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Alright, question for our resident spaceman. What is your opinion about Oumuamua? I know there's no scientific consensus on it yet, but do you have any gut feelings? Speculation? Is it just a rock, or an alien craft? Or maybe it's a sentient alien species of space rock?
I've been asked that a number of times. Occupational hazard I guess. There actually is scientific consensus on the basics. If you don't mind I'll do a cut-and-paste from a previous response for now. It is a bit long. Sorry, I don't have time to make it shorter. If you have any other questions or comments, please feel free to follow up.
(Disclaimer. I wrote the following, and it is my opinion only. I am an engineer, not a scientist. My employer doesn't speak for me or vice-versa. Etc, etc....)
'Oumuamua was great. There is almost nothing known for sure about it, and what is known makes it a huge target for speculation. Even the name is open to interpretation. "Ou" means "to reach out" in Hawaiian. "Mua" means "before", or an early instance of something. Repetition means emphasis, so literally "muamua" would translate as "early early", or maybe "first". Ok, so should "ou" mean something with intent (subtext aliens), and if so what kind of intent? Are we talking first contact here? Is this a scout? Scout could just mean exploration, but there's also the military use of the term "scout". The excitement builds!
Then there's the dimensions. All we really have to go on is brightness changes that were observed as it tumbled through the solar system and some spectral measurements. We never got closer than about 85 million kilometers to it, or 80 or so times the distance from the Earth to the moon. It was already on it's way out when we saw it too, so there was no chance to launch a probe.
It was going too fast to catch up with with current technology anyway. When it entered and left the solar system it was going over 26km per second. At closest approach to the sun it was going almost 90km per second. The fastest object ever made by humans was the New Horizons spacecraft that went to Pluto. When it left Earth it was going 16.26 kilometers a second. Even if we'd had another New Horizons on the launchpad ready to go and launched it as soon as we saw 'Oumuamua it still would never have been able to catch up.
So all we got were a whole lot of views with Earth-based telescopes and a few from the space-based telescopes Spitzer and Hubble. Based on that observational data and some speculative computer modelling the object is about 35mx35mx230m.
The error bars are really, really large however. The length has been estimated to be anywhere from 100 meters to 1000 meters for instance. The computer models assume that brightness is a direct function of a uniform surface albedo and the dimensions of the facing surface. A dark spot on the surface would make the estimated size smaller, while a light spot would make it larger. Without more observational data the estimate is as good as it is going to get at this point, but that's not saying much.
To some the dimensions look an awful lot like the dimensions they'd expect a space ship to have. And clearly the aliens planned their trajectory so that we could not catch them. Right?
The other thing that got everyone excited was the "non-gravitational acceleration" observed.
Let me back up a bit. It is quite common for things like comets to have a lot of volatiles in their makeup. Comets originate in the Oort cloud, which is very, very far from the heat of the sun. The Oort cloud, like the rest of the solar system is thought to have originated from a ginormous gas cloud that was more or less homogeneous and included materials like water, methane, and ammonia that are liquids or gasses at "normal" temperatures but are frozen solid and just part of the dust cloud out there away from heat sources like suns. When the sun started shining it heated up the inner solar system, turning most of the volatiles into vapor then pushing that vapor out of the inner system. Drying it out, in other words. Except for a small amount brought in later by comets the only volatiles left in the inner system are those that were captured in gravity wells (planets) and hidden away before the sun lit up.
However, stuff out beyond Neptune was never subjected to this heat treatment and so is expected to have retained the original allotment of volatiles. It is common for things like comets that come into the inner system from very, very far out to have some of those volatiles out-gas as they get closer to the sun and heat up. These out gassings act like little rockets firing off in random directions and those rockets cause the comet's course to veer off of what would otherwise be a deterministic trajectory through the solar system.
Thing is, 'Oumuamua apparently didn't have much in the way of volatiles in it. There was a bit of out gassing, but nowhere near what a "normal" comet would have in it. Yet it still "swerved" as it came in. Wait. What? Like I said, gotta be aliens, right? Especially with those "ship-like" dimensions.
Well, not really. There is actually quite a lot of real estate in the outer system, and there's been literally billions and billions of years for it to have mixed and interacted. Even just going by what we can see out there (which isn't much, considering that virtually none of the Oort cloud is directly observable from Earth) we can come up with plausible explanations for 'Oumuamua without having to invoke aliens.
No fun, right? Well...
(links for future reference)
I mentioned comets so let me talk about planets for a second. A "planet" is defined as an object that is large enough for its mass to have overcome the structural strength of the rocks and ice that it is made of. When this happens the object becomes round. Small objects like comets are just aggregations of "stuff" loosely piled in one place. If you look at something like 67P/Churyumov–Gerasimenko, the comet visited by the ESA's Rosetta spacecraft, then you will see that it looks something like a rubber-duck bathtub toy. (Well, that's what it looks like to me anyway.) Things bumped together and stuck. A comet is typically not round at all.
