B First Interstellar Asteroid Found

[CygnusX-1]

The first asteroid ever seen from another solar system is whizzing through our own, and astronomers are racing to observe the visitor before it slips away.

Links: Nature

http://www.skyandtelescope.com/astronomy-news/astronomers-spot-first-known-interstellar-comet/

 

[tony873004]

Here are a few animations and simulations of A/2017 U1 that I made that I posted on my Twitter account.
https://twitter.com/tony873004/status/923734271495254018
https://twitter.com/tony873004/status/923228678963658752
https://twitter.com/tony873004/status/923109368903622656
https://twitter.com/tony873004/status/925537689843023872

 

[Vanadium 50]

The first asteroid ever seen from another solar system

I don't think there is any evidence that this came from "another solar system". It could be from, e.g. our own Oort cloud.

 

[tony873004]

I don't think there is any evidence that this came from "another solar system". It could be from, e.g. our own Oort cloud.

It's traveling about 25 km/s too fast to have come from the Oort Cloud.
It came from the most likely direction that an interstellar object would come from. i.e. if the solar system were a car, and this was a raindrop, it hit our windshield.
It came in with a speed typical for what we would expect from an interstellar object.

 

[CygnusX-1]

It did not come from the Oort cloud.

As stated in the Nature story, the orbit is clearly hyperbolic; its eccentricity is 1.20, which is safely above 1.00. Furthermore, as shown in the animation in the Nature story, the object never passes close to a giant planet whose gravitational pull could have altered an elliptical orbit and made it hyperbolic.

Bottom line: this object did indeed come from another solar system.

 

[DrStupid]

Bottom line: this object did indeed come from another solar system.

Does it need a solar system to form an asteroid?

 

[OmCheeto]

It's traveling about 25 km/s too fast to have come from the Oort Cloud.
It came from the most likely direction that an interstellar object would come from. i.e. if the solar system were a car, and this was a raindrop, it hit our windshield.
It came in with a speed typical for what we would expect from an interstellar object.

I had loads of fun this morning figuring out where the "25 km/s" number came from.
I think I did my maths correctly, as I came up with a figure of 25.7 km/sec.

Another number I came up with was, that if it was from Vega, it would have taken 580,000,000 years to travel that distance.
Though, as indicated by another of your simulations, Vega was probably nowhere near where it came from, that far back.
(Our galaxy looks like a busy airport when you look at it in 1000years/increment timescales. Freakin' stars flying everywhere!)

ps. Any idea who runs "projectpluto.com"? They seem to know what they are talking about:"Pseudo-MPEC" for A/2017 U1

[edit: millions of millions sounded too much like Sagan]

 

[CygnusX-1]

Actually, it took only 290,000 years to travel the 25-light-year distance from Vega, although as correctly noted, the star wasn't "there" 290,000 years ago.

According to this paper, the asteroid's speed before feeling the Sun's pull was 26.2 kilometers per second.

 

[OmCheeto]

Actually, it took only 290,000 years to travel the 25-light-year distance from Vega, although as correctly noted, the star wasn't "there" 290,000 years ago.

According to this paper, the asteroid's speed before feeling the Sun's pull was 26.2 kilometers per second.

Thank you for the maths correction.

kilometer ≠ meter​

But now I'm still off by a factor of 2.

 

[Vanadium 50]

As stated in the Nature story, the orbit is clearly hyperbolic; its eccentricity is 1.20, which is safely above 1.00.

t's traveling about 25 km/s too fast to have come from the Oort Cloud.

These are both arguments that this body isn't in a permanent orbit with such parameters. I accept that. But why is it impossible to have been perturbed into this trjectory from the Oort cloud? Note that if you want to claim it's from a different solar system, you have the same problem - you need to give it enough velocity to eject it from that solar system.

Since you need this to happen either way, how can you use this to reject the source that is closer?

It came from the most likely direction that an interstellar object would come from. i.e. if the solar system were a car, and this was a raindrop, it hit our windshield.

If we had an ensemble of such objects and we could make a distribution, I would find this argument compelling. But I have seen statistical arguments applied to a single data point fail too often to put much stock in them.

 

[Astronuc]

Toward the bottom of the Nature article is the statement "The asteroid came from the direction of the constellation Lyra, which is roughly where our Solar System is heading."

Does it need a solar system to form an asteroid?

