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Interstellar Visitor Found to Be Unlike a Comet or an Asteroid (quantamagazine.org)
204 points by Bootvis 7 days ago | hide | past | web | favorite | 114 comments

This is a little misleading because it presents the idea that there is a firm separation between "comets" and "asteroids", each of which have very different properties. In reality we've discovered in recent decades that there is significant overlap. Pure comets were formed in the outer solar system beyond the frost line, and they are composed of a mixture of primarily chondritic grains and ices. As noted such bodies are not very structurally strong so if they approach the Sun too closely they will break up from the stress of gravity as well as from the intensity of off-gassing vaporizing gasses. On the other hand you have classic asteroids which are formed inside the frost line and are made up of either chondritic material, stone, or metal (depending on their histories). Such asteroids can be much stronger but they lack the ices which might off-gas during a close solar encounter.

Except, that dichotomy is a simplified fiction. The reality is that there are objects in the middle, sometimes called "iceteroids" which we've discovered over the years. These could be the cores of comets which have lost most of their ices or these could be objects with more complicated histories, such as asteroids which have accreted surface layers of ices through various processes.

So this is a mystery but it's not some enormous mystery it's a small mystery. The idea that an asteroid could have acquired a small amount of ices on its surface isn't exactly world shattering news but because we don't know the history of the object we don't know the process by which it ended up with its observed properties.

To me, the interesting thing is that this iceteroid (love the term!) is definitively extra-solar and that means that it is likely that other systems have both asteroids and comets. Though we've seen asteroids in other systems like Fomalhaut [0], we've not been up close and personal with the compositions of extra-solar material.

Here in the Sol system, we have a pretty strange arrangement of planets [1], with big ones all in the outer system and smaller ones in the inner system. So looking at just our system is not very indicative of 'typical' system evolution, in terms of iceteroids. Once we figure out where Oumuamua came from, that'll give us some idea of the compositions and evolutions of other systems (data point of 1 is better than 0), like the frost line. Oumuamua should be viewed as awesome and exciting, but as the tip of the ... iceberg ... when it comes to the real questions of stellar evolution and all the awesome stuff happening there.



Good point. One thing we learned from exoplanet hunting is that there is a much greater diversity out there than we had originally thought, implying that planet formation is a lot more of a dynamic process than our previous models indicated. Prior to the discovery of exoplanets our planet forming models were very much "over fit" to the example of our own Solar System, giving the impression that other stellar systems would end up looking similar to our own. The reality is that our Solar System is, as you say, probably unusual. One aspect of this is that it seems like planetary migrations are pretty common (both inward and outward). If that sort of thing happens early during planet formation then it could easily explain an "asteroid" (which we understand is almost certainly a planetessimal fragment, though many asteroids are just that) which has acquired additional material including ices.

The obvious followup here is that we need way, way more closeup observations of small bodies in our own Solar System, we probably also need to develop better techniques for observing interstellar interlopers, and we need to improve our studies of extrasolar planetary systems. Fortunately the first part is already covered, between the Hayabusa 2, Osiris-REX, Lucy, Psyche, and New Horizons missions we should collect a lot more information on such bodies. However, it's probably going to be a long time before we have answers even to simple questions like the formation histories of most small bodies in stellar systems and what the "average" stellar system looks like.

I have close friends working on Psyche! You are right that our understanding of small bodies is going up very quickly, it's quite exciting (in asteroid terms) to be alive now; a lot is being answered and re-questioned with actual evidence.

Hopefully (knock on wood) JWST gets up there soon and a LOT more questions can be answered.

One issue we have is that there is this 'gap' in planet formation modeling. We can figure out small bodies and large bodies, but things that are ~1m to ~1km in size don't work in the models; they should all just fly apart. Somehow planets do form, but getting there is a pickle, theoretically. Asteroids are likely THE key to understanding that gap, and that should help out with a LOT of system formation dynamics.

Also, I'm still bullish on finding a LOT of platinum/gold on asteroids, because space mining. Like, eat your heart out Bruce Willis.

Awesome! I'm constantly bummed out about the failure of the CONTOUR mission, I can only imagine what we'd know now. Psyche is one of the coolest missions in the pipeline though, it's kind of crazy how fast our knowledge of small bodies is growing.

