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Nailing down the nature of 'Oumuamua-it's probably a comet, but …



Image of an extraterrestrial spaceship from the film Encounters of the Third Kind.
Enlarge / I mean, maybe, right? Maybe? Probably not, though. Almost certainly not.

Shortly before Halloween, the chairman of Harvard's astronomy department openly declared that an interstellar object hurtling through our Solar System could just be part of an extraterrestrial craft. And then … crickets.

The astrophysics blog Centauri Dreams broke the story to the three days later. It presented an informed survey of the academic paper which raised this brash possibility, bolstered with quotes and commentary from the paper's co-author (and noted department chair), Avi Loeb. It was well into November before outlets like CNN, Time, and The Washington Post picked up the story, replete with the inevitable sarcastic quotation marks and snarky headlines. The object, named 'Oumuamua, had a number of weird and seemingly contradictory properties; it could be that those properties appeared the way they do because our observations were not all that great. There are also other possibilities.

I read Loeb's paper-which by then was speedily accepted for the publication by the respected Astrophysical Journal. A few days later, Loeb and I sat down for the longest and-by Loeb's own account of the most serious and in-depth interview he's given on this subject. The embedded audio player following the colon at the end of this very sentence features an hour-ish edit of it, including all the highlights:

If you're not into spoken word audio, we've got a transcript available as both plain text and as a PDF (which is probably a little easier to read).

"I'm not saying it's aliens, but …"

Avi Loeb is clearly nosing around one of the most extraordinary claims in astronomy. This of course requires extraordinary evidence-a requirement from which Loeb's fancy job title earns him no exceptions. But we must also avoid the inverted knee-jerk response, which goes like something, "just because Harvard's astronomy chairman says it could be an alien craft does not mean it is one; and in fact, it mean it is not one, because of the irony! Oh, and smugness, too. "

My interview with Loeb should not settle this debate in favor of aliens for you, me, or anyone (Loeb himself needs more evidence to come close to considering the case resolved). But the story of 'Oumuamua is inherently fascinating. Digging into it, non-astronomers can not help but learn a thing or three about the universe works. If you go down this path, you must be aware that alien technology has been considered, then ultimately dropped as explanations for many astronomical phenomena. 'Oumuamua will join this list definitively someday. But much is learned by chasing down leads-both by field of astronomy and by curious outsiders who follow the process.

If you listen to our interview (or read our probably-OK-ish transcript), you'll understand this debate on a subtler level than most of the people yammering about it. And the really cool thing? For 2022, when an important new telescope goes online.

For those in a hurry, I'll now provide a synopsis of our interview, punctuated with time stamps to help you zip to the parts that most interest you.

There's something about 'Oumuamua

Our story starts on 19th of last year (at timestamp 07:55 of the interview audio above, if you want to listen for more details than those are contained in this brief write-up). That's when the object that would come to be named 'Oumuamua was first spotted by Hawaii's Pan-STARRS system, which tracks and detects near-Earth objects.

Astronomers quickly established that 'Oumuamua was traveling too fast to be bound by our Sun, which meant it originated in a distant star system. This made it the first interstellar object definitively identified within our Solar System. Rightly intrigued, the astronomical community pointed a great deal of hardware towards the receding blip. Masses of observational data were obtained before 'Oumuamua vanished from sight in January.

An artist's impression of the oddly shaped interstellar asteroid 'src = "https://cdn.arstechnica.net/wp-content/uploads/2017/11/1I_eso_artistimpression-640x400.jpg" width = "640" height = " 400 "srcset =" https://cdn.arstechnica.net/wp-content/uploads/2017/11/1I_eso_artistimpression-1280x800.jpg 2x
Enlarge / An artist's impression of the oddly shaped interstellar asteroid 'Oumuamua.

'Oumuamua was bizarre on multiple fronts from the get-go. An interesting one is that it travels at the "local standard of rest" (timestamp 15:36) amongst our local pack of stars. For reasons Loeb explains, this is a fascinating attribute and an imposbable (though not impossible) one for a natural object to have.

