<p><strong>tl;dr:</strong> It's a hard problem. While <a href="https://astronomy.stackexchange.com/a/30051/7982">this answer</a> says </p>

<blockquote>
  <p>However, space is big "therefore ʻOumuamua will never return".</p>
</blockquote>

<p>I say <em>never say never!</em></p>

<hr>

<blockquote>
  <p>What natural mechanisms could lead to the unlikely case of the same rogue asteroid or planet passing through our solar system twice?</p>
</blockquote>

<p>I can think of three mechanisms.</p>

<h2>1. Stellar and solar-system pinball</h2>

<p>Under normal situations an asteroid or planet could not swing around a massive object, make a U-turn, and come straight back nearly along it's same path after a 180° deflection. However deflections by two or more large bodies can do it. Those two bodies could be</p>

<ul>
<li><strong>Two unrelated stars:</strong> This would make a triangular path; Sun, star-1, star-2, Sun again.</li>
<li><strong>A double star:</strong> Three-body unbound orbits are crazy and chaotic and just about anything can happen including a 180°U-turn. This is my favorite since it only requires one encounter and star systems of two or higher are pretty common.</li>
<li><strong>A star and its planet:</strong> Stars with Jupiter-size (and larger) planets are pretty common, and a hyperbolic orbit around a star doing much/most of the bend with the planet doing the rest of the 180°U-turn is possible.</li>
<li><strong>other variations on this theme.</strong></li>
</ul>

<h2>2. General Relativity (or Neutron Stars)</h2>

<ul>
<li><p><strong>A black hole:</strong> As explained in answer(s) to <a href="https://physics.stackexchange.com/q/467144/83380">Could a trajectory around a large mass ever deflect by more than 180 degrees due to general relativistic effects?</a> "...there is no limit on how many "laps" a hyperbolic orbit can make before it returns to infinity." That's in the limiting case of a zero-mass object. For large mass ratios such as an asteroid or planet versus a black hole, the maxiumum number of turns an unbound orbit can make is about 0.41 times the mass ratio, which would be greater than a million for an Earth-sized planet and a vanilla stellar-collapse black hole.</p></li>
<li><p><strong>A neutron star:</strong> I am not certain yet if the GR effects due to a neutron star are strong enough to produce a 180°U-turn without a destructively close approach; I'm currently working on this...</p></li>
</ul>

<h2>3. Galactic roller-coaster</h2>

<p>The solar system orbits the galactic center at about 220 km/s with a period of about 250 million years. That speed is an <em>order of magnitude or possibly two faster</em> than the orbital speeds of asteroids and planets within solar systems. Jupiter orbits at 13 km/s and (486958) 2014 MU69 (Ultima Thule) moves at 4.6 km/s. So no mater which direction our rogue friend leaves the solar system, it's going to be moving around the galactic center with a nearly parallel orbit to that of the solar system, and it's going to be doing a complicated dance above and below the galactic plane as our solar system is doing, due to the <em>restoring force</em> of the galactic disks's gravity. This can bring us and our rogue friend back together in a way that would at first surprise us both until we remember how stars move in the disk.</p>

<h2>Overall probability?</h2>

<p>For all of these the probability is small, but the single collision options (double stars, solar systems, black holes, neutron stars) may make the probability better than the two unrelated star sequential scattering scenarios, and the galactic plane oscillations don't require a near approach to any other body at all.</p>