From where would elements such as C, N, O, Si, Fe, and/or Ni be formed?

Also from the Nature article,

despite its excursion near the Sun, it did not develop a tail — as a comet would — and so astronomers are currently classifying it as an asteroid.

The object does not appear to be 'icy'. Wouldn't an Oort object be more likely 'icy'?

On the other hand,

If analyses of comets are representative of the whole, the vast majority of Oort-cloud objects consist of ices such as water, methane, ethane, carbon monoxide and hydrogen cyanide.[19] However, the discovery of the object 1996 PW, an object whose appearance was consistent with a D-type asteroid[20][21] in an orbit typical of a long-period comet, prompted theoretical research that suggests that the Oort cloud population consists of roughly one to two percent asteroids.[22]

Ref: https://en.wikipedia.org/wiki/Oort_cloud#Structure_and_composition

Given the dimness of such objects, perhaps we have missed such objects in the past, say more than 200 years ago.
https://en.wikipedia.org/wiki/History_of_the_telescope#Optical_telescopes

 

[CygnusX-1]

There are approximately pi x 10**7 seconds in a year. Try that and see if you get the right answer.

Take a look at the trajectory of the asteroid in the Nature story. Notice that the asteroid does NOT pass close to any giant planet; therefore, there's nothing in our solar system that could have perturbed an Oort cloud object that was originally on an elliptical orbit onto a highly hyperbolic orbit--unless you believe there's a Planet X located far above the solar system.

But there are plenty of giant planets in OTHER solar systems that could have ejected the asteroid.

 

[russ_watters]

I don't think there is any evidence that this came from "another solar system". It could be from, e.g. our own Oort cloud.

These are both arguments that this body isn't in a permanent orbit with such parameters. I accept that. But why is it impossible to have been perturbed into this trjectory from the Oort cloud? Note that if you want to claim it's from a different solar system, you have the same problem - you need to give it enough velocity to eject it from that solar system.

Since you need this to happen either way, how can you use this to reject the source that is closer?

This debate is very interesting to me. What I'm hearing is that the vast majority of Oort Cloud objects we encounter are thrown at us at relatively low velocity. This would be because when you have an interaction between an asteroid and object of much larger size, there is a range of delta-V's that can be applied, and that range starts at zero and goes up from there. So a high [delta-]V object coming from the Oort Cloud is unlikely.

But a high velocity object from another solar system is also rare because the same logic about velocity distribution applies. The only difference between the scenarios is that the object would have gotten thrown "up" from another solar system vs "down" from our Oort Cloud.

...of course, while there are a lot more "up" velocity vectors than "down" ones, there are also a lot more other solar systems out there at those other velocity vectors. So perhaps all velocity vectors are roughly equally covered, making the likelihood of this being from another solar system higher than from the Oort Cloud?

Take a look at the trajectory of the asteroid in the Nature story. Notice that the asteroid does NOT pass close to any giant planet; therefore, there's nothing in our solar system that could have perturbed an Oort cloud object that was originally on an elliptical orbit onto a highly hyperbolic orbit--unless you believe there's a Planet X located far above the solar system.

But there are plenty of giant planets in OTHER solar systems that could have ejected the asteroid.

My understanding is that it can either be very large and far away or smaller and closer. While there aren't believed to be any Jupiter-sized objects in the Oort Cloud, we already know there are many Pluto+ sized objects. So a close encounter would be able to fire an object away/toward us at high velocity.

 

[Vanadium 50]

But a high velocity object from another solar system is also rare because the same logic about velocity distribution applies.

Exactly.

there are also a lot more other solar systems out there

That is true. But they are also farther away. It's not clear to me which effect wins: more sources, or a closer source.

 

[russ_watters]

That is true. But they are also farther away. It's not clear to me which effect wins: more sources, or a closer source.

It isn't clear to me either, but looking at angles tells me the extrasolar origin is probably more likely:

In order to fire an asteroid into the inner solar system at high velocity, it has to be in a tight window, say 10 degrees. The other 99% of velocity vectors (made up numbers) eject it without reaching the inner solar system for us to see it...and of course, the one we se is also being ejected after its flyby.

So that would tell me if all solar systems are similar, that for every 1 we see at high velocity from our own, we should see 100 from other solar systems.

...i do suppose though that this assumes the galaxy is dense enough that given enough time, all asteroids ejected from one solar system will eventually pass through another. Its Olbers' paradox with asteroids.

