In 2017, a small lump of ice and rock made headlines around the world. Named 'Oumuamua - the Hawai’ian term for “scout” - this body was initially assumed to be a long period comet. However astronomers soon established that its hyperbolic trajectory was unlike any they had seemed before - ‘Oumuamua was reclassified 1I/2017 U1, the I standing for “Interstellar” and indicating that this was the first known object to have been observed within the Solar System having originated from outside it. In 2019 a second such object, 2I/Borisov, was observed, suggesting that these are likely relatively common, although it is only recently that we have possessed the technology to detect them.

When ‘Oumuamua was first observed, a great deal of speculation was generated by the interstellar visitor. It was shown to have an elongated physical shape unlike those of known asteroids and comets, and it also showed changes to its trajectory while close to the Sun that were unexpected at the time. It was speculated that such interstellar asteroids could be shaped and sent out as messengers or probes by extraterrestrial intelligences. While it is certainly possible to make such arguments, detailed study has shown repeatedly that all of ‘Oumuamua’s properties and behaviour could be explained through simple physical processes and the known behaviour of astronomical objects. As a result, the alien hypothesis is both unproven and unnecessary, leaving Occam’s razor heavily skewed towards this being a natural object, with no need for sentient manipulation.

Perhaps though it is unsurprising that a few astronomers and a great deal of the media leapt onto the slim possibility of an alien encounter in the form of these bodies. It is a premise that has formed part of the science fiction repertoire for many years.

(Image: The orbit of 'Oumuamua through the inner solar system, credit: NASA/IFA Hawaii)

A Rendezvous with Rama

Probably the most influential work in this area, and the one most cited by those discussing ‘Oumuamua is the novel Rendezvous with Rama (Clarke, 1973; later sequels coauthored with Gentry Lee, 1989-1993). Arthur C Clarke was not only one of the best known science fiction authors of the second half of the twentieth century, he was also a leading member of the British Interplanetary Society, an organisation which works to establish scientifically-plausible space science, largely from outside of mainstream academia. As such, and given his own training as a radio engineer, Clarke tended to bed his fiction in the plausible and to provide detailed technical explanations, which leant his speculations verisimilitude.

In Rendezvous with Rama he described a huge alien artifact - a megastructure in the form of a hollow cylinder fifty kilometers in length and twenty in diameter - which spun slowly around its long axis to generate apparent gravity on its inner surface through centripetal acceleration. Equipped with lights and biological robot servitors which activated as it approached the Sun, Rama was sufficiently large to support internal weather systems and hence to act as a self-sustaining artificial world for its inhabitants. This general premise was expanded as a serious proposal for a space-habitat by physicist Gerard O’Neill in 1976, and has become known as an O’Neill cylinder.

The origin and destination of Rama was not revealed in the initial book although it was explored in later novels of the series. The human explorers who entered the cylinder were left to speculate that it had either been sent out before being inhabited, or that life aboard had died out. An alternate possibility is that Rama is an automated probe designed to scan other systems and send that information back to its homeworld in some undetectable way. Certainly the systems of Rama were designed to go into automatic hibernation when the cylinder left the Solar System and were equally capable of reviving themselves when close to a star, although the need for a functional biosphere in the first place is unclear in that case.

The similarities of ‘Oumuamua are purely cosmetic and most are inferred from sparse data. ‘Oumuamua was no more than a few hundred metres long, and perhaps 70m wide. Unlike Rama it was also tumbling, rather than rotating about its long axis. And while, like Rama, ‘Oumuamua is made of rock, there is no indication from its mass or size that it could be hollow, and no indications of extraterrestrial modification.

(Image: Artist's impression of 'Oumuamua)

The Comet, the Cairn and the Capsule

Perhaps a closer analogy to ‘Oumuamua can be found in the short story “The Comet, the Cairn and the Capsule” (1972) by Duncan Lunnan, which can be found in the anthology “The Science Fictional Solar System (1980, eds. Asimov, Greenberg and Waugh). In this story, a crew of three is despatched to intercept a rare highly hyperbolic comet and find, to their surprise, that they are not the first to anchor a scientific analysis package into the interstellar visitor. As the character Paxton describes the discovery:

“It looks like the bottom half of a totem pole. I’d say there are three distinct sections, one on top of the other. The bottom one is gold, or covered in gold foil, cylindrical, with heat radiator panels projecting. The one above that is roughly spherical, black and silver, with solar cell panels on the surface and projecting antennas. The top section is hexagonal for three fourths of the way up, then it becomes a straight cylinder of lesser diameter. It too is gold, and some of the panels of the hexagon have solar cells. There are connecting rods from it to one side, anchoring it to the bottom section. I don’t think they touch the sphere at all.”