<p>So I think the statement:</p>

<blockquote>
  <p>...Therefore it will never return.</p>
</blockquote>

<p>could be considered cavalier and possibly scientifically unsupported.</p>

<p>It's a hard problem, so <em>never say never!</em></p>

CC BY-SA 4.0
2020-05-10T02:01:37.873Z
Why the down vote? Is this not in fact a correct answer to *What natural mechanisms could lead to the unlikely case of the same rogue asteroid or planet passing through our solar system twice?*
uhoh
CC BY-SA 4.0
2020-12-09T23:14:37.310Z
It's rare for me to answer my own question and rarer-still for me to accept it, but as OP I feel this is the best answer. *Please feel free to convince me otherwise!*
uhoh
CC BY-SA 4.0
2023-05-17T11:35:05.580Z
"Under normal situations an asteroid or planet could not swing around a massive object, make a U-turn, and come straight back nearly along it's same path after a 180° deflection. " This is only true from the perspective of the reference frame where the object is stationary, from other reference frames the asteroid can appear to turn 180. A rouge gas gianst travelling through the oort cloud could give what is a 90 degree trun from it's reference frame but a 180 degree turn from the perspective of the sun's reference frame.
blademan9999
CC BY-SA 4.0
2023-05-17T14:00:46.773Z
@blademan9999 I think that a "rouge gas gianst travelling through the oort cloud" would not be considered "under normal situations". I think *as written* my statement is fine. Why not add a supplementary answer beginning with something like "While [uhoh mentions 'normal situations'](https://astronomy.stackexchange.com/a/30058/7982) there are certainly other kinematic scenarios we can consider." I think that would be great! :-)
uhoh
CC BY-SA 4.0
2023-05-18T10:04:13.107Z
But my coment also applies to any passage near a star, Unless the star has a VERY low proper motion relative to the star, it will be possible for the asteroid to turn 180 degrees form the perspective of observers in the Sun's reference frame.
blademan9999
CC BY-SA 4.0
2023-05-18T10:48:09.593Z
@blademan9999 I think what you have to say is important enough, and will reach a lot more readers if you post it as a new answer.
uhoh
CC BY-SA 4.0
2019-03-19T09:14:48.577Z
2020-05-10T02:28:49.920Z
2020-05-10T02:28:49.920Z
uhoh
<blockquote>
  <p>Space is big. Really big. You just won't believe how vastly, hugely, mind-bogglingly big it is (Douglas Adams)</p>
</blockquote>

<p>The wikipedia page notes that 'Oumuamua "has circulated the Milky Way several times" so it was unlikely to have formed around any nearby star. While it would take 'Oumuamua  600,000 years to travel here from a local star, the time before it can be expected to have another stellar encounter would be much greater, hundreds of millions of years. And the next star that it meets is unlikely to be a currently local star, it could be almost any star in the galaxy. </p>

<p>Once an interstellar object has left the solar system it will travel in a roughly straight line, (slowly curving with the general gravity of the galaxy). For it to return to the Earth it would have to have a gravitational encounter with another star (or more likely a series of encounters) that send it back towards the solar system. Even given a vast mind-boggling long period of time like 10 billion years the chance of this happening is minute. </p>

<p>Such an object will probably orbit the galaxy several times between stellar encounters, perhaps it has an encounter every 250 million years. And it is equally likely to encounter any of the disc stars (conservatively 50 billion stars). This means that the mean time between encounters of the sun is on the order of ten quintillion years. </p>

<p>It isn't impossible.  It is quite easy to design a free-return flight to the moon.   There is no reason why a free-return flight to another star can't be done. However, space is big "therefore ʻOumuamua will never return".</p>

CC BY-SA 4.0
2019-03-19T00:50:46.900Z
How does one arrive at "250 million years" and "ten quintillion years"? If it's an estimate can you mention how it was done, if there's a source, a mention would be great. Thanks!
uhoh
CC BY-SA 4.0
2019-03-19T03:55:22.793Z
He was doing a back-of-the-envelope calculation. The point is that Oumuamua is really no more likely to come back around to the sun that it is to have an encounter with any other star. So he assumed an average number of years between stellar encounters, and approximately multiplied that by the number of stars.
Mark Foskey
CC BY-SA 4.0
2019-03-19T08:27:06.077Z
@MarkFoskey So where does the "average number of years between stellar encounters" come from? I'm asking how the values  "250 million years" and "ten quintillion years" are obtained. If you can reproduce them that would be great, thanks.
uhoh
CC BY-SA 4.0
2019-03-19T11:21:18.683Z
The problem with back-of-the-envelope calculations is that *because* they are not precise, each estimate is different. The *strength* of back-of-the-envelope calculations is that they give you good order-of-magnitude results. If you estimate 1 billion years, it *might* be 100 million years and it might be five billion years -- but it isn't going to be 106 years next Tuesday.
Mark Olson
CC BY-SA 4.0
2019-03-19T14:45:29.497Z
Well, actually I think 250 million years is a gross underestimate. 'Oumuamua is no more likely to have a close encounter with a star than the Sun is to have such an encounter with another star, and any such encounter in the past few billion years would have disrupted the solar system. But, rounding, 250 million ~ 10^8, 50 billion ~ 10^11, and their product is 10^19. A quintillion is 10^18. Since 250 million is a clear underestimate, this makes the case that a double encounter would be vanishingly rare.
Mark Foskey
CC BY-SA 4.0
2019-03-19T18:41:16.110Z
on the order of "ten quintillion years", would expansion of space start over-coming the Milky Way's gravity, ripping it apart, making it further unlikely for Oumuamua to return?
dmoody256
CC BY-SA 4.0
2023-05-17T11:56:03.253Z
@MarkFoskey And going by the formula here https://en.wikipedia.org/wiki/Stellar_collision#Stellar_collisions_and_the_Solar_System You'd expect a pass of less then 1000au once every 10 billion years.
blademan9999
CC BY-SA 4.0
2019-03-18T22:12:11.697Z
2019-03-19T16:42:13.527Z
2019-03-19T16:42:13.527Z
James K
<p>I find he following <a href="https://astronomy.stackexchange.com/a/30023/7982">answer</a> to the question <em>What's the soonest Oumuamua could return?</em> unsatisfying and oversimplifying and the last five words "Therefore it will never return." unsupported at a minimum.</p>