 

[tony873004]

These are both arguments that this body isn't in a permanent orbit with such parameters. I accept that. But why is it impossible to have been perturbed into this trajectory from the Oort cloud?

Objects in the Oort Cloud would move with speeds in the 100s of meter/second. There's no way two of them are going to interact, sending one of them off with a speed of 26 km/s.

It's possible that a star passed through the Oort Cloud and perturbed some objects. But it would have to get really close to an object to give it 26 km/s of speed. Objects in the Oort Cloud are so spread out that a star passing through would likely not get close to any individual object, just like our spacecraft pass through the asteroid belt without encountering any asteroids, unless we aim for them.

Also, the objects in the Oort Cloud are comets. This object is an asteroid. So it likely didn't come from the Oort Cloud.

Note that if you want to claim it's from a different solar system, you have the same problem - you need to give it enough velocity to eject it from that solar system.

Since you need this to happen either way, how can you use this to reject the source that is closer?

Because it could have been ejected from the star's planetary region, rather than from it's Oort Cloud. In our Solar System we've seen Jupiter do it (https://twitter.com/tony873004/status/913997533570985985), and have reason to believe that during the formation of the Solar System, lots of stuff got ejected into interplanetary space. It doesn't need to be ejected from the other solar system at high speed either. It could be ejected with a velocity at infinity of 1 m/s. But if this other solar system was moving at 26 km/s with respect to the Sun, that's how fast the asteroid will encounter the Sun. This asteroid's inbound velocity is typical to the velocity distribution of solar neighborhood stars.
https://arxiv.org/abs/1710.11364

If we had an ensemble of such objects and we could make a distribution, I would find this argument compelling. But I have seen statistical arguments applied to a single data point fail too often to put much stock in them.

It's not a statistical argument. Even with 0 data points it's to be expected that we will encounter interstellar objects more frequently in the direction of our motion. That's why when they manufacture cars, they put the wipers on the front window without ever having seen a single raindrop hit the car.

 

[Noisy Rhysling]

Pardon my poor eyes if I missed it, but given that we've detected large bodies in space not associated with any star why does it have to be from another system?

Two scenarios come to mind, and there are probably more, of course.

1. It was thrown out of another solar system, crossed interstellar space, and found its way here. (No "guiding" implied there.)

2. It was already in space, but not near another star. As Sol passed it was pulled in. (Does not imply the visitor was stationary itself.)

3. ...

 

[tony873004]

Large enough bodies like brown dwarfs can form the same way stars do, from clouds of hydrogen and other elements.
Smaller large bodies such as planets were probably formed in a solar system and then ejected.
Asteroids don't spontaneously assemble themselves from interstellar gas and dust.

It was not stationary. It was moving at 26 km/s. Sol accelerated it to 88 km/s. Now its on the way out, and sol is decelerating it. It will slow back down to 26 km/s as it leaves the solar system.

 

[Noisy Rhysling]

Large enough bodies like brown dwarfs can form the same way stars do, from clouds of hydrogen and other elements.
Smaller large bodies such as planets were probably formed in a solar system and then ejected.
Asteroids don't spontaneously assemble themselves from interstellar gas and dust.

There's no scenario where the body could have formed outside a solar system?

It was not stationary. It was moving at 26 km/s. Sol accelerated it to 88 km/s. Now its on the way out, and sol is decelerating it. It will slow back down to 26 km/s as it leaves the solar system.

"(Does not imply the visitor was stationary itself.) "

 

[russ_watters]

Objects in the Oort Cloud would move with speeds in the 100s of meter/second. There's no way two of them are going to interact, sending one of them off with a speed of 26 km/s.

It's possible that a star passed through the Oort Cloud and perturbed some objects. But it would have to get really close to an object to give it 26 km/s of speed.

Ok, so this is something I misunderstood about the gravitational slingshot. I thought you could gain more speed by getting closer to the massive object, with no limit. But googling, I see a limit of 2x the large object's velocity. That makes it very difficult for an object to gain such a large velocity while in the Oort cloud.

 

[OmCheeto]

There are approximately pi x 10**7 seconds in a year. Try that and see if you get the right answer.

