IT WAS our first known interstellar visitor when it was detected flashing past the Sun in October last year. Dubbed ‘Oumuamua’ — Hawaiian for messenger — it was quickly determined to be not of this solar system.
Its trajectory had been traced. And the track it was on could not have possibly been an orbit around our Sun. So it must have come from deep space.
Follow-up observations after the Pan-STAARS-1 telescope in Hawaii announced its discovery revealed the object to be odd.
It was elongated. It was about a kilometre long. It was a strange reddish colour. And it appeared to have properties that belong to both comets and asteroids.
Naturally enough, some speculated it could be an alien spacecraft. SETI (the Search for Extraterrestrial Intelligence) was interested enough to turn one of its electronic ears in its direction.
It found nothing. There were no observable omissions from the tumbling interstellar visitor.
And, as Oumuamua was moving so fast as to already whip it into the outer reaches of our Solar System, interest waned.
There just wasn’t much more we could do to check it out.
now, a new study by two Harvard Smithsonian Center for Astrophysics astronomers has postulated that it may, after all, have been an alien object.
A light sail.
The study, titled Could Solar Radiation Pressure Explain ‘Oumuamua’s Peculiar Acceleration, was presented by Shmuel Bialy and Professor Abraham Loeb.
Professor Loeb is a committee member of the Breakthrough Starshot committee — a project announced by the late astrophysicist Stephen Hawking and funded by Russian billionaire Yuri Milner that seeks to speed up our search for extraterrestrial intelligence.
Any examination of the unusual nature of Oumuamua must include “the possibility that it might be a lightsail of artificial origin,” he writes
Oumuamua was found to have a high density. Normally, this would indicate it was an object made of rock and metal.
This seemed to be supported as it skimmed past the Sun. No comet-like clouds of gas spun out as a tail in the solar wind.
But a spectral analysis — where light is broken down into its components to identify the chemicals influencing its colours — indicated it was much icier than expected.
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But the big eyebrow raiser was Oumuamua’s speed.
After passing the Sun, it actually sped up. It should, by all accounts, have slowed down …
Unless it was a comet, venting gas from the warmer face closest to the Sun. This could give it the boost needed to increase its velocity.
So, where was that comet-like tail?
It’s still not been found.
The Harvard astronomers also point out that any such ‘outgassing’ would have quickly changed the nature of Oumuamua’s spin.
This also was not observed.
With these anomalies, Bialy and Loeb suggest there is only one other viable alternative: that it is a mechanical light sail, designed to use starlight to propel it through space.
“The first artificial relic might have just been discovered over the past year when the Pan STARRS sky survey identified the first interstellar object in the solar system, ‘Oumuamua,” Professor Loeb writes.
It’s a concept similar to one Breakthrough Starshot is working on.
“We explain the excess acceleration of `Oumuamua away from the Sun as the result of the force that the Sunlight exerts on its surface,” they write. “For this force to explain measured excess acceleration, the object needs to be extremely thin, of order a fraction of a millimetre in thickness but tens of meters in size. This makes the object lightweight for its surface area and allows it to act as a light-sail. Its origin could be either natural (in the interstellar medium or proto-planetary disks) or artificial (as a probe sent for a reconnaissance mission into the inner region of the Solar System).”
The Harvard astronomers attempted to compute the probable shape, size and mass any such interstellar light-sail would need.
It would need to survive the intense cold and extreme radiation of deep space. It would also need to be structurally rigid enough to cope with the stresses of its spin.
Their calculations state it could be achieved by an incredibly thin — just a fraction of a millimetre — thick sheet of metal.
“For a thin sheet this requires a width of ≈ 0.3−0.9 mm,” the study reads. “We find that although extremely thin, such an object would survive an interstellar travel over Galactic distances of about 5 kpc, withstanding collisions with gas and dust-grains as well as stresses from rotation and tidal forces.”
Professor Loeb states similar lightsails have already been designed and built here — such as the Japanese IKAROS and his own Starshot Initiative.
Why would such an alien ship be here?
Bialy and Loeb speculate it could be flotsam — a jetissoned solar sail floating at the whim of the interstellar winds. That would explain the lack of transmissions, they say.
“Finding evidence for space junk of artificial origin would provide an affirmative answer to the age-old question “Are we alone?” This would have a dramatic impact on our culture and add a new cosmic perspective to the significance of human activity.”
Equally, Loeb said, Oumuamua could be a space probe.
“The alternative is to imagine that `Oumuamua was on a reconnaissance mission,” he told Universe Today. He said the objects path was simply too convenient.
It passed within 0.25 AU (Astronomical Units, the distance of the Earth from the Sun) of the Sun which avoided the worst of its solar radiation. It then crossed within just 0.15AU of Earth.
Both astronomers concede we simply know too little about Oumuamua to reliably guess its nature. But, at the very least, for it to have the observed characteristics it has it must be an entirely new type of material or object.