<blockquote>
  <p>Oumuamua as an object is remarkable, because it has a positive net energy, which means it is not bound to the gravitational well of our sun. Therefore it will never return.</p>
</blockquote>

<p>So I'd like to ask: *What natural mechanisms could lead to the unlikely case of a rogue asteroid or planet passing through our solar system twice?"</p>

<ul>
<li>Stick to accepted scientific facts and theories</li>
<li>Let's give an ample time limit of 10 billion years between visits.</li>
<li>For the purposes of this question, the solar system includes the Kuiper belt (50AU) but not the Oort cloud</li>
<li>Don't include intervention by artificial means or intervention (divine or otherwise)</li>
</ul>

CC BY-SA 4.0
2019-03-18T09:50:12.140Z
Gravity from an *extremely* well-placed body? Actually, it'd probably take more than 1 body to get the direction right.
PM 2Ring
CC BY-SA 4.0
2019-03-18T10:12:14.603Z
@PM2Ring you've got me thinking: [Could a trajectory around a large mass ever deflect by more than180 degrees due to general relativistic effects?](https://physics.stackexchange.com/q/467144/83380)
uhoh
CC BY-SA 4.0
2019-03-18T10:34:34.513Z
@PM2Ring there are 8 (or 9) planets
Robert Frost
CC BY-SA 4.0
2019-03-18T10:35:58.850Z
If I could I'd flag this as a duplicate of my question https://astronomy.stackexchange.com/questions/30019/whats-the-soonest-oumuamua-could-return
Robert Frost
CC BY-SA 4.0
2019-03-18T10:51:53.753Z
@user334732 I think that the edit you just made to your question makes them a little more similar, but still your asking there about the asteroid turing itself around at least partially under its own power and possibly intelligent control. Here I'm asking only about conventional orbital mechanics and no propulsion. "Don't include intervention by artificial means or intervention (divine or otherwise)"
uhoh
CC BY-SA 4.0
2019-03-18T11:44:30.300Z
@user334732 ['Oumuamua's trajectory isn't in the ecliptic plane](https://commons.m.wikimedia.org/wiki/File:Oumuamua-solar_system-ecliptic-normals.png) . Even though its current distance from the Sun is roughly the same as Saturn's radial distance, 'Oumuamua is already well above the main plane of the solar system. So it's *very* unlikely to encounter anything of significant mass until it reaches the outer Oort cloud in several thousand years. And that cloud is rather sparse, Wikipedia says the distance between bodies there is on the order of tens of millions of kilometres.
PM 2Ring
CC BY-SA 4.0
2019-03-18T22:02:09.383Z
"It is unknown how long the object has been traveling among the stars.The Solar System is likely the first star system that ʻOumuamua has closely encountered since being ejected from its birth star system, potentially several billion years ago." I mean it surely is possible that the object has a close encounter with another star that alters its trajectory significantly, and then again with a third which throws it back at the original system, but that is so extremely *contrived* that you might as well call it magic or divine intervention. Saying it doesn't come back is imho justified.
Polygnome
CC BY-SA 4.0
2019-03-19T08:00:52.713Z
@uhoh I get where you're coming from but this question has the exact same intent as mine and those extra factors you mention were only the declared premise behind the astronomical question. It's a sad state for stack exchange if we have to cloak the premise behind the question by asking the same question again.
Robert Frost
CC BY-SA 4.0
2019-03-19T08:24:17.483Z
@user334732 *steering Oumuamua via thrust*, intelligent or otherwise, is central to your question whereas this question is about what might happen to any rogue asteroid or planet, without thrust, and is looking for a science-based answer. "the exact same intent" doesn't properly characterize the situation.
uhoh
CC BY-SA 4.0
2019-03-19T08:35:51.257Z
@user334732 *steering Oumuamua via thrust*, intelligent or otherwise, is central to your question whereas this question is about what *natural mechanisms* might affect any unspecified rogue asteroid or planet, *without thrust*, and is looking for a *science-based* answer. When I wrote this question, your question said **The question is motivated by considering the (unlikely) eventuality that Oumuamua is a weaponised asteroid with powers to sense life and crash into planets.** So I'm pretty sure that "the exact same intent" doesn't properly characterize the situation.
uhoh
CC BY-SA 4.0
2019-03-19T08:56:13.230Z
@uhoh the only difference is one of probability. Mine asks "suppose favourable circumstances (such as minimal thrust applied at the correct times)", yours asks about "the unlikely case". Both probe as to the soonest it could return, given a favourable path among gravitational bodies..
Robert Frost
<asteroids><orbital-mechanics><mathematics><rogue-planet>
What natural mechanisms could lead to the unlikely case of the same rogue asteroid or planet passing through our solar system twice?
uhoh
# What natural mechanisms could lead to the unlikely case of the same rogue asteroid or planet passing through our solar system twice?

