I/’Oumuamua as an N₂ Ice Fragment of an Exo-Pluto Surface: I. Size and Compositional Constraints,’, Journalof Geophysical Research: Planets, 2021;  ‘1I/’Oumuamua as an N₂ Ice Fragment of an Exo-Pluto Surface II: Generation of N2 Ice Fragments and the Origin of ’Oumuamua’, Journalof Geophysical Research: Planets, 2021;  Anon, ‘News:  Was ’Oumuamua a Nitrogen Iceberg from a Pluto-like Planet?’, Astronomy Now, May 2021.

18.  Fraser Cain, ‘Interview:  On ’Oumuamua, Aliens and Astrophysics, with Dr. Avi Loeb’, online,  Universe Today, April 9^th, 2021.

19.  William A. Hartmann, personal communication, June 14^th 2021.

20.  G. David Nordley, ‘Empress of Starlight’, Analog Nov/Dec 2018, reprinted in G. David Nordley, Around Alien Stars, Brief Candle Press, 2019;  ‘Astronomical and Astronautical Concepts Used in Empress of Starlight’, personal communication, 2021.

21.  Duncan Lunan, ‘Keep Watching the Skies’, Analog, October 1994;  Incoming Asteroid!  What could we do about it?, Springer, 2013, Chap. 5.

(To be continued)