In my universe, there are apparently only 30 seconds per minute; "seconds/year ≠ 365.2422×24×60×30"

Final travel time from Vega-ish distance answer: 291,097 years

Take a look at the trajectory of the asteroid in the Nature story. Notice that the asteroid does NOT pass close to any giant planet; therefore, there's nothing in our solar system that could have perturbed an Oort cloud object that was originally on an elliptical orbit onto a highly hyperbolic orbit--unless you believe there's a Planet X located far above the solar system.

But there are plenty of giant planets in OTHER solar systems that could have ejected the asteroid.

I was scanning the Lyra constellation area on Google Earth the other day and found a "most likely" candidate: NGC 6745

These galaxies did not merely interact gravitationally as they passed one another; they actually collided. [ref]​

And per wiki;

NGC 6745 (also known as UGC 11391) is an irregular galaxy about 206 million light-years (63.5 mega-parsecs) away in the constellation Lyra. It is actually a trio of galaxies in the process of colliding.​

The three galaxies have been colliding for hundreds of millions of years.​

The only problem with this is:

NGC 6745: 1.9E+24 meters distance to Earth
travel time: 2,398,637,430,644 years
age of the universe: 13,820,000,000 years
complicating factor: 173.6 (=travel time/age of universe)
conclusion: either the universe is older than we think, or this object is not from NGC5745​

So, are there closer candidate "colliding" galaxies?
(google google google)

Wow!

WHAT IS THE CLOSEST GALAXY TO THE MILKY WAY?

Closest Galaxy:

At present, the closet known galaxy to the Milky Way is the Canis Major Dwarf Galaxy – aka. the Canis Major Overdensity. This stellar formation is about 42,000 light years from the galactic center, and a mere 25,000 light years from our Solar System. This puts it closer to us than the center of our own galaxy, which is 30,000 light years away from the Solar System.
...
The Milky Way became the size it is now by eating up other galaxies like Canis Major, and it continues to do so today. And since stars from the Canis Major Dwarf Galaxy are technically already part of the Milky Way, it is by definition the nearest galaxy to us.​

At only 25,000 light years, and from my interpretation, that "the Milky way is currently colliding with the Canis Major dwarf galaxy", my guess is that we will never know the origin of this asteroid.

Kind of reminds me of what Zed said in Men in Black; "We're not hosting an intergalactic kegger down here."

Oh yes we are. The galaxies are all drunk, crashing into each other, and sending debris flying every which way.

Other globular clusters that orbit the center of our Milky Way as a satellite – i.e. NGC 1851, NGC 1904, NGC 2298 and NGC 2808 – are thought to have been part of the Canis Major Dwarf Galaxy before its accretion.​

 

[DrStupid]

From where would elements such as C, N, O, Si, Fe, and/or Ni be formed?

From supernovae or neutron star collisions million or even billion years before the asteroid has been formed. Maybe my question was not precise enough. What I want to know is this:

There's no scenario where the body could have formed outside a solar system?

 

[dipole]

What kind of measurements of the composition of the asteroid are possible? It would be very interesting to be able to get an idea if the ratios of heavy elements contained in it are consistent with what we find in other known asteroids.

 

[Noisy Rhysling]

From supernovae or neutron star collisions million or even billion years before the asteroid has been formed. Maybe my question was not precise enough. What I want to know is this:

There's no scenario where the body could have formed outside a solar system?

Whether PSO J318.5-22 was flung from the Beta Pictoris moving group or formed independently from it by some still-unknown process remains unknown. Be that as it may, Liu and his colleagues believe the planet presents a rare opportunity to do some science far from the blinding light of a nearby star.

https://io9.gizmodo.com/astronomers-say-theyve-found-a-rogue-planet-with-no-su-1443571329

 

[stefan r]

But a high velocity object from another solar system is also rare because the same logic about velocity distribution applies. The only difference between the scenarios is that the object would have gotten thrown "up" from another solar system vs "down" from our Oort Cloud.

An object can leave a star at painfully slow speed. Barely nudged out of orbit. As an object's orbit goes further it becomes easier and more likely that the object gets a nudge.

The sun is moving at around 20 km/s relative to the average velocity of nearby stars. So the parent star could, for example, be moving at 5.999 km/s relative to average stars in our neighborhood and the asteroid could leave orbit at 0.001 km/s. Then we could add the velocities to get 26 km/hr. More likely there are some angles involved and the host parent star had a higher velocity relative to the sun's neighborhood and/or the object exited a little faster. No need for a high velocity exit.

