Scientists and stargazers alike were transfixed last year when ‘Oumuamua, the first object known to come from outside our solar system, passed close by our sun. Astrophysicists had long believed it was possible for such objects to exist, but none had ever been observed before.
On October 19th, 2017, the Panoramic Survey Telescope and Rapid Response System-1 (Pan-STARRS-1) in Hawaii announced the first-ever detection of an interstellar asteroid, named 1I/2017 U1 (aka. ‘Oumuamua). In the months that followed, multiple follow-up observations were conducted that allowed astronomers to get a better idea of its size and shape, while also revealing that it had the characteristics of both a comet and an asteroid.
Oumuamua's peculiar, seemingly cigar-like shape and unusual orbital characteristics prompted some people to wonder whether it was an interstellar spacecraft. To find out, astronomers in the outback of Western Australia used the Murchison Widefield Array telescope to eavesdrop on the rocky visitor.
In particular, the astronomers checked for radio transmissions coming from the roughly quarter-mile-long (400 meters) 'Oumuamua between the frequencies of 72 and 102 megahertz, a range that is similar to the frequencies used in FM radio broadcasts. The study found no evidence of signals with non-terrestrial origins.
But something strange happened. As ‘Oumuanua began to leave the Solar System, the Hubble Space Telescope snapped some final images of that revealed some unexpected behaviour, it accelerated. The object is speeding up, in a way that we wouldn’t expect if it were simply being driven by the force of gravity.
The most likely explanation, they claimed, was that ‘Oumuamua was venting material from its surface due to solar heating (aka. outgassing). The release of this material, which is consistent with how a comet behaves, would give ‘Oumuamua the steady push it needed to achieve this boost in velocity.
A new paper from a pair of astronomers Shmuel Bialy and Abraham Loeb from the Harvard Smithsonian Center for Astrophysics (CfA) has taken it a step further, suggesting that ‘Oumuamua may actually be a light sail of extra-terrestrial origin.
This isn’t just baseless speculation, either. The authors of the paper started with a simple idea: What if pressure from the sun’s radiation caused the unexpected acceleration ‘Oumunanua exhibited? Seems reasonable enough.
A light sail is a form of spacecraft that relies on radiation pressure to generate propulsion – similar to what Breakthrough Starshot is working on. Similar to what is planned for Starshot, this light sail may been sent from another civilization to study our Solar System and look for signs of life.
But for the sun’s radiation to cause the acceleration that scientists observed, ‘Oumunanua would have to be a very strange shape. Paul Gilster, a blogger on peer-reviewed astronomy research, explained:
We can work out constraints on the object’s area through its observed magnitude. The paper proceeds to show that a thin sheet roughly 0.3 mm thick and some 20 meters in radius will allow the non-gravitational acceleration computed in the Micheli paper.
He added:
Thus, considering the object as a thin surface, we could imagine a conical or hollow cylindrical shape. “You can easily envision that by rotating a curved piece of paper and looking at its net surface area from different viewing angles,” Loeb told me.
So let’s back up a moment. We are asking what properties ‘Oumuamua would have to have if its non-gravitational acceleration is the result of solar radiation pressure. We do not know that solar radiation is the culprit, but if it is, the object would need to be a thin sheet with a width in the range of 0.3 mm. This scenario explains the acceleration but forces the question of what kind of object could have these characteristics. A major problem is that, as mentioned above, there are too many degrees of freedom in our observations to nail down what ‘Oumuamua looks like. We did not have observations sensitive enough to produce a resolved image.
As for what an extra-terrestrial light sail would be doing in our Solar System, Bialy and Loeb offer some possible explanations for that. First, they suggest that the probe may actually be a defunct sail floating under the influence of gravity and stellar radiation, similar to debris from ship wrecks floating in the ocean. This would help explain why Breakthrough Listen found no evidence of radio transmissions.
Loeb further illustrated this idea in a recent article he penned for Scientific American, where he suggested that ‘Oumuamua could be the first known case of an artificial relic which floated into our Solar System from interstellar space.
Humans have already constructed such objects of similar dimensions, they note, and such vessels “might be abundantly used for transportation of cargos between planets or between stars.
Vision of a future light sail credit MV
There’s another even more exciting possibility, ‘Oumuamua' could be an active piece of alien technology that came to explore our Solar System.
“The alternative is to imagine that `Oumuamua was on a reconnaissance mission. The reason I contemplate the reconnaissance possibility is that the assumption that `Oumumua followed a random orbit requires the production of ~10^{15} such objects per star in our galaxy. This abundance is up to a hundred million times more than expected from the Solar System, based on a calculation that we did back in 2009. A surprisingly high overabundance, unless `Oumuamua is a targeted probe on a reconnaissance mission and not a member of a random population of objects.”
According to Loeb, that there’s also the fact that ‘Oumuamua’s orbit brought it to within 0.25 AU of the Sun, which is a good orbit for intercepting Earth without experiencing too much solar irradiation. In addition, it came to within 0.15 AU of Earth, which could have been the result of orbital corrections designed to facilitate a flyby.
Frustratingly, it seems most likely we’ll never get a definitive answer about what ‘Oumuamua was. It has left our solar system, and it’s too distant to observe now. Even while it was nearby, our imaging technology could only capture hazy pictures of the object. Oumuamua is sure to remain the subject of intense study for many years to come. And you can bet astronomers will be on the lookout for more of them!