This article is excellent. I wondered about this exact issue before. Prograde/forward acceleration only from outgassing from a constant surface just seemed difficult to visualize. It would make some sense for an object with minimal spin since the side facing the sun would heat up more than the rest of the surface and so you'd have an outward acceleration primarily from the rear, but when you're spinning how could the outgassing not also [strongly] affect that spin, excepting some highly improbable scenario where your axis of spin is directly perpendicular to the sun and the 'thing' is perfectly balanced, like a propeller facing the rear of a plane?

I also wish more media ran articles like this. It's fun to let your imagination run wild, apply a bit of math and visualization, and also learn lots in the process. And there's no division. Even if somebody says something completely wrong, people can have fun correcting and adjusting their views like a collaborative project instead of some competition to 'prove' your subjective interpretation right, as so many other topics devolve into.

I am truly surprise that we don't get more interstellar visitors into our solar system. It kind shows how small our solar system is, or other way, how big the universe is.

There's a good chance these sorts of things are whizzing past us all the time, and we're just not detecting them.

I mean, we just discovered ten new moons of Jupiter this year: http://earthsky.org/space/10-new-moons-discovered-jupiter-1-...

And two the year prior, bringing the count to 12 since 2016, for a total of 79 moons around Jupiter.

How many moons are discovered per-year on average? Is there something about these recent ones that made it difficult to discover earlier, eg size?

We could have many more and not know. This one was seen because it came so close to the earth.

We have telescope technology to thank. The quality and the number of telescopes is constantly increasing. We don't just look in the sky any more, we watch the same spot over time and look for changes. We are seeing more spots, more time, and more changes. An investment in telescopes is an investment in physics.

I wouldn't be so sure we don't... Just because we don't see them with our 5 senses doesn't mean they are not here yet.

You do realize we're not limited to "our 5 senses" when studying the universe, right?

You're totally right. I actually didn't think of that, but just meant that maybe other forms of being could exist and manifest in dimensions we're not even looking at. Is that why my comment was downvoted?

It's too bad we're not ready to launch probes to visit and explore such transient objects at a moment's notice.

I think it would make sense to plan a probe mission where the probe would be put into storage to stand by for these kind of events. It would still be a challenge and might be impossible to find a launch window and reach the passing object in time, but it would be worth trying.

Just imagine how tragic it would be, if an ancient artifact of an alien civilization drifts by earth and we don't manage to have at least a look at it.

Yeah and then you'd need the delta-V to match the object's velocity as well, which would be solar escape velocity and then some. Although having probes scattered around the solar system that could be trained onto the object to get observations from multiple simultaneous angles would be neat.

I hope one day we'll get a solar system wide observation network. Like the current arrays of radio dish observatories, but on a solar system scale.

The delta-V would be a big problem. Though, if the probe is reansoably small and you would have reservation for a F9 Heavy flight, you could get some impressive delta-V. Detecting a candidate for such a mission early on would help a lot too.

Wouldn’t such an object be emitting radio signals that earth could receive?

Not if it's something like another civilization's Tesla Roadster.

> Not if it's something like another civilization's Tesla Roadster.

'Oumuamua is red and headed toward Pegasus (the winged horse) after a very long journey starting longtime in spacetime ago. It is wildly tumbling off-kilter and potentially creating a magnetic field that would be useful for interplanetary spacetravel.

They're probably pointing us to somewhere else from somewhere else.

If this is any indication of the state of another civilization's advanced physics, and it missed us by a wide margin, they're probably laughing at our energy and water markets; and indicating that we should be focused on asteroid impact avoidance (and then we will really laugh about rockets and red electromagnetic kinetic energy machines and asteroid mining). https://en.wikipedia.org/wiki/Asteroid_impact_avoidance


[We watch it fly by, heads all turning]

Maybe it would've been better to have put alone starman in the passenger seat or two starpeoples total?

Given the skull shape of October 2015 TB145 [1] (due to return in November 2018), maybe 'Oumuamua [2] is a pathology of Mars and an acknowledgement of our spacefaring intentions? Red, subsurface water, disrupted magnetic field.

[1] https://en.wikipedia.org/wiki/2015_TB145

[2] https://en.wikipedia.org/wiki/%CA%BBOumuamua

In regards to a red, unshielded, earth vehicle floating in solar orbit with a suited anthropomorphic creature whose head is too big for the windshield:

"What happened here?"

"That's not a knife... This is a knife." -- Crocodile Dundee

Right. As unlikely it is to run on other civilizations remainders, there is quite some chance for future humans to have a surprise find of a 2008 Tesla roadster :)

Only if it is actively emitting them. So if the aliens sent an active probe, it might emit radio signals. But if it is some junk from an alien civilization, it would just be trash drifting by. It would still be increadibly interesting to catch such an artifact.

