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Aliens vs comet junk: NASA scientist weighs in on Oumuamua debate

Please be aliens, please be aliens.

This artist's impression shows the first interstellar asteroid: Oumuamua. 
European Southern Observatory / M. Kornmesser

A respected Harvard scientist still says aliens might be responsible for the mysterious object that has stirred up so much drama since visiting from another stellar mama in 2017. Now, a longtime NASA scientist says you can save that idea for Futurama. His new research suggests it might have broken apart before it even arrived.

At this point there are just a few things everyone agrees on when it comes to the space object known as Oumuamua. The consensus is that it came from outside the solar system, it appears to have had an unusual, oblong shape, and it unexpectedly accelerated on its way out of the solar system.

Trying to explain all these pretty unusual characteristics has led scientists to produce a number of hypotheses, including Oumuamua as a dead comet, a piece of an obliterated planet and even a mysterious hunk of dark matter.  

But the most earth-shattering notion so far has come from Harvard astronomy chair Avi Loeb. The astronomer suggested in November that the speedy, cigar-shaped space rock might actually be an alien space sail of sorts. The conclusion reached in a paper by Loeb and researcher Shmuel Bialy quickly went viral, and Loeb has doubled-down on the idea over and over in the face of criticism and smirks from colleagues.

The most baffling thing about Oumuamua is the fact that it seemed to accelerate on its way out of town. This would be expected from a comet that gets heated up by the sun, causing trapped gas in its nucleus to expand and shoot through holes and weak spots in the crust, kind of like steam jetting out of the hole in a teapot. The effect can act just like thrusters on a rocket. 

But there were no signs of outgassing on Oumuamua, meaning something else was pushing on it to speed it up. 

Loeb suggested that the radiation pressure from the sun itself provided the boost. But for this explanation to work, Oumuamua would have to be quite thin, like a sail on a ship. The Breakthrough Starshot project (which Loeb is a part of) is actually working on a similar design to send a tiny spacecraft to Alpha Centauri. So perhaps Oumuamua is evidence that another intelligent civilization beat us to the idea, according to Loeb.

But longtime NASA scientist Zdenek Sekanina has another idea that's not nearly as tantalizing as an alien space sail: Oumuamua was really just the leftover debris cloud from an interstellar comet that broke apart shortly before it was first spotted by astronomers. 


What if Oumuamua is really just a cloud of junk from a cracked-up comet?


"This model of 'Oumuamua provides an opportunity to consistently explain its nongravitational acceleration as an effect of solar radiation pressure," he writes in a paper posted to Cornell's pre-print Arxiv server (PDF) last week. "The rest of the parent dwarf comet's debris is expected to have escaped detection."

The idea here is that a cloud of dust and debris from an interstellar comet that went to pieces when it came too close to our sun is obviously less dense than a big rock cigar. This notion of a porous cloud could explain why the relatively weak radiation pressure exerted by the sun was enough to accelerate it. 

Imagine the effect a breeze would have on three different objects: a cloud, a plastic bag and a rock. Solar radiation pressure is like the breeze and Oumuamua is one of the three objects. If it's a cloud, as Sekanina suggests, or something very thin, lightweight and manufactured like a plastic bag, as Loeb posits, then its behavior makes sense. 

But Loeb isn't buying Sekanina's idea at the moment.

"My main concern with the cloud of debris idea is that in order for radiation pressure from sunlight to provide the observed push on Oumumua, the thickness of the object needs to be less than a millimeter," he told me via email. "This would be very difficult to arrange in a natural setup of a porous debris cloud."

But Sekanina, who was recently called the "grandmaster of comet decay," believes his model adds up and that a cloud of comet crap wouldn't have to be that well organized to explain Oumuamua's movements. He conceptualizes the object "as a monstrous, very irregularly shaped and devolatilized fluffy aggregate of loosely bound dust grains."

So the debate rages on. 

We may never know for sure what Oumuamua is or was, but one thing is clear: My 2019 Halloween costume will be "a monstrous, very irregularly shaped and devolatilized fluffy aggregate of loosely bound dust grains." Sounds like the perfect mix of timely and terrifying but just a little bit cute, too.

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