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The Mystery of 'Oumuamua

by Christopher Seymour about a month ago in space
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Could it be an alien space probe?

Professor Loeb

Unidentified Flying Objects, Aliens, Space Invaders, and suchlike apparitions are typically the domain of either science fiction or crackpots. Indeed, the sceptics frequently point to the fact that astronomers never report UFOs. After all astronomers have an ongoing professional interest in the sky. Some spend most of their careers examining images of the sky. If anyone was seeing flying saucers, it should be astronomers. But astronomers are familiar with the sky and know what they are looking at. Invariably they can find mundane explanations for the hyperbolic assertions of excitable amateurs. It’s always explained by the planet Venus, a meteorite, a satellite, ball lightening, or some other entirely natural phenomena.

Thus, it was quite a surprise when a distinguished Harvard astronomer reported in 2017 that our solar system had been visited by an alien spacecraft. Professor Abraham Loeb is neither a fiction writer nor a crackpot. Born in Israel, from 2011 to 2020 he was Chair of Harvard’s Department of Astronomy. Currently he is the Frank B. Baird, Jr., Professor of Science at Harvard University. He has written eight (non-fiction) books and more than 800 scientific papers. His papers have an h-index of 117, showing they are held in high regards by his peers.

However, his hypothesis of an alien spacecraft has provoked scorn among many of those peers. One recent technical paper1 said of Loeb’s claims “While provocative, this argument is baseless” – a pretty strong indictment in the world of science. The social and popular media responses have been more direct. " 'Oumuamua is not an alien spaceship, and the authors of the paper insult honest scientific inquiry to even suggest it," tweeted Paul Sutter, an astrophysicist at Ohio State University. "A shocking example of sensationalist, ill-motivated science," theoretical astrophysicist Ethan Siegel wrote in Forbes. North Carolina State University astrophysicist Katie Mack suggested Loeb was trolling for publicity. "Sometimes you write a paper about something that you don't believe to be true at all, just for the purpose of putting out there," she told the Verge.

Loeb has authored several scientific papers and a popular book2 explaining his theories about the object known as 'Oumuamua. He backs up his writings with numerous facts and observations which confirm his conclusion that ‘Oumuamua demonstrates remarkable features for which there is no natural explanation. He quotes the Sherlock Holmes principle that “when you have eliminated the impossible, whatever is left, however improbable must be the truth.” In this case the most likely explanation is that it is a spacecraft designed and launched by an alien civilization. His critics seem not to have read his work because they base their objections on a general philosophy that aliens cannot exist, rather than countering Loeb’s arguments one by one.


The object that provoked the controversy was found by astronomer Robert Weryk on 19th October 2017. Weryk was examining images taken the previous evening by the PanSTARRS 1 (PS1) telescope. The PanSTARRS project was set up to search for earth crossing asteroids that might constitute a threat to Earth. Established on the top of the dormant Haleakala volcano on the island of Maui in Hawaii, it boasts the largest digital camera on the planet, able to record 1.4 billion pixels in each image.

The images that Weryk was examining had been taken with 45 second exposures. The telescope pans with the Earth’s movement so that stars show as single points. Anything moving however will leave a streak. And there in the image was a definite streak. Weryk referred back to images taken the previous night. And there was the object again. Over the 24-hour period between the images the object had moved a massive 6.2 degrees. It was either very close to Earth or moving very fast.

The PanSTARRS’ team notified other observatories around the world to check on their observations. The Catalina Sky Survey, set up in the Catalina mountains near Tucson Arizona, discovered they had imaged the object five days earlier on 14th October 2017. This remains the earliest known image. Soon other observatories were reporting they had images. These included the San Marcello Pistoiese observatory in Italy, the European Space Agency observatory in Tenerife, the Klet observatory in the Czech Republic and the Siding Spring Observatory in Australia.

It soon became obvious that this was a very strange object indeed.


Small objects in the solar systems are either comets or asteroids. Comets come from the outer solar system and are composed of water and carbon dioxide ices, mixed with dust, and are sometimes referred to as dirty snowballs. Asteroids originated in the inner solar system and are composed of rocky materials or sometimes iron fragments from the core of a failed planet.