Just another roadside picnic?

Let's say it takes us another 50 years to fund and prepare a mission, could we still catch up with it?

In 50 years it would be some 41bn kms away (it travels at a speed of 26km/s). Say you want to reach it within a five year span that would mean we'll need to build a spacecraft that could travel 0,1% the speed of light. Probably within our future capabilities considering all the theories that have been proposed for interstellar travel. For example the Breakthrough Starshot project has predictions for 10%-20% of the speed of light.

Of course reaching it and landing on it, especially at these speeds, are two completely different things.

We could revive Project Orion[1] and use up some of the world's nukes in a productive way. If there was high confidence that ’Oumuamua was an alien constructed object, I would support it.


So far we've managed to accelerate three probes to solar escape velocity using a lucky constellation of planets for gravity assists. Catching up with an object that has a 50 year head start seems very unlikely to me unless you assume very advanced propulsion systems, e.g. nuclear pulse propulsion or something similar. Even then catching up would take a very long time, longer than anything we built survived in space so far. How do you power a probe that's supposed to travel for fifty years give or take?

This reminds me of Rendevous with Rama, which I’m currently reading with my daughter.

’Oumuamua’s “mysterious thrust” [1][2] is obviously from rocket thrusters which undoubtedly come in threes.

1: [’Oumuamua] was being slightly accelerated by an unseen force, which they argued could only be attributed to comet “outgassing” acting like a thruster.

2: Now in a new study that is currently under peer review, Roman Rafikov, an astrophysicist at the University of Cambridge, argues that the same forces that appeared to have accelerated ’Oumuamua — the same forces that should have also produced a tail — would have also affected its spin. In particular, the acceleration would have torqued ’Oumuamua to such a degree that it would have spun apart, breaking up into smaller pieces. If ’Oumuamua were a comet, he argues, it would not have survived.

I'll ask what everyone is thinking but won't say. If the acceleration is in fact being caused by artificial thrusters, what kind of thruster configuration can produce prograde acceleration while maintaining the observed tumble? And why would someone engineer this configuration?

It's maybe worth pointing out the tumbling is a hypothesis used to explain variable brightness measurements and not something that was observed directly.


For all anyone knows the intrinsic brightness was variable.

(This is probably not very likely without some supporting spectroscopic anomalies. But still.)

As for thrusters - that's the same problem as the comet and outgassing hypothesis. No known mechanism can explain the acceleration.

And if you were alien race and wanted to look at a planetary system without attracting attention, something that looks like a tumbling rock with a plausibly improbable trajectory would be ideal.

It could just be a coincidence that the first large proven extra-solar object slingshotted around the Sun in a way that happened to pass close by the Earth.

But considering the size of the Solar System and the convenient phasing of the Earth's orbit, what are the odds? It's certainly very curious.

The odds that the first extra-solar object found is close to earth is really high, because they're easier to see if they're close.

If aliens could do this to make the object look inconspicuous, then why not make it look not suspicious as well?

Someone just f'ed up the config files?

They deployed on a Friday. They should have known better :-)

Sorry couldn't resist and hey, it's also Friday today.

I wonder if it was aliens and they made it just slightly interesting without making it look completely obvious. Then they'd monitor and see how we would react. Would we be able to detect. Would we go chase it? Beam microwaves at it. Do we have the capability to fire projectiles at it.

So you're saying it's possibly an interstellar alien cat toy to test our curiosity and willpower? Plausible.

Who knows what kinds of things are flying through "their" systems? Maybe this thing is the most not suspicious object they could imagine ;-)

> And why would someone engineer this configuration?

To make its being engineered seem implausible, surely.

You're saying instead of magic cloaking devices, the aliens used camouflage? Makes sense to me!

Because it wasn't an important project so they made the fresh put of college ME do it.

couldnt a thruster at its axis spin in the opposite direction to offset? some crude nonsense - https://i.imgur.com/8jj2hWy.png

The Dig comes to my mind from 1995.

“I miss you too darlin’...” such an epic soundtrack. Bah, I gotta listen to it now!

Would gravitational pull account for the acceleration? Meaning if it were made (or part of it) of some very dense element like Iridium, there would be an unexpected attraction to our sun?

Nope; gravity is very predictable and very constant (see also the difficulty in detecting gravitational waves), they calculated how much it should accelerate under influence of gravity and found the actual acceleration to be different.