(Asked by: username_0 on 2019-03-18)

I find he following [answer](https://astronomy.stackexchange.com/a/30023/7982) to the question *What's the soonest Oumuamua could return?* unsatisfying and oversimplifying and the last five words "Therefore it will never return." unsupported at a minimum.

> Oumuamua as an object is remarkable, because it has a positive net energy, which means it is not bound to the gravitational well of our sun. Therefore it will never return.

So I'd like to ask: \*What natural mechanisms could lead to the unlikely case of a rogue asteroid or planet passing through our solar system twice?"

* Stick to accepted scientific facts and theories
* Let's give an ample time limit of 10 billion years between visits.
* For the purposes of this question, the solar system includes the Kuiper belt (50AU) but not the Oort cloud
* Don't include intervention by artificial means or intervention (divine or otherwise)

**Tags:** asteroids, orbital-mechanics, mathematics, rogue-planet
> **Comments:**
> **username_1**: Gravity from an *extremely* well-placed body? Actually, it'd probably take more than 1 body to get the direction right.  
> **username_0**: @PM2Ring you've got me thinking: [Could a trajectory around a large mass ever deflect by more than180 degrees due to general relativistic effects?](https://physics.stackexchange.com/q/467144/83380)  
> **username_2**: @PM2Ring there are 8 (or 9) planets  
> **username_2**: If I could I'd flag this as a duplicate of my question https://astronomy.stackexchange.com/questions/30019/whats-the-soonest-oumuamua-could-return  
> **username_0**: @user334732 I think that the edit you just made to your question makes them a little more similar, but still your asking there about the asteroid turing itself around at least partially under its own power and possibly intelligent control. Here I'm asking only about conventional orbital mechanics and no propulsion. "Don't include intervention by artificial means or intervention (divine or otherwise)"  
> **username_1**: @user334732 ['Oumuamua's trajectory isn't in the ecliptic plane](https://commons.m.wikimedia.org/wiki/File:Oumuamua-solar_system-ecliptic-normals.png) . Even though its current distance from the Sun is roughly the same as Saturn's radial distance, 'Oumuamua is already well above the main plane of the solar system. So it's *very* unlikely to encounter anything of significant mass until it reaches the outer Oort cloud in several thousand years. And that cloud is rather sparse, Wikipedia says the distance between bodies there is on the order of tens of millions of kilometres.  
> **username_3**: "It is unknown how long the object has been traveling among the stars.The Solar System is likely the first star system that ʻOumuamua has closely encountered since being ejected from its birth star system, potentially several billion years ago." I mean it surely is possible that the object has a close encounter with another star that alters its trajectory significantly, and then again with a third which throws it back at the original system, but that is so extremely *contrived* that you might as well call it magic or divine intervention. Saying it doesn't come back is imho justified.  
> **username_2**: @uhoh I get where you're coming from but this question has the exact same intent as mine and those extra factors you mention were only the declared premise behind the astronomical question. It's a sad state for stack exchange if we have to cloak the premise behind the question by asking the same question again.  
> **username_0**: @user334732 *steering Oumuamua via thrust*, intelligent or otherwise, is central to your question whereas this question is about what might happen to any rogue asteroid or planet, without thrust, and is looking for a science-based answer. "the exact same intent" doesn't properly characterize the situation.  
> **username_0**: @user334732 *steering Oumuamua via thrust*, intelligent or otherwise, is central to your question whereas this question is about what *natural mechanisms* might affect any unspecified rogue asteroid or planet, *without thrust*, and is looking for a *science-based* answer. When I wrote this question, your question said **The question is motivated by considering the (unlikely) eventuality that Oumuamua is a weaponised asteroid with powers to sense life and crash into planets.** So I'm pretty sure that "the exact same intent" doesn't properly characterize the situation.  
> **username_2**: @uhoh the only difference is one of probability. Mine asks "suppose favourable circumstances (such as minimal thrust applied at the correct times)", yours asks about "the unlikely case". Both probe as to the soonest it could return, given a favourable path among gravitational bodies..  