Initially this object was classified as a comet and given the designation 1C 2017 U1 indicating that it was the first comet discovered in the second half of October 2017. However solar system comets when they reach into the inner solar system usually develop pronounced tails as the heat of the sun evaporates their ices. The lack of any discernible tail caused astronomers to reclassify the object as an asteroid and redesignate it as 1A 2017 U1. Then later, when its identity was still ambiguous, the International Union of Astronomer’s (IUA) developed the new classification of Interstellar, and the designation changed to 1I 2017 U1.


Meanwhile at the urging of an Hawaiian language support group, the IAU object was given the name 'Oumuamua. The punctuation mark at the beginning is an Hawaiian okina, representing a glottal stop. The name in Hawaiian means “Scout” or “Messenger from afar.” The siting of numerous telescopes in the clear air at the top of mountains in Hawaii has become controversial and the astronomers were anxious to placate native Hawaiian opinion.


The mounting number of observations enabled an orbit to be calculated. It was discovered that ‘Oumuamua was moving too fast to be in orbit around the sun. It was the first object ever discovered to have entered our solar system from interstellar space. The calculations showed that ‘Oumuamua had passed close to the sun on 9th September 2017 at about the same distance as Mercury. It had come closest to Earth on 14th October and was now heading out past the orbits of Mars and Jupiter and back into interstellar space.

The period during which ‘Oumuamua could be observed was very short. As it moved away from the sun its brightness dropped and the last observation was by the Hubble Space telescope on 2 January 2018. There was thus an observation period of just eighty days, during which ‘Oumuamua was observed about 120 times. In all these observations, no actual images of ‘Oumuamua were obtained – it was never more than a single spot of light.

Its passage so close to the sun had changed its direction very significantly, slowing its orbit around the galactic centre and sending it away from that centre.

Brightness Variation

‘Oumuamua was found to be changing in brightness to a remarkable degree on a roughly four- hour cycle. The change in brightness was said by many publications to be 10 to 1. But actually, it was 15 to 1. Astronomers use the magnitude scale to define brightness. It is a logarithmic scale. A magnitude difference of 2.5 is equivalent to a change in brightness of 10 times. A magnitude difference of 3 is equivalent to a brightness variation of 15 times.

NASA has obtained light curves for about 18,000 small natural bodies in the solar system. There are only a few disputed and dubious objects whose brightness varies as much as ‘Oumuamua. However, there are hundreds of man-made objects in space with similar light patterns. These include spent rocket stages and spacecraft such as the Parker Solar Probe, which includes a highly reflective shield so that it can safely pass close to the sun.

The chart shows that the actual light variation was three magnitudes – a brightness variation of fifteen times.

The actual light variation is three magnitudes - equivalent to a brightness variation of fifteen times.

There are three possible explanations for the regular variation in brightness:

• ‘Oumuamua could have a long cigar shape. As it rotated it would show first a long axis and then a short one. However, its long axis would have to be quite closely aligned with Earth to have displayed the full magnitude of such an extreme effect.

• ‘Oumuamua could be disk shaped, showing first its thin edge and then its flat disk. The magnitude of the variation means that it would have to be a very thin disk.

• ‘Oumuamua could have one side much brighter than the other, similar to the Parker Solar Probe. This is the simplest explanation but means that ‘Oumuamua would have to be artificial since there is no natural explanation for such an extreme variation in reflectivity.

Spitzer fails to see it

In November NASA directed the Spitzer Infra-red Space Telescope to observe ‘Oumuamua. It devoted 30 hours to this task. This is a very significant amount of time and meant that other projects lost their observing time. After having passed so close to the sun, ‘Oumuamua should still have been warm and glowing brightly in the infra-red. Also, if ‘Oumuamua were a typical comet, it would be emitting carbon dioxide, which also glows brightly in the infra-red.

However, Spitzer failed3 to detect any trace of ‘Oumuamua. This non -observation proves that ‘Oumuamua cannot possibly be a rocky asteroid.

Anomalous Acceleration

In December 2017 it was found that ‘Oumuamua was not adhering to its calculated path. As it moved away from the sun it was slowing down, but something was opposing that slowing down – allowing it to move a little faster than calculated.