As for the unexpected attraction, as one of the moon astronauts displayed, objects of different mass are attracted (fall) equally quickly by gravity in a vacuum.

Thanks for your comments, they have kicked me out of my head space. I suppose I got it into my head thinking of massive densities, like densities of neutron star. Anything dense enough to make a difference would be very loud and easy to spot... as I'm writing this, even if it were very dense. The effect would only be seen when it is much closer to another gravity well. Ok, how about this (I'm still stuck on this gravity idea), there is an unseen "cloud" of rock orbiting (very) slowly round the main body. Seemingly slowing down and accelerating as it orbits. A small body or cloud of bodies could easily be small enough for us to not see it, but enough to cause this effect. Sound reasonable?

Gravity doesn't work that way, I'm afraid, as Galileo demonstrated.

Based on the comments in this thread, I'm sure this will soon be an episode of 'Ancient Aliens' on the "History" Channel. If it isn't already.

How Oumuamua inspired the Egpytians to use logs and slave labor to build stone pyramids, like any advanced ancient alien would.

I'm still convinced this is an alien probe. I mean look at that trajectory. 0.163 AU from Earth - what are the odds? The spinning is irrelevant surely - I mean, it's all relative, so as far as ’Oumuamua is concerned, we're spinning. Maybe it was spinning that way to stay aligned with a distant companion for communication.

I prefer a complementary theory -- that it's a spent stage of a multi-stage spacecraft. If the primary stage were decelerating for a rendezvous with Earth, then any stage that made it to full acceleration would reach us first, probably getting close and then flinging back out of the solar system, just like this did.

Easy enough to test -- we'll eventually see another object following the same trajectory, give or take, but that's decelerating.

(Cue the dramatic music.)

"And on far-off Earth, Dr. Carlisle Perera had as yet told no one how he had wakened from a restless sleep with the message from his subconscious still echoing in his brain: The Ramans do everything in threes."

Great series of books! Every time I see a thread about this big rock I think of Rama.

Beg to differ. Great book. Awful cheap hacks posing as sequels. I regret reading them - they've nearly ruined the original for me.

I agree with you, they were pretty awful and lost so much of the original's sense of wonder and incomprehension.

One favourite daydream of mine is to think about how the Rama aliens' second and third probes would be received. I read Rama around the same time as Eon by Greg Bear, so I've always had an affinity for spin-gravity habitats filled with unknown stuff :)

With The Expanse TV series showing us how good hard scifi can be exciting and wonderful, I'm hoping we one day get an Eon or Rama series. Rama especially would work well in a hard scifi miniseries format.

(That's actually a great thought: that many shows thought impossible a couple of decades ago will now be made. Let's just hope they're made with love and devotion like The Expanse!)

My personal theory is that Clarke's writing partners sedated and tied him inside a closet until the book was finished.

That's the only way to explain how awful some of his last books were.

Having said that, I liked Richter 10. Too bad the co-author died before they could write another one.

My heuristic is that anything co-written by Gentry Lee is not worth my time.

I even wonder why they insisted on it so much.

The final reveal was mind-numbingly bad.

IIRC, the only reason the sequels exist is that fans (or maybe his editors) pointed out out that if the Ramans "do everything in threes" then there needed to be two more parts to the story. It was never planned as such, though.

For anyone curious as I was, this is a quote from "Rendezvous with Rama" by Arthur C. Clarke.


What if the object was already traveling in our rough direction and aliens decided to simply take advantage of the free ride? They tweaked the direction with a few thrusters to pass by Earth, and they left the rotation to help camouflage the object with its natural tumble through space. Finally, they deboarded the object when they reached their desired location nearest to Earth.

It's fun to speculate, but I'm sure the reality of the situation is far more boring.

Maybe it was intended to be a booby trap for the Zentraedi?

Looking at the images on that site, the trajectory doing a "fly by" of a lot of the planets did look remarkedly like the sort of thing we've being doing for decades in our own solar system [1]- line up a bunch of planets and use gravity slingshots to do a close fly-by to get a closer look.

The thought makes me have goose-bumps. Did some alient race just take a close look at us? Was there a voyager-style plaque on it? I guess we'll never know now.

Occam tells us that in reality it is most likely just some space debris from some ancient rock-smashing incident, but the fact that this thing continues to defy classification allows us some space to imagine!