 

[snorkack]

Ok, so this is something I misunderstood about the gravitational slingshot. I thought you could gain more speed by getting closer to the massive object, with no limit. But googling, I see a limit of 2x the large object's velocity. That makes it very difficult for an object to gain such a large velocity while in the Oort cloud.

Obviously you could.
Provided the massive object itself is in Oort cloud, but NOT part of it.

 

[Vanadium 50]

Thanks for the replies. I find the quantitative velocity argument the only compelling one.

• The composition argument (rocky vs. icy) is, at best, probabilistic. There exist a small number of tail-less comets.
• The "easier to eject from an inner solar system" argument is not true kinematically (sqrt(2) * circular orbit speed will eject an object). It's easier only in the sense that the density of objects is higher.
• As I said before, I would believe the directional argument if I had a statistical ensemble of observations, but not for a single event.
• Getting to 26 km/s from the outer Solar System is hard. An interaction with Jupiter would do it, but the trajectory doesn't allow it. However, this is still puzzling when coupled with the direction argument. That means the object was almost at rest (~8 km/s, about 40% of the sun's velocity) with respect to the Local Standard of Rest, Not impossible, just puzzling.

 

[tony873004]

You don't need an ensemble of observations.
You don't need to see it rain to know that if it does rain, a rain drop is more likely to hit the windshield than the side windows.

 

[Vanadium 50]

You don't need to see it rain to know that if it does rain, a rain drop is more likely to hit the windshield than the side windows.

But if you have a single drop of water on your windshield, will you conclude that it must be rain? (As opposed to from a sprinkler, or a garden hose, or something else)

 

[tony873004]

If there was a single drop on my windshield, I would only conclude that the drop of water likely originated from outside the car. Sprinkler and garden hoses are external to the car. Water drops from them are also more likely to strike a moving car's windshield than its side windows.

 

[snorkack]

• Getting to 26 km/s from the outer Solar System is hard. An interaction with Jupiter would do it, but the trajectory doesn't allow it. However, this is still puzzling when coupled with the direction argument. That means the object was almost at rest (~8 km/s, about 40% of the sun's velocity) with respect to the Local Standard of Rest, Not impossible, just puzzling.

What is the local standard of rest?
Is Sun, at 20 km/s, an unusually fast-moving star moving through stars whose speeds relative to each other/local standard of rest are slower than 20 km/s, an unusually slow-moving star sitting among stars which are moving at high speed at all directions but slightly more in one direction, or an average star?

 

[OmCheeto]

What is the local standard of rest?

My guess is, that there isn't one. [see below]

Is Sun, at 20 km/s, an unusually fast-moving star moving through stars whose speeds relative to each other/local standard of rest are slower than 20 km/s, an unusually slow-moving star sitting among stars which are moving at high speed at all directions but slightly more in one direction, or an average star?

As might be common knowledge, Barnard's star is moving pretty fast relative to us.

per wiki's entry on Barnard's Star;

The radial velocity of Barnard's Star towards the Sun is measured from its blue shift to be 110 km/s. Combined with its proper motion, this gives a space velocity (actual velocity relative to the Sun) of 142.6 ± 0.2 km/s. Barnard's Star will make its closest approach to the Sun around AD 11,800, when it will approach to within about 3.75 light-years.​

I did some rough calculations, based on an image on that page:

, and came up with similar numbers for Barnard's star. So my confidence level is high that I've gotten the maths correct this time, and have included the relative velocities of other local stars:

_closest__  rel vel
  AD   ly    km/s     star(s)
30000  3.2     25     Proxima & Alpha Centauri
12000  3.7    130     Barnard's star
22000  4.7     82     Lalande 21185
39000  3.0     76     Ross 248
48000  3.5     27     Ross 128
76000  5.2    106     Gliese 445

Now, I just have to figure out where they are/were, and how Oort cloud "outer borders" are determined.

ps. I'm really bad at maths, so everyone is welcome to jump in here.

 

[Bandersnatch]

What is the local standard of rest?

My guess is, that there isn't one. [see below]

LSR is an astronomical term for the mean motion of galactic matter in the solar neighbourhood. It's what you'd think about if you wanted to treat galaxies as composed of matter following idealised regular orbits, where all stars and gas at a given distance have mostly identical velocities governed by the dynamics of the entire galaxy, without whatever motion local interactions induce.
It's what peculiar motions are measured against.

