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Interstellar Interloper, Coming in from the great beyond
HSchirmer
post Dec 21 2017, 01:12 AM
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QUOTE (JRehling @ Dec 20 2017, 11:08 PM) *
The velocity of 'Oumuamua is far beyond the escape velocity of the Sun.
There is no circumstance of orbital mechanics in our solar system that could accelerate an object that was initially in solar orbit to this velocity.


Hmm, then it couldn't have come from a yellow dwarf star with a layout like our solar system?
So, Is this more about the star it's being ejected FROM or the planet DOING the ejecting?

QUOTE (JRehling)
the largest boosts leading to escape will happen close to the star and could explain a baked-Alaska outcome.


So, would that boost be something like a hot jupiter around a red dwarf?
Sorry, for a string of questions, just trying to get a range-idea of how much of the velocity is due to being ejected, and how much is due to relative motion.

Curious, some nice model simulations
QUOTE (COULD JUPITER OR SATURN HAVE EJECTED A FIFTH GIANT PLANET?)

seem to suggest that a gas giant ejecting an ice giant might scatter the moons at very high speed?
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nprev
post Dec 21 2017, 02:52 AM
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We can't really infer anything about the spectral type or even the mass of the host star. Additionally, there are a very large number of possible scenarios that could result in ejection from its host system, so I don't see any way to constrain even that.

What I do wonder is whether it would be possible to find out if it came from a 'nearby' star, defined as one within a few hundred light-years. Hipparcos and the future ESA astrometric mission (can't recall the name) have produced and will produce extremely high-quality data, and since we know the trajectory parameters well enough to project its path back over a few million years, and since intermediate close stellar encounters are possible but extremely unlikely, it may be possible to identify a candidate system...if it's close.

Of course, if it came from MUCH farther away this becomes a hopeless task, at least with the current state of technology and our astrometric dataset.


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A few will take this knowledge and use this power of a dream realized as a force for change, an impetus for further discovery to make less ancient dreams real.
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JRehling
post Dec 21 2017, 06:02 AM
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Per Gerald's point above, we don't know that the object was ejected from another system at high velocity relative to that system, because it could have been ejected from another system that was already moving at high velocity relative to our system. So the observation seems to be compatible with it being ejected from another system at modest velocity relative to that system.

But for any hypothesized origin in the solar system, that doesn't work. It would have to have been accelerated at a high velocity somewhere in the outer solar system, which is not possible via gravity assists. Gravity assists cannot accelerate an assistee by more than 0.6 times the velocity of the assister. In the outer solar system, there are no such high velocities.

Note that Oumuamua approached from the general direction from which one would expect an interstellar object to approach, and far from the ecliptic.
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HSchirmer
post Dec 21 2017, 02:00 PM
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QUOTE (JRehling @ Dec 21 2017, 07:02 AM) *
Per Gerald's point above, we don't know that the object was ejected from another system at high velocity relative to that system,
because
it could have been ejected from another system that was already moving at high velocity relative to our system.
...


That's an interesting point, I'll try and ask a follow up question more clearly-
If U1 was ejected at low speed, as some sort of "scattered disc" Xena-Gabrielle analog, don't we need to consider that it might have been a binary?
However, that raises a really interesting possibility, a binary capture-ejection scenario. (Interstellar equivelent of Neptune - Triton capture).
In that case, we can't just project the U1 outbound orbit backwards to determine origin, we'd need pre-covery images of the inbound orbit.


U1 looks similar to some KBOs we see around our star;
"Our" KBOs have alot of binaries; a fair percentage of these binaries survive being kicked into orbits as "hot KBOs" or scatterd disc orbits;
U1 was ejected from its parent solar system, but we're not sure whether it was ejected by interaction with-
the star, a hot gas giant, a cold gas giant, an ice giant.

So, considering assumptions that
1) We're the first solar system that U1 encountered,
2) U1 was ejected by the most distant ice giant in it's solar system, i.e. it's a TNO/scattered disc analog from another solar system.
3) U1 may have started as a binary object, consistent with our KBOs.

Given those (modest) assumptions, should we consider the possibility that U1 entered our solar system as a loose binary?

If that is the case, we can't accurately determine it's direction from the outbound orbit of one body; we're only tracking one member of the initial binary.
We'd need pre-covery images to figure out the actual inbound orbit of both bodies, which would also provide an idea of where the other half went...