1 - https://duckduckgo.com/?q=voyager+gravity+slingshot+diagram&...

I wonder if it's speed increased on it's way out. Maybe it's on a multi-system route, using gravity assist as it goes to cut the route time down by a few thousand years here or there as it circles back around to it's home system. Must be very patient aliens, assuming no FTL messaging tech.

> 0.163 AU from Earth - what are the odds?

Look at it from a sampling bias perspective. The object at its brightest was very dim (19th magnitude) and moving fast. The ability for rapid, repeating full sky surveys is recent (PanSTARRS had been online for around a decade). The limiting magnitude of PanSTARRS is 24th. So if such other such objects exist, of course we are going to find the closest ones first since they are the brightest.

> The spinning is irrelevant surely - I mean it’s all relative

That is incorrect. All motion is not relative, only intertial motion is relative. Spinning implies acceleration on a body which is measurably different from any reference frame compared to a body that is not accelerating

What I meant was, the origin of the object may have been in a different inertial frame of reference to our solar system, so the 'spin' as far as it is concerned is not spin.

This is not a thing though. Spin is spin, regardless of reference.

I really misunderstand my relativity then.

You've got the right general idea, but the principle of relativity only applies to constant velocity motion. If two astronauts float past each other in space, either of them can claim that they're motionless and the other is moving past them. However, if one of them is accelerating or spinning, they'll both agree on who it is.



To add on to this, here's a simple thought experiment to explain why spinning is different from constant velocity:

Imagine you're off somewhere space. You're holding a ball in each hand.

1. You're motionless with reference to the Earth. You let go of the balls. They stay put.

2. You're moving away from the earth with a constant velocity. You let go of the balls. They continue to move at the same velocity as you do: This is indistinguishable from case 1.

3. You're accelerating away from the Earth (using rocket boots). You let go of the balls. They do not continue to accelerate with you.

4. You're motionless with respect to Earth, except that you're spinning. Think of the fastest you've ever spun. You'll be really dizzy when you stop. You let go of the balls. They fly off into space, because in order to stay spinning, they need acceleration toward the center of the spin plane. By letting them go, you stop that acceleration, and their trajectory changes (or really, their trajectory change stops).

Velocity is all relative, but acceleration is not. Spinning is acceleration.


Here's another example:

You occupy a cosmos consisting only of you and an apple. You accelerate away from the apple for a moment, then stop accelerating. You now have a constant velocity away from the apple, but it's meaningless to say which of you is actually moving. You each perceive the other as moving away at constant velocity.

Now imagine you start spinning. You feel like you're spinning, and visually, you perceive the apple orbiting you at a bizarrely fast rate. From the apple's perspective, you are spinning. These two perceptions are not symmetric.

I’m really interested in understanding this. So please indulge me... So, I’m Oumuamua rocking my way through interstellar space. I see nothing around me so as far as I know I’m not spinning. Then, all of a sudden I happen upon a solar system that is spinning around me. Who is spinning? Me or the solar system?

Is the point here that I’m spinning with reference to my direction of motion?

Yes. You are aware that you are spinning even without external reference because your outer parts are accelerating toward your inner parts. This is the feeling of spinning in a circle on earth too.

This is how plausible simulated gravity works too. A ship spins. It doesn’t require a reference to know whether it’s spinning.

You seem to know that the rules of physics should hold in any inertial (i.e. non-accelerating) reference frame. But the thing you missed is that a spinning reference frame cannot be inertial. There is centripetal acceleration when something spins.

If one were so inclined, they could argue that the spinning is in fact very relevant, as it would allow a static camera or other observation device a full 360° view along all axes.

Any civilization capable of building an interstellar craft would certainly have mastered the ability to move a camera by then ;-)

Stealth. If you wanted to reveal yourself, you wouldn't need to disguise your probe as space debris.

This has always fascinated me. We can actually build giant spaceships (space stations) inside asteroids and use them for interstellar travel. All the resources we need, including ice would be at our disposal. Like traveling with your own mini-planet.

Dirk Schulze had taken that as a subject into his sci-fiction book - Alien Encounter.

From an energy perspective, it's likely better to build your own station (from an asteroid - or alternative, remove all the mass from the asteroid that you don't need) - that way, you're only accelerating exactly the mass that you need, not wasting any energy accelerating the rest of an asteroid.

If it's a space station, then accelerating it is less of a concern than keeping it livable, and a huge amount of matter surrounding you goes a long way to protect you from things that would otherwise kill you.