The reason for using round-ness as a determinant for calling something a "planet" or not is because if something is big enough to be round then it is also big enough for geologic processes to have occurred on or in it. Gravitational heating from so much mass pressing down can cause ices to melt and to flow or outgas. Large masses can also retain and build up significant amounts of radioactive decay heat. Flows of material can lead to layering, weathering, and all sorts of other interesting behaviours and features.
So, there's a "dwarf" planet named Haumea. Even in it's current state it is the third-largest known dwarf planet, after Eris and Pluto. It is much bigger than Churyumov–Gerasimenko. So big that it should be a planet, i.e. round. Instead it is football-shaped. It is also surrounded by a ring of material. Spectral analyses of the football vs the ring(s) indicates that the rings are more icy while the football is more rocky, or even metallic by some measures. The theory is that Haumea was once a bigger (though still "dwarf") planet. The planet had differentiated into a rocky core and an icy surface. Then something came along and smacked it pretty hard, knocking off a lot of the ice and a big chunk of the core. Where other chunk(s) of core is is currently unknown, but it would be pretty hard to find. It would be a small, darker object. Maybe somewhat like Oumuamua. It would be on some strange, unpredictable orbit somewhere out in the outer solar system. It might even have been hit hard enough to be knocked out of the solar system entirely, like Oumuamua. Especially if it were closer to the "edge" of the system in the first place and not (relatively) close in like Haumea.
(In case you are wondering 'Oumuamua is not part of Haumea. The spectrum of the core of Haumea and 'Oumuamua are very different, and there's no way to put the missing part(s) of Haumea on the course that we observed for 'Oumuamua. 'Oumuamua came in on a course and at a speed that indicated it came from interstellar space. If a piece of Haumea had been hit hard enough to put it where Oumuamua came from then it would have left our solar system at greater than escape velocity. It might have eventually become some other solar system's Oumuamua but it would never have come back to our own solar system.)
Ok, so getting back to that "non-gravitational accelleration"? Turns out that was just the media going nuts. There was some acceleration observed, but it was consistent with a push from solar radiation pressure. "Solar radiation pressure" is what makes things like solar sails work. It is a real force with observable effects, but not really significant in terms of absolute force. At the distance that 'Oumuamua was observed the force could even have been explained by normal, boring outgassing. It is true that 'Oumuamua didn't do a lot outgassing that was observable from Earth. It didn't have a composition making it possible to do large amounts of it and it did not have a "tail" of dust characteristic of comets that more or less continuously outgas as they move through the solar system either. However, a small amount of outgassing might have been occurring that was below our ability to observe with telescopes, and that could have caused the observed effects.
Still, given the paucity of real data on 'Oumuamua there is nothing in the observations to indicate that it could /not/ have been an interstellar spacecraft using a solar sail. And the speculation continues.
Wikipedia says, 'In July 2019, astronomers reported that ʻOumuamua was an object of a "purely natural origin".' It is a little odd and that makes it fun to speculate about regarding the nature of its oddities, but ultimately I would agree with that assessment.
Honestly when I find something curious relating to space, I have recently been thinking "What does efaardvark think of this?" Blame it on not knowing anybody else with inclination on the subject. But anyway, I appreciate the comment on it. It actually gave me a lot to think about (the length did not intimidate me, was more than happy to read it!)Quote
getting back to that "non-gravitational accelleration"? Turns out that was just the media going nuts.There was some acceleration observed, but it was consistent with a push from solar radiation pressure.
I suspected that was hyperbole, but this is the first time I've seen a counter-arguement that actually made any sense. Everyone seems to be out there trying to prove it's an alien ship.
The idea it may possibly be a chunk of a planet is less exciting than aliens, of course, but it's also somewhat comforting? I also think if there were any evidence that it were intelligent in any way, it would probably be tightly under wraps and most of us wouldn't even have heard about it to begin with.
As always, thank you for humouring me! I hope my curiosity isn't too bothersome lol
What does efaardvark think of this?
Maybe I should start a blog under that title.
I actually seriously thought of doing something like that back in the early days of the 'net. There is a magazine I subscribe to called "analog: Science Fact & Science Fiction" (yes, some people still read things written on dead trees) that sometimes runs articles where somebody with actual science & engineering knowledge examines a topic like `Oumuamua and tries to come up with ideas that are crazy but still fit the observed facts and known science. Like maybe the author would start with the premise that `Oumuamua actually was an alien mission that had to come in close to the sun for some reason but didn't want to disturb us for some other reason so they just pretended to be a rock. Then the writer would try to flesh out the plot so to speak by coming up with scientifically valid reasons for why they might need to get close to the sun and why they might not want to disturb us. The goal was to inflate the story as much as possible without actually crossing the "science" line. Kind of the reverse of Occam's razor. Fun stuff. My idea was to do something similar online, but I just never had the time to do the research for something like that. I'd have had to quit my day job and I could not figure out a way to make something like a blog bring in enough income to replace the paycheck.
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