Here's an example paper discussing recent attempts at measuring Sun's peculiar velocity w/r to the LSR, which may shine some light on what's being done and how:
https://arxiv.org/abs/1411.3572
edit:
this lecture presentation is probably more accessible:
http://astroweb.case.edu/ssm/ASTR421/lecture11.pdf
/edit

Regarding that, though...

That means the object was almost at rest (~8 km/s, about 40% of the sun's velocity) with respect to the Local Standard of Rest, Not impossible, just puzzling.

Why is this puzzling any more than any other number? I really don't get the argument here. Would 15 km/s be not puzzling? 1 km/s? A thousand?

 

[Vanadium 50]

Why is this puzzling any more than any other number?

What does one expect? I'd expect ejected bodies to be going a little faster (relative to the LSR) than the bodies they were ejected from. So maybe 30 km/s would be most probable. Now, 8 km/s is not an impossible number, but it is unusual - phase space considerations (an idealization - maybe even an oversimplification - to be sure) would suggest maybe 1 in 10 or so such objects would be going that fast or slower.

So 1km/s would be more puzzling. Not impossible, but slower than ~99.9% of the expected objects. 15 km/s would be less puzzling - maybe a quarter of the objects would be that slow or slower. 1000 km/s would be very surprising, as it is faster than the galaxy's escape velocity.

 

[snorkack]

LSR is an astronomical term for the mean motion of galactic matter in the solar neighbourhood. It's what you'd think about if you wanted to treat galaxies as composed of matter following idealised regular orbits, where all stars and gas at a given distance have mostly identical velocities governed by the dynamics of the entire galaxy, without whatever motion local interactions induce.
It's what peculiar motions are measured against.

Here's an example paper discussing recent attempts at measuring Sun's peculiar velocity w/r to the LSR, which may shine some light on what's being done and how:
https://arxiv.org/abs/1411.3572
edit:
this lecture presentation is probably more accessible:
http://astroweb.case.edu/ssm/ASTR421/lecture11.pdf
/edit

Regarding that, though...

Why is this puzzling any more than any other number? I really don't get the argument here. Would 15 km/s be not puzzling? 1 km/s? A thousand?

Vanadium explained why 1000 would be puzzling, and 1 likewise. Arguments which I thought of myself.
250 would NOT be puzzling, because that happens to be the peculiar speed of Kapteyn´s star, for example.
The lecture presentation gives velocity dispersion of 9 km/s for A stars, compared to 20 of Sun.
Does this suggest that the asteroid is likely to be that of some A star, such as Sirius, Altair, Vega or Fomalhaut, and not of a G star, such as Rigil Kentaurus, tau Ceti, sigma Draconis or eta Cassiopeiae?

 

[CygnusX-1]

The discovery of the interstellar asteroid is now on the radio: Listen free at The John Batchelor Show.

 

[Bandersnatch]

What does one expect? I'd expect ejected bodies to be going a little faster (relative to the LSR) than the bodies they were ejected from. So maybe 30 km/s would be most probable. Now, 8 km/s is not an impossible number, but it is unusual - phase space considerations (an idealization - maybe even an oversimplification - to be sure) would suggest maybe 1 in 10 or so such objects would be going that fast or slower.

So 1km/s would be more puzzling. Not impossible, but slower than ~99.9% of the expected objects. 15 km/s would be less puzzling - maybe a quarter of the objects would be that slow or slower. 1000 km/s would be very surprising, as it is faster than the galaxy's escape velocity.

First, there seems to be a matter of language use: when you say puzzling, it implies that the cause is somehow unknown, perplexing, completely unexpected. As in, 'how in the world did it get to have that velocity?' kind of puzzling.
But from your post it looks like what you meant is that it is just an outlier in some distribution. I was objecting to the former (all of those velocities can be produced by known interactions, hence they're not puzzling), not to the latter.

But let's look at whether 8 km/s w/r to LSR is really an outlier. There are two things to take into consideration here:
1.
Ejection events are more likely to be caused by gradual (over many orbits) changes to orbital angular momentum of an asteroid whose orbit is coupled to some massive planet, rather than a one-off close encounter resulting in large delta V - simply because there will always be more asteroids affected at long range than in close encounters.
Since the most likely ejection is by incremental boosting of orbital momentum, the velocities of ejecta should be clustered around the escape velocity, i.e. one would expect rogue asteroids to have velocities close to the peculiar velocity of their parent star, with similar distribution.

