In October 2017 the first interstellar visitor ever spotted by human astronomers passed through our solar system. It had been passing through for years and was on its way out at high speed, having slingshotted around the Sun, when the Hawaii-based Pan-STARRS telescope noticed the interloper. Even so, astronomers were able to figure out it was no spaceship; it was a weirdly-shaped asteroid they christened `Oumuamua, which in Hawaiian means “a messenger from afar arriving first”.
Too bad for alien enthusiasts. But what if it had been an extraterrestrial spaceship? What tell-tale signs would give it away?
Chasing far-flung stars
We know there are at least a few spaceships exploring outer space because we sent them. In 2012 the Voyager 1 spaceship made history when it became the first man-made object to enter interstellar space, and Voyager 2 is close on its heels. New Horizons reached Pluto in July 2015 and is on its way to rendezvous with an object way out in the Kuiper belt by 2019.
One day our probes will explore nearby star systems, and they might do more than just beam back data. One possibility, proposed by mathematician John von Neumann in the 1940s, would be to send robotic probes that clone themselves using raw materials mined from asteroids and then spread out across the galaxy.
Other intelligent, spacefaring civilisations – if they exist – would surely come up with the same elegant idea. So if an alien probe does turn up in our corner of the galaxy, how would we recognise it?
Identifying an interstellar visitor
It’s hard to interrogate a faint, fast pin-prick of light in the vast blackness of space. The only reason we have any chance of spotting interstellar objects is thanks to new automated surveys like Pan-STARRS, the Catalina sky survey and the ATLAS survey, which scour the sky for moving objects.
So what can we find out about such alien objects? The first thing that stood out about `Oumuamua was its orbit. Though it passed through our solar system, it was not captured by the gravitational pull of the Sun.
“It is the only object seen so far with a strongly hyperbolic orbit, meaning that it is travelling so fast that the Sun’s gravity cannot hold it back,” explains astronomer David Jewitt at the University of California, Los Angeles.
This immediately indicated `Oumuamua could be something novel, according to Jonti Horner, an astrobiologist at the University of Southern Queensland. But “extraordinary claims require extraordinary evidence, so people across the planet went into a frenzy to get more observations and lock things down.”
One key observation was to determine whether the object was surrounded by a fuzzy coma of dust and gas, the signature of a comet heating up and releasing gas as it approaches the Sun. `Oumuamua didn’t show any signs of such comet-like activity.
Another is to track how its brightness changes over time. Asteroids have irregular shapes and tend to spin through space, so they appear brighter or dimmer as they tumble in the sunlight. The brightness of a spaceship, on the other hand, would be likely to be more stable.
`Oumuamua showed significant fluctuations in its brightness, suggesting it was an asteroid. Computer analysis of the shifting brightness pattern concluded the rock was highly elongated, roughly ten times as long as it is wide, leading some armchair scientists to draw comparisons to science-fictional artefacts like the monoliths from 2001: A Space Odyssey and the starship from Arthur C. Clarke’s Rendezvous with Rama.
Olivier Hainaut, astronomer at the European Southern Observatory, is quick to point out that such speculation was quite a stretch: “We don’t have anything that hints it is not what it seems to be: a big chunk of rock.”
`Oumuamua seems to be exactly what we would expect of an interstellar asteroid.
But what if it wasn’t? What would reveal the fact it was an alien spaceship?
An obvious giveaway could be found by listening for radio transmissions across a range of wavelengths. “Narrow radio emissions, especially if they are modulated in some way, don’t really happen in nature,” says Hainaut.
Astronomers can also extract information about the surface properties of the object by analysing the spectrum of reflected light. `Oumuamua, for example, was found to have a dark reddish hue, perhaps indicating that its surface is covered with a carbon-rich material.
Unexpected signatures in the spectrum could point to materials such as spacecraft paint. Seeing bright, short flashes could also indicate an artificial polished surface.
If an object is rotating, that might be a hint that it is creating artificial gravity — think the rotating ring of the Hermes spacecraft in Andy Weir’s The Martian, or the Discovery in 2001: A Space Odyssey. `Oumuamua is rotating, but way too slowly to be useful. At 7-8 hours per rotation, “it would not help anybody living in there,” say Hainaut. It would need to rotate more like once a minute.
A spacecraft might also give off a heat signature from an engine or an internal energy source, visible to us in the thermal infrared. Its engine might also give off detectable emissions; American aerospace engineer and author Robert Zubrin suggested in 1993 that theoretically we could detect exhaust from antimatter engines.
If the object strays off the path of a natural gravitationally-driven orbit, this might be another indication of an engine. However, outgassing can also slightly disturb the orbits of comets, so it would take a large variation for the orbit to truly signal an artificial spaceship.
Would we spot an alien probe at all?
In Horner’s opinion, it is enormously unlikely that we would spot any interstellar object, regardless of origin: “We’re only just reaching the technological level to have a good chance of catching these things.”
For interstellar objects to be spotted by automated surveys like Pan-STARRS, they need to come close enough to the Sun and Earth and need to be in convenient places in the sky. Horner believes that if `Oumuamua had come along just a fortnight earlier or later, we probably would have missed it, as it would have been too far from Earth or too close to the Sun in the sky.
“We could be missing most objects of this sort of size,” British space scientist Duncan Steel agrees. “If there are many such objects that are smaller still – as is to be expected – then the vast majority will not be detected by present surveys.”
Future technology will expand our abilities to spot and study interstellar objects. According to Jewitt, in 2022 Pan-STARRS will “be eclipsed by the much more powerful Large Synoptic Survey Telescope (LSST), which should pick [interstellar objects] up by the bucket-load.”
Astronomers estimated at least one interstellar asteroid similar to `Oumuamua passes through our Solar System every year.
According to Hainaut, every one of these objects will be a challenge to study. “No object is ever exactly like any other that we have observed before,” he says. “Each object is an opportunity to learn something more.”
Detecting and carefully studying these objects will allow us to build up a database of their properties. If an artificial visitor does arrive, we’ll have a better chance of recognising its true nature. And then the real fun will begin.
Related reading: Looking for extraterrestrial life