Now for a space ship, I agree. Though even then you might benefit from that extra matter in the same way.

I think he is talking about chilling on the asteroid while it follows its known trajectory undisturbed, simply because they are packed with useful resources we can gobble up while waiting.

Asteroid are per definition not interstellar though (until Oumuamua?).

An interesting approach would be to send an automated ship builder robot fleet to dismantle an asteroid and board the fully furnished ship when it makes another close approach, and then boosting it to its final destination.

I'd love to see a scenario where we'd build robot terraforming fleets that'd land on an uninhabited planet, make it habitable, complete with spaceports, cities, factories, crops (all permanently tended for robots), build a couple extra fleets and launch to their next most promising targets.

And then, for some reason, humans lose interest in all that and someone else wakes up to find a universe full of 60's futuristic utopian planets nobody ever lived in.

someone else wakes up to find a universe full of 60's futuristic utopian planets nobody ever lived in

Reminds me of the paperclips game [1]. Would be horrid to find those people wake up and find the universe transformed into nothing but paperclips. [1]

[1] http://www.decisionproblem.com/paperclips/index2.html

For the book, the reviews on Goodreads aren't very good :-/


That's true, but I personally found it pretty engaging. In fact, I read the last 150+ pages in one night. I'd give it at least 4 stars.

Also mentionned in Legend of Galactic Heroes by Yoshiki Tanaka. The founder of the Free Planet Alliances escape from its prison world by using ice asteroids as vessels for interstellar travel.

They’d make great protection from high speed space debris.

This is explored in Seveneves by Neal Stephenson.

You'd still be better off by restructuring all the mass of the asteroid around your spaceship instead of building your ship inside the asteroid.

The odds we expect depend on our detection ability. Maybe there are thousands of these objects but we saw this one only because it passed so close to Earth.

There is fairly strong consensus in the astronomy community that possibly hundreds of Sednoid class objects exist, and those are in relatively normal orbits compared to things which are beyond solar escape velocity. But so far we've only found 3 or 4 Sednoids.


Sednoids orbiting in the kuiper belt are totally different from extrasolar objects on hyperbolic trajectories. Oamuamua passed inside Mercury's orbit.

Yes, but both classes of objects are very dificult to detect, due to distance, size and lack of brightness. We only detected the extrasolar object once it had already passed perihelion and was on its way out.

"Such an intrinsically faint body could be discovered only thanks to the close Earth flyby in October 2017, which created a tremendous opportunity for the first detailed characterization of a minor object of exosolar origin."


If you're right about the spin for alignment, we should be looking into deep space in the direction of it's axis. Presumably, this direction is erratic but there may be orientations that dominate over time.

A tumbling probe that doubles as a cosmic pointer to home. Cool.

There's another little fun point here. If this was a probe, the point where you'd accelerate to have the greatest effect on your trajectory would be exactly where it did - at one of its semi-major points - where its trajectory turns.

What’s special about 0.163 AU? Does it have a mathematical significance?

Just that it's unusually close.

As we are in paranoid mode (and it's fun to be, isn't it?): not only it's unusually close, but it seems that Earth is the planet it pass closer in all the Solar System.

Maybe somebody should check if that "slightly acceleration" bring it closer or farther to Earth.

Maybe we have been photographed. I hope it's not that somebody shoot us and failed.

Aliens got us just before GDPR, sneaky bastards!

There is a bit of sampling bias, given that we are looking for space objects from Earth, and so it easier to detect something that comes closer to Earth. Not sure how significant the bias is for an object of this size and speed, but I'm guessing that it is probably a relevant factor.

AU stands for Astronomical unit, and it's used for measuring distances in space smaller than light-years. 1 AU is the distance between the Earth and the Sun. 0.163 AU tells you how close it came to earth.

I'd prefer to use light seconds/minutes for that. Not only it's easier to grasp (0.163 AU seems awfully close, but 81 light seconds sounds reassuringly far) but it'll probably be more helpful when we have to communicate across these distances.

How could spinning rapidly possibly help for communication? Unless this distant companion is moving at many times the speed of light in a massive ellipse. Also, as someone else mentioned, rotational motion isn't intertial, it's not just relatively different from us on Earth.

Two more of them, and I'd say you have an excellent case.

They do everything in threes.

A Arc, B Arc...

I want this to be an alien ship so badly!

> what are the odds?

100%, because it happened

’Oumuamua, I’ve come to bargain!

There was no other way.

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