In June (timestamp 23:22), Nature released a rigorous analysis of 'Oumuamua's trajectory. Its authors determined-with 30 standard deviations of confidence-that the object was accelerating as it receded from the Sun. This was interpreted as proof that it was a comet, rather than an asteroid (the other likely candidate). Comets are typically accelerated in this manner, propelled by the gases released by the heat of the Sun, which create their signature tails.

However, many observations ran counter to this. (timestamp 25:44). For instance, no tail was ever observed on 'Oumuamua. Neither was a coma (a comet's fuzzy head). There was no sign of water on it, and comets typically carry water. And 'Oumuamua's surface reflection is far beyond the bounds associated with comets.

These and other quirks can be explained or justified on their own. But for Loeb, the last straw was a September paper by Cambridge University's Roman Rafikov (timestamp 28:39). It argues that 'Oumuamua's spin rate (which was quite zippy-another oddity) was observed throughout the span of observations, while outgassing should have perturbed the spin significantly.

Loeb concluded that outgassing could not have caused 'Oumuamua's acceleration. He considered alternate forces and settled on one that astronomers understood well: the pressure of radiation beaming out from the sun. But this is a very weaker force than outgassing. If it was responsible, 'Oumuamua would have to be far smaller than the quarter-mile-plus hunk of rock astronomers envisaged. Specifically, Loeb pegged it as being as small as 20 meters in diameter. And-here's the clincher-less than a millimeter thick.

Close encounters of some kind or another

No known natural process can produce anything remotely this thin in space. But this sounds a terrible lot like a solar sail. And Loeb has spent many long hours modeling the physics of solar sails, in helping to lead Yuri Milner's Breakthrough Starshot project (timestamp 18:55). Yes, the cliché about hammer owners mistaking non-nails for nails instantly leaps to mind, and Loeb acknowledges this (30:07). But hammerers have also been known to identify nails accurately.

The most exotic possibility in the Loeb's paper33:55) is that 'Oumuamua was on a targeted reconnaissance mission (not necessarily singling out Earth-but perhaps generally cruising the habitable zones of star systems). This is based on legacy calculations concerning the relative abundance of interstellar objects, and other factors.

Loeb and I then discuss the online archive where he and his co-author, postdoctoral fellow Shmuel Bialy, initially place their paper36:58) and the uncommon rapidity with which Astrophysical Journal both accepted and published it (40:35). I then present Loeb with the jabs of some of his critics, to which he responds (44:51). This leads to a discussion of Loeb's philosophy about the roles and responsibilities of academics.

The object's unusual approach suggests it came from our Solar System. "Src =" https://cdn.arstechnica.net/wp-content/uploads/2018/06/Screen-Shot-2018-06-28-at-1.51 .22-PM-640x379.png "width =" 640 "height =" 379 "srcset =" https://cdn.arstechnica.net/wp-content/uploads/2018/06/Screen-Shot-2018-06- 28-at-1.51.22-PM.png 2x
Enlarge / The object's unusual approach suggests it came from our Solar System.

We close on the fascinating prospect of a large telescope debuting in 2022 may be the fastest answer questions that elude current hardware (56:56). This gets back to the abundance of interstellar objects like 'Oumuamua. If they are as rare as earlier calculations indicated, the more powerful new gear will only discover a small handful of new ones. But if they are common enough to make Oumuamua's discovery unsurprising, the new telescope should rapidly spot thousands of them.

This argument is too involved to fully explore here (I am a podcaster, not a journalist). So I urge you to listen to this section. It all but gives Loeb's controversial explanation a sell-by date, and that date is just a few years off.

Personally, I can not wait to follow closely as they approach. Listen to this section, and you'll know the underlying issues as well as I do. However, there's at least a small chance that 2022 will bring jarringly suggestive evidence that 'Oumuamua is an artificial relic. And whatever the outcome, would not it be cool to follow that story as it unfolds?

This interview is the most recent episode of my podcast After On. If you enjoy it, a full archive of my episodes can be found on my site or via your favorite podcast by searching the words "After On." The broader series is built around deep-dive interviews with world-class thinkers, founders, and scientists, and it tends to be tech- and science-heavy.


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