More interesting, given the close approach to the sun, perhaps we should be considering an "interstellar triton" capture scenario,
where one member of the binary is captured, the other is ejected?
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Gerald
post Dec 21 2017, 04:38 PM
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The time for interstellar dust to be captured has been has been estimated to about 2.5 billion years:
QUOTE
From the instant of the formation in stellar ejecta on, stardust
grains are subject to destructive processes in the ISM by
sputtering and shattering processes induced by supernova (SN)
shocks (cf. Jones et al. 1996, and references therein). They are
finally incorporated into newly formed stars and their planetary
systems after about 2.5 Gyrs residence time in the ISM,

For simplicity, assume a travel time of 3 billion years for the presumed travel time of U1.
Just as a starting point of model scenarios, assume a travel distance of 1000 light years relative to its presumed source star.
Divide the 1000 light years by 3 billion years. You get a mean velocity of 1000e-9 c/3 with c the speed of light, hence about 100 m/s.

The escape velocity is proportional to the inverse of the square of the distance to the central body.
The escape velocity is also the speed of a circular orbit times the square root of 2.

Take 30 km/s as Earth's orbital velocity. That's 300-times faster than our assumed mean velocity of the interloper relative to the source star.
Hence it corrresponds to the orbital velocity of an object at 90000 au, or about 1.4 light years.

Presumed, stars are usually surrounded by an Oort-type cloud, a star "slowly" passing in a distance of a few light years would disturb such a cloud, and some objects may be slowed down to eventually fall into the interior of the hosting solar system. Even a very small gravity assist of an inner planet could add sufficient kinetic energy, that the object would escape, due to the multiplication by the Oberth effect.
As an example, suppose a closest approach of 1 au relative to a star of the mass of the sun. At periapsis, we have a circular orbital velocity of 30 km/s (like Earth), hence a parabolic velocity of about 42.4 km/h, which is also the escape velocity. Apply the approximative formula of the Oberth effect for parabolic orbits and small delta-v to a delta-v of 1 m/s to see, that the velocity gain outside the gravity well will be near sqrt(42,400) m/s = 206 m/s. Crudely subtract the 100 m/s of an object orbiting at 1.4 light years distance, in order to see, that even this very benign scenario is sufficient to convert a distant Oort object into an interstellar object.

Close encounters with inner planets would cause much higher travel velocities, of course, in addition to the velocity between source and target system.
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fredk
post Dec 21 2017, 05:47 PM
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QUOTE (Gerald @ Dec 21 2017, 05:38 PM) *
Even a very small gravity assist of an inner planet could add sufficient kinetic energy that the object would escape, due to the multiplication by the Oberth effect.

Only if the object could provide thrust, since the Oberth effect refers to the efficiency of thrust. So are you thinking of cometary jets? What delta v would be realistic?

Also, we know the body's asymptotic velocity: around 26 km/s relative to the sun, much higher than 100 m/s, though it's not clear what you meant by that.
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Gerald
post Dec 21 2017, 06:17 PM
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When you get a delta-v by a freefall component towards some planet in the inner part of the source planetary system, this adds much more kinetic energy with respect to the central star than the same delta-v in outer parts of the same planetary system, assuming the same energy sum of kinetic and potential energy, hence a standard Kepler orbit. Hence, the Oberth effect applies to more scenarios than just thrust.
A cometary jet would do so, too. But I've been thinking at a gravitational slingshot in first place. The principle is related to the one applied to New Horizons with the Jupiter slingshot.
But all you need for a former Oort object in the source system is a very small delta-v provided by some inner planet, once the trajectory has been changed to quasi-parabolic by some external source, in order to obtain escape velocity.
The remainder of the relative velocity to the target system is still mostly provided by the velocity difference between the source an the target planetary system.
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HSchirmer
post Dec 21 2017, 08:44 PM
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QUOTE (Gerald @ Dec 21 2017, 07:17 PM) *
When you get a delta-v by a freefall component towards some planet in the inner part of the source planetary system,
this adds much more kinetic energy with respect to the central star than the same delta-v in outer parts of the same planetary system, ...


But isn't that offset by being deeper in the star's gravity well?