---

## 2 answers

### Answer by username_0 on 2019-03-19. Score: -2 | Accepted answer

**tl;dr:** It's a hard problem. While [this answer](https://astronomy.stackexchange.com/a/30051/7982) says

> However, space is big "therefore ʻOumuamua will never return".

I say *never say never!*

---

> What natural mechanisms could lead to the unlikely case of the same rogue asteroid or planet passing through our solar system twice?

I can think of three mechanisms.

## 1. Stellar and solar-system pinball

Under normal situations an asteroid or planet could not swing around a massive object, make a U-turn, and come straight back nearly along it's same path after a 180° deflection. However deflections by two or more large bodies can do it. Those two bodies could be

* **Two unrelated stars:** This would make a triangular path; Sun, star-1, star-2, Sun again.
* **A double star:** Three-body unbound orbits are crazy and chaotic and just about anything can happen including a 180°U-turn. This is my favorite since it only requires one encounter and star systems of two or higher are pretty common.
* **A star and its planet:** Stars with Jupiter-size (and larger) planets are pretty common, and a hyperbolic orbit around a star doing much/most of the bend with the planet doing the rest of the 180°U-turn is possible.
* **other variations on this theme.**

## 2. General Relativity (or Neutron Stars)

* **A black hole:** As explained in answer(s) to [Could a trajectory around a large mass ever deflect by more than 180 degrees due to general relativistic effects?](https://physics.stackexchange.com/q/467144/83380) "...there is no limit on how many "laps" a hyperbolic orbit can make before it returns to infinity." That's in the limiting case of a zero-mass object. For large mass ratios such as an asteroid or planet versus a black hole, the maxiumum number of turns an unbound orbit can make is about 0.41 times the mass ratio, which would be greater than a million for an Earth-sized planet and a vanilla stellar-collapse black hole.
* **A neutron star:** I am not certain yet if the GR effects due to a neutron star are strong enough to produce a 180°U-turn without a destructively close approach; I'm currently working on this...

## 3. Galactic roller-coaster

The solar system orbits the galactic center at about 220 km/s with a period of about 250 million years. That speed is an *order of magnitude or possibly two faster* than the orbital speeds of asteroids and planets within solar systems. Jupiter orbits at 13 km/s and (486958) 2014 MU69 (Ultima Thule) moves at 4.6 km/s. So no mater which direction our rogue friend leaves the solar system, it's going to be moving around the galactic center with a nearly parallel orbit to that of the solar system, and it's going to be doing a complicated dance above and below the galactic plane as our solar system is doing, due to the *restoring force* of the galactic disks's gravity. This can bring us and our rogue friend back together in a way that would at first surprise us both until we remember how stars move in the disk.

## Overall probability?

For all of these the probability is small, but the single collision options (double stars, solar systems, black holes, neutron stars) may make the probability better than the two unrelated star sequential scattering scenarios, and the galactic plane oscillations don't require a near approach to any other body at all.

So I think the statement:

> ...Therefore it will never return.

could be considered cavalier and possibly scientifically unsupported.