2.
Peculiar velocities of stars w/r to LSR are, by definition, directed every which way. An asteroid ejected with some velocity in a random direction w/r to its parent system will then have its velocity w/r to LSR be a nett result of the two.
E.g. even in a fantasy scenario where all stars have peculiar velocities equal to 20 km/s, and all asteroids are ejected with 20 km/s over escape velocity, the expected LSR velocity of an asteroid would be anywhere in the range of [0, 40] km/s.
That is, one doesn't even need a slow-moving star, nor slow ejection, to get a slow-moving rogue asteroid.

This is the velocity distribution in the galactic plane of some 20 thousand stars in the solar neighbourhood:

Where the first dark contour contains approx 50% of stars. Vx is radial, Vy is in the direction or galactic rotation. The triangle is LSR.
(Taken from: https://arxiv.org/abs/0912.3262, fig 1. This earlier paper contains contours for plane-normal direction as well: https://arxiv.org/abs/0905.2980 - they're narrower, but similarly clustered around 0 km/s w/r to LSR)

So, taking into account both points mentioned above, as well as the velocity distribution of nearby stars, I think the 8 km/s is not only within the expected range (i.e. not-puzzling), but also in the most likely range.

250 would NOT be puzzling, because that happens to be the peculiar speed of Kapteyn´s star, for example.
The lecture presentation gives velocity dispersion of 9 km/s for A stars, compared to 20 of Sun.

The dispersion you mentioned is for plane-normal velocities only. It is not the nett peculiar velocity which you compare it with. The plane-normal velocity of the Sun is approx. +7 km/s.

Same thing as with Vanadium's post, if by 'not puzzling' you mean 'likely', then 250 km/s would be unlikely, as velocities like that of Kapteyn's star are strong outliers.

 

[Vanadium 50]

Yes, by "puzzling" I mean "an outlyer". An it's not a yes or no thing. A 1 in 2 outlyer doesn't bother me at all. A 1 in 10 bothers me some, 1 in 100 more, and so on.

E.g. even in a fantasy scenario where all stars have peculiar velocities equal to 20 km/s, and all asteroids are ejected with 20 km/s over escape velocity, the expected LSR velocity of an asteroid would be anywhere in the range of [0, 40] km/s.

Yes, but in this fantasy I would expect the distribution to peak at ~30 km/s (20 and 20 in quadrature). 8 would be relatively unlikely, 4 even more so, and so on.

 

[Bandersnatch]

Yes, but in this fantasy I would expect the distribution to peak at ~30 km/s (20 and 20 in quadrature). 8 would be relatively unlikely, 4 even more so, and so on.

But that's a fantasy scenario, meant to illustrate a specific point. In reality, stellar velocities cluster around 0 km/s w/r to LSR, as shown on the graph above. Couple that with ejection velocities predominantly being expected to be close to escape velocity, and 8 km/s is within 1 sigma for the population. As is 1 km/s, or 0.

 

[stefan r]

Yes, by "puzzling" I mean "an outlyer". An it's not a yes or no thing. A 1 in 2 outlyer doesn't bother me at all. A 1 in 10 bothers me some, 1 in 100 more, and so on.
Yes, but in this fantasy I would expect the distribution to peak at ~30 km/s (20 and 20 in quadrature). 8 would be relatively unlikely, 4 even more so, and so on.

Wikipedia says oxygen has a critical ionization velocity of 12 km/s. Atoms in interstellar space frequently have thermal velocities of several km/s. Silicon and iron have lower ionization energies and more atomic mass. Higher velocity objects will build up more surface charges. Ionizing radiation and plasma can levitate dust. The effect has been observed on the moon. Charged particles interact with plasma in a wider radius than the particle radius. Asteroids with velocities 30 km/s would build up a lot more charge.

The milky way averages around 5 x 10⁵ molecules/m³ of interstellar gas. That is not enough to drag down the velocity of planets or stars. Suppose we guestimate the asteroid to have mass 3 x 10⁶ kg and 2 x 10⁴ m² cross section. If it moves at 10⁴m/s it will hit 10¹⁴ molecules per second. Assuming inelastic collisions _{(probably wrong)}, 1.7 x 10^-13 kg/s at 10⁴ m/s should decelerate a 3 x 10⁶ kg asteroid at 5.5 x 10^-16 ms^-2. In one billion years that becomes 17 m/s.