QUOTE (On the Consequences of the Detection of an Interstellar Asteroid Gregory Laughlin and Konstantin Batygin)
https://arxiv.org/pdf/1711.02260.pdf
Among the known extrasolar planets (Figure 1), neither the hot Jupiters, nor the far more numerically dominant population of super-Earths
– which typically have M∼10M⊕, R∼3M⊕, and a∼0.2 (Winn & Fabrycky 2015) – can eject planetesimals...

Upon substitution of the relevant constants, we find that for solar-mass stars,
the characteristic semi-major axis – beyond which ejection of planetesimals is readily accomplished by relatively low-mass planets – lies at a∼5 AU.
(This “throw line” diminishes to a∼1 AU for M?= 0.2M M-dwarfs).
Coincidentally, these values roughly correspond to the ice-sublimation lines of the respective stars,
and strongly hint at the ubiquity of sub-Jovian planets residing at stellocentric radii of order a few astronomical units...


While this paper is extrapolaing from one asteroid to consider the orbital mechancs needed to eject larger planetismals,
shouldn't the efficient ejection of larger bodies at "the throw line" apply equally to ejection of smaller objects such as U1?
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Gerald
post Dec 21 2017, 09:22 PM
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By conservation of energy, an Oort object has almost the energy to be ejected. If it gets into the inner of a planetary system, a small slingshot effect is sufficent to add the small amount of energy needed to accelerate it to escape velocity.
Even objects starting from the inner parts of planetary systems have the potential to be ejected from the planetary system, although with a much smaller probability than objects starting from the Oort cloud. It's a standard method for the exploration of our solar system with space probes. For random trajectories, it's just more likely to end up in the star or on a planet. But ejection of planetesimals is mostly possible as soon as there is a planet around a star. It's just a matter of probabilities of the sometimes very complex trajectories.
The simplest such case is the three body problem. These systems already behave chaotic, in general, such that you get occasional near-misses with the potential of slingshots. Several such slingshots in a sequence may shift the smallest object to higher and more elliptical orbits, until it escapes. But such sequences are less probable in a random settings the more accurately they need to be designed.
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HSchirmer
post Dec 21 2017, 11:01 PM
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QUOTE (Gerald @ Dec 21 2017, 09:22 PM) *
By conservation of energy, an Oort object has almost the energy to be ejected.


But before you can have Oort objects, don't you need a Nice catastrphe to throw them out there in the first place?
They didn't quite escape because passing stars or galctic edge effects circularized their orbits and bent them back?

If the Oort cloud are the sub-set of objects that were scattered but-not-quite-ejected, should we look at the objects that were actually ejected?
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Gerald
post Dec 22 2017, 12:10 AM
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I don't think, that a large fraction of the Oort objects are a result of a catastrophic event, but remanants of protoplanetary discs, as described in the Wikipedia article. A catastrophic event wouldn't have left many objects in the very narrow energy range the Oort objects are living in. And especially the outer Oort cloud doesn't appear to show much structure, infered from comets with approximately parabolic orbits. If they would be a result of an ejection from the inner solar system, the cloud would be strongly correlated to the ecliptical plane. And more important, supervolatiles couldn't have condensed to a solid. But they have been found sublimating on 67P.
I'm looking at this set of objects, since we have observational evidence from our own planetary system.
Protoplanetary disks and debris disks appear to be pretty common, and so may be a source of planetesimals.
We don't know, whether the material ejected early during planetary system formation, i.e. during or immediately after Jeans instability contains bodies larger than gas and dust. Hence to be sure, that we are talking about evidently existing minor bodies, I'm refering to objects we are knowing of, like comets with origin in the presumed Oort cloud.
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HSchirmer
post Dec 22 2017, 04:02 PM
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QUOTE (Gerald @ Dec 22 2017, 12:10 AM) *
I don't think, that a large fraction of the Oort objects are a result of a catastrophic event, but remanants of protoplanetary discs, as described in the Wikipedia article.


Ah, I thought that, generally, the mass of a protoplanetary disc, and mostly of the planetismal forming, occurs near the snowline, only a few AU from the star.
How does a protoplanetary disc get organized when it's 1 or 2 lightyears away from the parent star?

QUOTE (Gerald @ Dec 22 2017, 12:10 AM) *
A catastrophic event wouldn't have left many objects in the very narrow energy range the Oort objects are living in.


Why not?
A narrow energy range for Oort cloud objects imply that they're a sub-set of a wide range of energies for objects that are scattered by formation of gas giants.

How does a very narrow energy range for Oort objects imply protoplanetary disc with objects 2 lightyears away?