It's a hard problem, so *never say never!*

> **Comments:**
> **username_0**: Why the down vote? Is this not in fact a correct answer to *What natural mechanisms could lead to the unlikely case of the same rogue asteroid or planet passing through our solar system twice?*  
> **username_0**: It's rare for me to answer my own question and rarer-still for me to accept it, but as OP I feel this is the best answer. *Please feel free to convince me otherwise!*  
> **username_4**: "Under normal situations an asteroid or planet could not swing around a massive object, make a U-turn, and come straight back nearly along it's same path after a 180° deflection. " This is only true from the perspective of the reference frame where the object is stationary, from other reference frames the asteroid can appear to turn 180. A rouge gas gianst travelling through the oort cloud could give what is a 90 degree trun from it's reference frame but a 180 degree turn from the perspective of the sun's reference frame.  
> **username_0**: @blademan9999 I think that a "rouge gas gianst travelling through the oort cloud" would not be considered "under normal situations". I think *as written* my statement is fine. Why not add a supplementary answer beginning with something like "While [uhoh mentions 'normal situations'](https://astronomy.stackexchange.com/a/30058/7982) there are certainly other kinematic scenarios we can consider." I think that would be great! :-)  
> **username_4**: But my coment also applies to any passage near a star, Unless the star has a VERY low proper motion relative to the star, it will be possible for the asteroid to turn 180 degrees form the perspective of observers in the Sun's reference frame.  
> **username_0**: @blademan9999 I think what you have to say is important enough, and will reach a lot more readers if you post it as a new answer.  

---

### Answer by username_5 on 2019-03-18. Score: 2

> Space is big. Really big. You just won't believe how vastly, hugely, mind-bogglingly big it is (Douglas Adams)

The wikipedia page notes that 'Oumuamua "has circulated the Milky Way several times" so it was unlikely to have formed around any nearby star. While it would take 'Oumuamua 600,000 years to travel here from a local star, the time before it can be expected to have another stellar encounter would be much greater, hundreds of millions of years. And the next star that it meets is unlikely to be a currently local star, it could be almost any star in the galaxy.

Once an interstellar object has left the solar system it will travel in a roughly straight line, (slowly curving with the general gravity of the galaxy). For it to return to the Earth it would have to have a gravitational encounter with another star (or more likely a series of encounters) that send it back towards the solar system. Even given a vast mind-boggling long period of time like 10 billion years the chance of this happening is minute.

Such an object will probably orbit the galaxy several times between stellar encounters, perhaps it has an encounter every 250 million years. And it is equally likely to encounter any of the disc stars (conservatively 50 billion stars). This means that the mean time between encounters of the sun is on the order of ten quintillion years.

It isn't impossible. It is quite easy to design a free-return flight to the moon. There is no reason why a free-return flight to another star can't be done. However, space is big "therefore ʻOumuamua will never return".

> **Comments:**
> **username_0**: How does one arrive at "250 million years" and "ten quintillion years"? If it's an estimate can you mention how it was done, if there's a source, a mention would be great. Thanks!  
> **username_6**: He was doing a back-of-the-envelope calculation. The point is that Oumuamua is really no more likely to come back around to the sun that it is to have an encounter with any other star. So he assumed an average number of years between stellar encounters, and approximately multiplied that by the number of stars.  
> **username_0**: @MarkFoskey So where does the "average number of years between stellar encounters" come from? I'm asking how the values  "250 million years" and "ten quintillion years" are obtained. If you can reproduce them that would be great, thanks.  
> **username_7**: The problem with back-of-the-envelope calculations is that *because* they are not precise, each estimate is different. The *strength* of back-of-the-envelope calculations is that they give you good order-of-magnitude results. If you estimate 1 billion years, it *might* be 100 million years and it might be five billion years -- but it isn't going to be 106 years next Tuesday.  
> **username_6**: Well, actually I think 250 million years is a gross underestimate. 'Oumuamua is no more likely to have a close encounter with a star than the Sun is to have such an encounter with another star, and any such encounter in the past few billion years would have disrupted the solar system. But, rounding, 250 million ~ 10^8, 50 billion ~ 10^11, and their product is 10^19. A quintillion is 10^18. Since 250 million is a clear underestimate, this makes the case that a double encounter would be vanishingly rare.  
> **username_8**: on the order of "ten quintillion years", would expansion of space start over-coming the Milky Way's gravity, ripping it apart, making it further unlikely for Oumuamua to return?  
> **username_4**: @MarkFoskey And going by the formula here https://en.wikipedia.org/wiki/Stellar_collision#Stellar_collisions_and_the_Solar_System You'd expect a pass of less then 1000au once every 10 billion years.  

---