Comets typically show similar behavior. The sun’s heat causes volatile substances on their surface such as water ice or carbon dioxide ice to vaporise and create a rocket effect. Dust particles released from the ice create a visible tail. But no emissions had been seen from ‘Oumuamua, and the Spitzer observations had ruled out carbon dioxide. Moreover, the acceleration was at the extreme limit seen in natural comets and was more than sixty times the average acceleration seen in comets. The acceleration was so extreme that Cambridge astrophysicist Professor Roman Rafikov calculated that it should have completely disrupted a cigar shaped object.4 And the randomly located vaporisation jets from a natural comet should have completely changed the rotation rate. ‘Oumuamua’s rotation rate did not change at all.

Is it a Solar Sail?

At this point Professor Loeb advanced his solar sail hypothesis5. If ‘Oumuamua was a solar sail designed by an alien civilization, it would have an extremely thin disk shape. Professor Loeb was very familiar with the concept of solar sails because he had proposed one himself. In an initiative launched by Stephen Hawking and billionaire Yuri Milner the Starshot project sought ideas for a probe to reach our nearest star, Alpha Centauri within a human lifetime. Loeb had proposed a sail – powered not by wind but by light. Under Loeb’s proposal the sail would be propelled by light from powerful Earth based lasers and would reach 20% of the speed of light.

If ‘Oumuamua was a solar sail it would explain many of its features, including its brightness variation and anomalous acceleration.

Or a Fluff Ball?

Other astrophysicists were working on developing natural models.

Astronomer Amaya Moro-Martin from the Space Telescope Science Institute proposed6 that ‘Oumuamua might be a cosmic fluffball with a density 100 times lighter than air. She says such porous aggregates can naturally arise from the collisional grow of icy dust particles beyond the snowline of a protoplanetary disk. Such a fluffball could also be accelerated by the sun’s light, but there is no explanation for the brightness variation.

A Hydrogen Iceberg?

Two astrophysicists, Darryl Seligman and Gregory Laughlin, the latter from Harvard’s rival Yale, proposed7 that ‘Oumuamua was composed of frozen molecular hydrogen. Such a hydrogen iceberg could have arisen in the cold centre of a giant molecular cloud in interstellar space. This solution explains the acceleration since the low density of hydrogen would require less outgassing, dust particles would be far fewer and the escaping hydrogen gas would not be detectable by Spitzer. Seligman and Laughlin say that the light curve is explained by a disk shape produced from a more circular body eroded by its close passage to the sun. They do not explain why comets which pass by the sun do not get similarly shaped into a disk.

Moreover, this solution implies that ‘Oumuamua would have been much larger as it approached the sun. Some of the images that have been intensively reviewed since ‘Oumuamua’s discovery are those from the SOHO and STEREO space telescopes.

The Solar and Heliospheric Observatory (SOHO) is a joint project between NASA and the European Space Agency. SOHO orbits the sun, roughly between the Earth and the Sun. SOHO is designed to study the sun, but as an unintended consequence, one of its instruments has been a prolific discoverer of comets – responsible for roughly half of the comets discovered since SOHO’s launch in 1996. Images from SOHO’s LASCO camera are publicly available. Despite being intensely scrutinized by numerous astronomers, no trace of ‘Oumuamua has been found in LASCO’s images. It should have been visible on and around 9 September 2017 when ‘Oumuamua is calculated to have been at perihelion. If it was then much larger, as hypothesized by the hydrogen theory, it should have been very obvious.

NASA’s Solar Terrestrial Relations Observatory (STEREO) is also in orbit around the sun. Originally two identical spacecraft, one was lost in 2014. STEREO A remains in operation with five cameras focused on the sun and its immediate environs. STEREO images too have been intensively reviewed, but no trace of ‘Oumuamua has been found.

The final blow to the hydrogen iceberg theory was a technical paper8published by Professor Loeb and Thiem Hoang of the Korea Astronomy and Space Science Institute that showed that a hydrogen iceberg would not survive exposure in interstellar space.

Or Pure Nitrogen?

In a variation on the hydrogen iceberg theory, Steve Desch and Alan Jackson proposed9 that ‘Oumuamua is composed of pure nitrogen ice, released from a collision with a planet similar to Pluto, which has frozen seas of nitrogen. Once again, the acceleration is explained but the light curve is a more difficult obstacle to overcome. And like the hydrogen example, much of the nitrogen would have been lost while close to the sun, making the non- detection by SOHO and STEREO hard to explain.