They soon realise the implications:

“We’re looking at objects from outside the Solar System altogether, like the comet itself. Sometime in the past, when this nucleus swung past another sun, there was another landing here - perhaps more than one.”

“If that’s true,” said Scherner, “then the object might be millions of years old. This is a fast comet, but over interstellar distances…”

“Not less than a million years,” Paxton agreed.

Realising that this construct indicates the cumulative efforts of not just one alien culture but three, the astronauts are left with a dilemma. In the face of calls from Earth to shatter the comet, destroying it in order to recover the devices for further study, instead they choose to anchor their own package at the top of the cairn, adding to the legacy of the interstellar passage.

This story interests me in several ways. First, and most relevant in the context of this discussion, the decision to place the tale on an interstellar comet, rather than one native to the Solar System, was arguably a narrative device, but the difficulty of intercepting such a body and its unusual orbit are well described. Like Clarke, Lunan was a member of the British Interplanetary Society, with an interest in human space flight and astronomy, so perhaps his astronomical detail shouldn’t be wondered at. He has also since been involved with astroarchaeological sites, suggesting an interest in the culture as well as the practice of astronomy [1].

Interestingly, in deciding to add to the cairn, the crew of his story improvise an engraving that will serve as a message to alien intelligences, including scientific representations, a picture of the Solar System and a picture of human beings. I've already written about the use of scientific notation as a common language and its use on probes expected to leave the Solar System. But as the editors note in their introduction to the story, this was written virtually simultaneously with the launch of the Pioneer probes - the first to carry SETI communications plaques. It is unclear whether the idea was generated independently by Lunan, whether his work predated that of Sagan and the SETI community, or whether it was an idea in broader circulation at the time. It is intriguing to note that rather than pulsars, Lunnan’s astronauts chose to anchor their Solar System map with regard to the Milky Way disk and its satellite galaxies - a less precise but arguably more accessible guide to position.

This story is also notable for its commentary on the conflicting demands of scientific discovery and cultural heritage. Here, this is played out in the minds of the astronauts who must decide whether the destructive needs of scientific investigation outweigh the symbolic, emotional and artistic merit of adding a symbol of humanity to an alien monument. The same basic questions arise in a range of different environments, but have been raised in the astronomical community in the context of building telescopes on mountain observing sites that have cultural or religious significance. While the dilemma of what to do when confronted with an alien artefact may be less down to Earth, it poses the same questions of where true value is to be found.

Into the Future 

Perhaps unsurprisingly, the prominent news reports which greeted ‘Oumuamua’s passage through the solar system (and to a lesser extent that of Borisov), appears to have inspired more recent science fiction. Author David Wellington used the premise of an interstellar alien probe, explicitly compared to ‘Oumuamua, on collision course for Earth in his 2019 novel “The Last Astronaut” in which he hypothesised that the second interstellar object would not be identified until 2044 (it’s dubbed 2I/2044 D1) in the novel - very different from the true date of 2019 itself. The covid-delayed disaster movie Greenland (2020, dir Waugh) also apparently features an interstellar comet (dubbed Clarke in a knowing nod) impacting with Earth. 

While only two interstellar objects have been identified to date, it is likely that soon-to-be-comissioned survey telescope facilities, such as the Legacy Survey of Space and Time (LSST) at the Vera Rubin Observatory, will identify dozens or even hundreds of such objects over the coming decade. Such visitors will likely never be entirely routine, as their range of surface properties, compositions and properties will continue to yield valuable direct information about solar systems beyond the range of human travel. And as long as these remain unvisited by human probes of even crewed ships, speculation will continue about the possibility of alien civilisations leaving their marker on these interstellar messengers, or even the possibility that deep in their cores, alien lifeforms themselves may remain to be found.