Spaceship ‘Oumuamua?
DAVID HAMBLING reports on the latest controversy caused by our recent interstellar visitor
‘Oumuamua, pronounced “Oh-mooahmooah”, was the first known object from interstellar space to pass through our Solar System (see FT346:14). Literally as soon as it appeared in 2017 some suggested that it might be an alien spacecraft; the name Rama was proposed after a giant interstellar vessel visiting the Solar System in a 1973 Arthur C Clarke novel. Instead it was given the equally suggestive Hawaiian name meaning Scout, having been first spotted form an observatory in Maui. Harvard astronomer Avi Loeb has held on to the spacecraft theory, and is now releasing a book claiming it is “The First Sign of Intelligent Life Beyond Earth”.
This claim put Loeb at odds with the astrophysics community. The conflict is over interpretation rather than data. Loeb says other scientists are not being sufficiently open-minded; they believe he is focusing too much on one improbable explanation.
‘Oumuamua is strikingly different in appearance to most space objects, being roughly cigar-shaped and hundreds of metres long and some tens of metres across. It appears to be tumbling through space rather than simply rotating. Astronomers do not know whether it is a single monolithic object like an asteroid, a ‘dusty snowball’ of water ice like a comet, or even an iceberg made of solid hydrogen. Early suggestions that it could have come from a nearby star having now been discounted. From its trajectory ‘Oumuamua has probably been travelling through the Milky Way for millions of years.
What really excited astronomers, Loeb in particular, was that ‘Oumuamua accelerated as it moved away from the Sun. This could not be accounted for by gravitational forces. The best guess was that it was caused by outgassing, frozen hydrogen boiling off to produce a rocketpropulsion effect. This is seen in comets, although ‘Oumuamua did not leave a visible tail like a comet.
Loeb proposes a quite different cause. He sees ‘Oumuamua not as a massive, three-dimensional object as other astronomers assume, but as a flat sheet less than a millimetre thick. He writes: “One possibility is that ‘Oumuamua is a lightsail, floating in interstellar space as debris from advanced technological equipment.”
The concept of the lightsail was introduced by Russian scientist Friedrich Zander in 1924. Light exerts a small but definite pressure when it is reflected from a surface. Zander suggested that a spacecraft with a large enough sail could be carried by sunlight just as a schooner is driven by the breeze. It takes a very large sail though: sunlight only exerts a force of about a thousandth of a gram per square metre, so a sail the size of a football pitch would only lift the weight of a 20p piece.
Lightsails will never replace rockets, but they could be useful for interplanetary travel away from Earth’s immediate gravity. Because lightsails do not require propellant, unlike rockets they never run out of fuel. And the slow, steady acceleration from a lightsail adds up over time. In 2015 the Planetary Society launched a satellite with the first lightsail able to carry out manoeuvres by light pressure alone. NASA has a more ambitious project on the launch pad, the Near-Earth Asteroid Scout. This will use the thrust from a lightsail to get a closeup view of an asteroid that could not be approached with conventional propulsion.
‘Oumuamua’s slight acceleration was the right order of magnitude for a lightsail, and it is not surprising that Loeb should be thinking in those terms. He was part of the team working on Breakthrough Starshot, a private initiative funded by Mark Zuckerberg among others to send a lightsail-powered craft to Alpha Centauri. Sunlight alone would not be enough to make the journey in a reasonable time, so it will be boosted by gigawatt-powered space-based lasers.
While others insist that seeing ‘Oumuamua as something artificial requires a stretch of the imagination, Loeb responds that the problem is scientists’ lack of imagination. “If you show a cellphone to a caveman who looked at rocks all of his life, the caveman would conclude that the cellphone is just a well-polished rock... You need to be open-minded in order to find wonderful things,” Loeb told CNET news.
Others are not convinced by these “wonderful things”.
“A shocking example of sensationalist, ill-motivated science,” astrophysicist Ethan Siegel wrote in Forbes magazine. North Carolina State University astrophysicist Katie Mack described Loeb’s theory as simple trolling for publicity. A research team from the International Space Science Institute found no compelling evidence for an alien explanation for ‘Oumuamua, and dismissed Loeb’s theory as not based on fact.
The argument echoes the debate about whether the Viking lander found chemical evidence of life in 1976 (see FT389:12). Both theories fit the facts, but one requires alien life: the disagreement is over whether this is a wild leap or a logical extrapolation of known laws.
The best way to resolve any scientific argument is usually by gathering more data. Unfortunately, ‘Oumuamua has left our Solar System and is no longer around to be scrutinised. It if was a scout, it passed by without stopping. There was not enough warning to train all of the best instruments on it, and there is no realistic possibility of sending a space probe in pursuit to examine it more closely.
As the first interstellar object, it was unprecedented. We simply have nothing to compare it to. With an asteroid or comet, astronomers have an idea of what constitutes normal, but in this case there is no benchmark for comparison.
However, the world’s astronomers are now primed to detect and inspect objects from outside the Solar System, and a second one, Borisov – or 2I/Borisov, indicating the second interstellar object – was found in 2019. This time the object was positively identified as a comet, albeit an unusual one. As more interstellar visitors are catalogued and studied, we may discover whether ‘Oumuamua was a typical example of the type, or an anomaly that could only be the product of alien manufacture.