In October 2017, some astronomers realised that our solar system had a visitor. For the first time in recorded history, an interstellar object was being observed. It wasn’t much—a small, dim pinpoint of light only able to be seen by our most powerful telescopes—but its impact on the astronomical community was astronomical in itself.
Alien artefact or Rolling Stone:
Debunking ‘Oumuamua
Disagreeing with one astronomer’s theory has led to a Te Herenga Waka academic studying a surprise visitor to our solar system.
The nature and origins of the object, eventually named ‘Oumuamua (meaning ‘scout’ in Hawaiian), were heavily debated. Although its exact size and shape were impossible to confirm from observations, the change in its reflection as it travelled allowed for its length to be estimated as approximately 100 to 1,000 metres, and its shape described as either like a cigar, a disc, or an oblate spheroid.
By 2019, a majority of astronomers agreed on at least one thing: ‘Oumuamua is a natural object—be it the remnants of a comet, a piece of a Pluto-like planet, or a ‘cosmic dust bunny’. However, a very small number of astronomers had a different theory, which was that ‘Oumuamua was made by aliens. One such astronomer was Dr Avi Loeb, who released a book in 2021, Extraterrestrial: The First Sign of Intelligent Life Beyond Earth, where he presented his theory that ‘Oumuamua was a solar sail built to carry a spacecraft across the vastness of space.
Enter Dr Stephen Curran, a senior lecturer in physics at Te Herenga Waka, who was alerted to a Radio New Zealand (RNZ) interview with Dr Loeb and found himself distinctly unimpressed.
“I thought … this is absolute nonsense. I don’t know what’s going on here—this isn’t science.” He contacted RNZ and was invited to talk about his views on air.
In preparation, Dr Curran did some sums on what it would mean if ‘Oumuamua was a light sail and realised that the results were publishable. “So I went ahead and did it.”
A solar sail isn’t a new idea. It is one of many theorised propulsion systems for carrying a spacecraft through interstellar space. While solar radiation is massless, it can create pressure, and with a large enough sail, it could be used to push a spacecraft away from a star without any need for fuel. With no friction in space, a solar sail is theoretically capable of accelerating to close to the speed of light.
“My original issue was that, according to Loeb, if it’s a light sail, it must be artificial. And I thought, ‘No, that’s like saying a butterfly has wings, we use wings on planes, therefore butterflies must be artificial.’ We copy from nature, not the other way around.”
Dr Curran was flabbergasted to see that Loeb had gone into detail discounting the other explanations for ‘Oumuamua but had defaulted to ‘it must be aliens’.
While in theory a solar sail accelerates forever, Dr Curran realised that they all have an effective terminal velocity, similar to how skydivers reach a maximum speed as they fall. In the case of solar sails, this terminal velocity was much lower than what was claimed by their proponents.
“I thought … this is absolute nonsense. I don’t know what’s going on here—this isn’t science.”
“The thing is, the quicker you accelerate away from the star, the quicker you accelerate away from your power source, to the point that acceleration approaches zero. While never quite reaching this point, it doesn’t take long to reach a speed where it would take longer than the lifetime of the universe to achieve your next metre per second.”
Dr Curran’s calculations showed that ‘Oumuamua would have a terminal velocity of 610 metres per second. At this speed, it would take two million years to reach Proxima Centauri, the nearest star to our sun. In astronomical terms, two million years is not long, but ‘Oumuamua didn’t come from Alpha Centauri. While astronomers are yet to identify exactly which star it might have come from, even the closest candidate is still so far away that it would have taken ‘Oumuamua half a billion years to reach us—a short time by the standards of the universe, but a long one by the standards of any civilisation. It’s just one more piece of evidence to answer the question of the origins of our mysterious visitor.
For Dr Curran, the answer is obvious. “Even if it was a light sail, which it probably isn’t, it’s unlikely that ‘Oumuamua has been sent by an extraterrestrial civilisation and more likely that it’s just an unusually shaped rock that happened to wander into our solar system.”