The Scientific Community is hostile

The increasingly exotic nature of the natural theories on the origin of ‘Oumuamua and their failure to explain all its features, has not made Professor Loeb and his alien spacecraft theory any more popular. Most scientists in the field just assume it must be natural despite all the evidence to the contrary. For example, a July 2019 paper10 in Nature Astronomy by fourteen authors blandly asserts “Despite all these surprises ‘Oumuamua’s properties can be readily and naturally explained”. However, the authors don’t actually give any naturalistic explanations of the anomalous observations and non- observations.

The near universal resistance to even considering an alien explanation is hard to understand. It is generally understood that there are billions of potentially inhabitable planets in our galaxy. Surely somewhere among them are civilizations more technologically advanced than our own? Indeed, a few scientists have expressed surprise that we have not been visited. In what is now known as the Fermi paradox, Italian physicist Enrico Fermi in 1950 exclaimed over lunch "But where is everybody?”. Perhaps we have seen them but just didn’t take notice.

But perhaps Loeb’s explanation has problems?

Not that Loeb’s explanation is without problems. If the light sail was contributing to acceleration, it’s hard to see how it could have maintained its orientation.

Perhaps a more conventional spacecraft similar to the Parker solar probe is easier to understand. The brightness variation would be due to rotation of its reflective and dark sides. It could have been designed to pass close to the sun to take advantage of a slingshot from the sun’s gravity. This would be similar to the maneuvers NASA used around Jupiter and Saturn to allow their Pioneer and Voyager spacecraft to escape the solar system. The anomalous acceleration would have been due to course corrections. The reflective shield that we see as a brightness variation would have prevented any heat buildup and kept ‘Oumuamua invisible to Spitzer. This seems to me a simpler explanation than the exotic solutions proposed by scientists intent on natural explanations.

Unfortunately, we will probably never know the true nature of ‘Oumuamua. Its moving too fast out of our solar system to catch, and even if we could catch it, it would probably be too hard to find in the vast extent of space.


1 ‘Oumuamua ISSI team (2019, July 3). The Natural History of 'Oumuamua. Retrieved from: https://arxiv.org/abs/1907.01910

2 Loeb, A. (2021). Extraterrestrial: The First Sign of Intelligent Life Beyond Earth. New York: Houghton Mifflin Harcourt.

3 DE Trilling, M. M. (2018, November 20). Spitzer Observations of Interstellar Object 1I/`Oumuamua: Retrieved from https://arxiv.org/abs/1811.08072

4 Roman Rafikov (2018) Spin Evolution and Cometary Interpretation of the Interstellar Minor Object 1I/2017 'Oumuamua Retrieved from https://arxiv.org/abs/1809.06389

5 Sheerina, T.F. and Loeb, A. (2018) Could 1I/2017 U1 ‘Oumuamua be a Solar Sail Hybrid? Retrieved from https://lweb.cfa.harvard.edu/~loeb/TL.pdf

6 Moro-Martin, A. (2019, April 4). COULD 1I/’OUMUAMUA BE AN ICY FRACTAL AGGREGATE? Retrieved from Arvix: https://arxiv.org/pdf/1902.04100.pdf

7 Seligman, D. and Laughlin, D. S. (2020, April 14). Evidence that 1I/2017 U1 ( ‘Oumuamua) was composed of molecular hydrogen ice. Retrieved from Arvix: https://arxiv.org/pdf/2005.12932.pdf

8 Loeb, A. and Hoang, A. L. (2020, June 14). Destruction of Molecular Hydrogen Ice and Implications for 1I/2017 U1 ( ‘Oumuamua). Retrieved from https://iopscience.iop.org/article/10.3847/2041-8213/abab0c/pdf

9 S. J. Desch, Jackson, A. P. (2021, March 16). 1I/ ‘Oumuamua as an N2 Ice Fragment of an Exo-Pluto Surface II. Retrieved from JGR planets: https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2020JE006807

10 Michele T. Bannister, M.T., Bhandare, A., Dybczyński, P.A., Fitzsimmons, A., Guilbert-Lepoutre, A., Jedicke, R., Knight, M.M., Meech, K.J., McNeill, A., Pfalzner, S., Raymond, S.N., Snodgrass, C., Trilling, D.E., Ye, Q.. (2019, July 3)The Natural History of 'Oumuamua. Nature Astronomy.


About the author

Christopher Seymour

In my career as a mining engineer, I have lived in California, New Mexico, South Africa, Australia and the UK. I am now retired in Australia

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