Better theory is, Oort objects are the objects ejected by coalescing gas giants, which happen to be on assymptotic orbits.
Low energy objects fell back into the inner solar system as the late heavy bombardment.
High energy objects were flat-out ejected onto interstellar trajectories.
Oort objects are the knife edge of assymptotic orbits that take thousands, millions or billions of years to return.

Unless there's a clear mechanism to restrict planetismal ejection velocities to that narrow assymptotic energy range, why assume a narrow energy range?


Here's an analogy.
Imagine going to Talledaga raceway, and hitting baseballs towards the banked portion of the track.




Under your scenario,
The majority of the baseballs will be less energetic, below escape velocity, hit the sloped track, and roll back immediatly. That's the late heavy bombardment.
The other marjor group of baseballs are above escape velocity, go over the wall, and never come back.
The Oort cloud represents that small fraction of balls that are perfectly balanced on the wall, waiting for an external force to send them back into the solar system.

QUOTE (Gerald @ Dec 22 2017, 12:10 AM) *
I'm refering to objects we are knowing of, like comets with origin in the presumed Oort cloud.


Eh, not sure about "known objects" like comets, being boostrapped into "presumed structures" like the Oort cloud (which was proposed pre-Nice model)
A "narrow range" of secattering energies creates a population of objects on not-quite assympotic orbits,
those objects take thousands, millions or billions of years to fall back into the inner solar system.

Occams' razor-
Why assume an Oort cloud structure that requires that these objects have their orbits modified twice?
Once to circularize, a second time to return to an almost-paraboilc orbit.

Isn't it simpler to acknowledge that the Nice model give you a broad population of almost-assymptotic objects that return after millions or billions of years?

A) Scattered object is put into into a long term return orbit.

or

B1) Scattered object is put into assymptotic orbit
B2) Assymptotic orbit is circularized by first highly unlikely event.
B3) Asymptotic orbit de-circularized by second highly unlikely event.
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Explorer1
post Dec 22 2017, 11:07 PM
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I suppose the best way to get answers is find one and see how it's moving. But what would it take for Oort cloud objects to be observed in situ, and have their orbit calculated? Could any (practical) telescope on Earth or in orbit detect them, given their size, distance, and albedo, or would some sort of dedicated mission need to go out into the darkness and survey them (if they exist, of course)? I read about the Whipple mission proposal, could it actually work?
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Gerald
post Dec 23 2017, 02:32 AM
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QUOTE (HSchirmer @ Dec 22 2017, 05:02 PM) *
Isn't it simpler to acknowledge that the Nice model give you a broad population of almost-assymptotic objects that return after millions or billions of years?

As long as we don't have a good explanation of how molecular nitrogen can sublimate from a comet, it's more plausible, that it formed far away from the sun:
QUOTE
This depletion suggests that cometary grains formed at low temperature conditions below ~30 K...

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HSchirmer
post Dec 26 2017, 03:43 PM
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QUOTE (Explorer1 @ Dec 22 2017, 11:07 PM) *
Could any (practical) telescope on Earth or in orbit detect them, given their size, distance, and albedo,


Possible, but really difficult.
You're not going to detect oort cloud objects with a "normal telescope" that directly images an object based on light reflected from it.
From what we know of comets, they are small, dark, incredibly cold, and slow moving.
That frustrates most observation mechanisms.

Instead, you'd need a telescope that is setup for occlusion / microlensing, point it at a bright part of the sky, and wait for Oort bodies
to pass in-between.



QUOTE (Explorer1 @ Dec 22 2017, 11:07 PM) *
I suppose the best way to get answers is find one and see how it's moving.
But what would it take for Oort cloud objects to be observed in situ, and have their orbit calculated?
snip
...or would some sort of dedicated mission need to go out into the darkness and survey them (if they exist, of course)?


Well, something that gets out to 2k-5k AU within a reasonable fraction of a human lifetime would be a good start

For comparison, Voyager 1 should get to the area where the Oort cloud is supposed to be in perhaps 250 years, and through it in another 25,000 years or so.

So, first problem is that any probe using current technology would need a large and heavy nuclear power source, a cutting edge drive, (e.g. TAU project)
and that only gets you half-way or one-fifth of the way to the edge.
And you have "budget issues" planning a mission that may tke a quarter of a millenium to get to the edge of the Oort cloud...
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