A one micro gram piece of dust orbiting (or following) the asteroid 7500 meters (100x radius) would have gravitational acceleration F=G(m₁m₂)r^-2= 6.674 x 10^-11 x 3 x 10⁵ x 1 x 10^-6 x 1.8 x 10-8 = 3.6 x 10^-19 Newtons.
The piece of dust could have diameter 10^-4 m, cross sectional area 8 x 10^-9 m². The dust hits about 40 molecules per second or 6.6 x 10-26 kg/s. Traveling at 10⁴ m/s (simplified) drag force would be around 6.6 x 10-22 Newtons. The force of interstellar gas on an orbiting grain of dust is 500x less than the force of the asteroid's gravity. So the grain could stay in orbit. A comet's dust tail could have much higher drag force than the comet's body.
If the dust tail has 1000x the total surface area then it could drop velocity from 10 km/s to 8 km/s in 120 million years. I am not sure if a dust trail would be detectable. It also would not need to be currently present.

I have not seen any evidence that A/2017 U1 has a dusty regolith. Just throwing out the possibility that high velocity meteors could slow down.

 

[Vanadium 50]

But that's a fantasy scenario, meant to illustrate a specific point.

But it was your scenario. That's why I responded to it.

In reality, stellar velocities cluster around 0 km/s w/r to LSR

But stellar speeds do not.

 

[r8chard]

Good discussion all around.
I am of the opinion that for all the highfalutin' mathematics and continuously evolving technology of the last few centuries?

It still remains a viable rule of thumb "That all action produces an equal and opposite reaction".

 

[Vanadium 50]

r8chard, was this the thread you intended to post this to?

 

[Bandersnatch]

But stellar speeds do not.

I've been trying to understand what you meant by that, but couldn't. No matter how I look at the graph in post #37, I see low speeds in the most likely range.
Same when I look at the W velocity dispersion from the presentation linked to in post #33.
Maybe I'm missing something obvious here.

 

[mfb]

The difference here is between speed and velocity.

As an example, consider a uniform velocity distribution from -20 km/s to +20 km/s in both directions. What is the speed distribution for speeds below 20 km/s?It is a linearly increasing function. Regions of fixed speed are circles, and the circle corresponding to 1 km/s is much smaller than the circle corresponding to 10 km/s. Slower speeds are less likely. This stays true even if the velocity distribution is not completely uniform. The probability that both velocities are very close to zero at the same time is very small.

 

[Bandersnatch]

Ok, I can appreciate how there would be a dip around the zero point. But if the distribution in the x-y plane is such that 50% of the population falls within a circle of radius 20, then any speed of less than 20 still makes the star fall within that 50% range, no?

 

[mfb]

But if the distribution in the x-y plane is such that 50% of the population falls within a circle of radius 20, then any speed of less than 20 still makes the star fall within that 50% range, no?

In that case 20 km/s is the median speed, sure. The mean can be higher.

 

[tony873004]

This asteroid, now called ʻOumuamua, is thought to be about 150 meters across.
If it was a red dwarf star instead, with a mass of 0.1 solar masses, it would have perturbed all the planets' orbits.
Earth would have a semi-major axis of over 7 AU.

Here is a simulation of our new solar system:
http://orbitsimulator.com/gravitySi...ns/1510265575945_afterInstellarRedDwarf..html

 

[Vanadium 50]

Nice! (Pun intended)

At 7 AU global warming becomes something we aspire to.

What happens to the moon in this scenario?

Why does A2017U1 keep coming back? I thought its orbit was hyperbolic.

Did you run this with a Jupiter-sized object? Anything interesting happen?

 

[tony873004]

The Moon stays with the Earth, but its eccentricity is nearly quadrupled.

In the simulation, the "camera" is in an unrealistically-fast orbit around the solar system just to show it to you from all sides. That makes it appear as if A2017U1 keeps coming back. If you check "Trails" and run it, you will see that's its spiraling away in this rotating frame of reference.

With a Jupiter-mass, it wasn't nearly as interesting. Earth's orbit got ever-so-slightly rounder. The Moon's eccentricity jumped from about 0. 55 to about 0.1.