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Interstellar Interloper, Coming in from the great beyond
Phil Stooke
post Oct 31 2017, 07:25 PM
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I'm afraid that's not quite how it works. To get from Mercury to the Moon you are increasing heliocentric velocity. To get from the Moon to Mercury you are reducing it. No net gain from bouncing back and forth.

More seriously, you need big objects to make this work best. Massive objects. So you might attempt a close solar flyby (probably allowed only by a Jupiter flyby to begin with... note that we are reducing heliocentric velocity here). The Sun gives a big boost (especially if you can add a rocket burn at closest approach), targeted to give a second Jupiter flyby. That is designed to give another boost - it will need to be low above the cloudtops, but Juno shows us we can do it). Then it's off out of the solar system.

I'm not sure that this would really work especially well given the need for thermal shielding mass at the Sun. Maybe lightsails would give better final velocity.

Phil


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fredk
post Oct 31 2017, 07:45 PM
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The sun also doesn't help. The slingshot mechanism relies on the three-body scenario: slinging around one object (eg Jupiter) in orbit around another (eg the sun). When you approach the sun all you'll do is continue outwards in the same orbit you were on moving inwards (apart from any slowing due to friction). That's a two-body problem and the only solutions are the conic sections.
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HSchirmer
post Oct 31 2017, 08:26 PM
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QUOTE (fredk @ Oct 31 2017, 08:45 PM) *
The sun also doesn't help. The slingshot mechanism relies on the three-body scenario: slinging around one object (eg Jupiter) in orbit around another (eg the sun). When you approach the sun all you'll do is continue outwards in the same orbit you were on moving inwards (apart from any slowing due to friction). That's a two-body problem and the only solutions are the conic sections.


Hmm, guess I have my frames of reference crossed up.

Thinking of Cassini, Venus-Venus-Earth-Jupiter gravity assist, curious that you can get a gravity assist from a planet that has less heliocentric speeed.
Guess I'm missing something.

I had though that, (way oversimplifying) with an idealized "hairpin orbit" you get a sligshot boost of roughly 2x the approach velocity between the object and the planet.
That way, you "shoot an arrow in the air" and grab "2U" each time you hairpin around the moon or mercury.
Guess that doesn't quite work.

So, if we want "ludicrous speed" to intercept something, we have to fall back to Pascahal-B, where US cold war nuclear testes (may have) launched a manhole at 6x escape velocity...
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fredk
post Oct 31 2017, 09:58 PM
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For the sun, the outgoing speed will clearly be the same as the ingoing, in the frame of the sun (ie, for the frame in which U = 0 in your sketch). But the point is that that's the relevant frame, since it is the speed relative to the sun that we're talking about maximizing.

The sun won't help, but I don't know how large a boost you might practically achieve given multiple passes of planets such as Jupiter etc...
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Paolo
post Nov 1 2017, 06:49 AM
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QUOTE (HSchirmer @ Oct 31 2017, 08:08 PM) *
I'm curious whether the outbound path will take it anywhere near VGR1, VGR2 or New Horizons...


2017 U1 was coming from the general direction of the solar apex and V1, V2 and NH (and Pioneer 11) are all going in the general direction of the apex, so they must have come close (like tens of AU close) in the inbound leg
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Paolo
post Nov 1 2017, 07:05 AM
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and a second arXiv paper:
Kinematics of the Interstellar Vagabond A/2017 U1

QUOTE
The initial Galactic velocity vector for the recently discovered hyperbolic asteroid A/2017 U1 is calculated for before its encounter with our solar system. When the velocity is compared to the local stars, A/2017 U1 can be easily ruled out as co-moving with any of the dozen nearest systems, i.e. it does not appear to be associated with any local exo-Oort clouds (most notably that of the Alpha Centauri triple system). The object's velocity is within 5 km/s of the mean Galactic velocity of the stars in the solar neighborhood (<25 pc), so its velocity would appear to be typical for that of a body whose velocity was drawn from the Galactic velocity distribution of the local stars. These calculations strengthen the interpretation that A/2017 U1 has a distant extrasolar origin, but not among the very nearest stars.

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Gerald
post Nov 1 2017, 08:02 PM
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QUOTE (JRehling @ Oct 31 2017, 08:25 PM) *
... Perhaps lithobraking solves the hard problem of the delta-v. Have a small payload encased in concentric shells of crushable shield to spread out the impact delta-v over a longer time. The failed Deep Space 2 probes to Mars would be the closest analogue – 2.4 kg per probe, intended to survive impact. There, though, the delta-v was much less, there was some aerobraking, and – again – they failed despite that. But if you had many hundreds of kg of shield devoted to permitting the survival of a ~3 kg probe, maybe this gets easier.

Overcoming the speed of sound within the solid material of the shield would certainly be a challenge. Longitudinal forces in solids usually are transmitted with about the speed of sound of the solid medium. So, the mechanism would need something faster, like electromagnetic fields.
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nprev
post Nov 1 2017, 11:39 PM
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Going back to the object itself, interesting that it seems to be fairly red. First thing that brings to mind is thiolins, so maybe this thing used to be an Oort Cloud denizen somewhere else. A thick layer of that stuff may have been enough to prevent significant outgassing as well (plus it's definitely not spending much time in the inner Solar System to get really warmed up).

Hopefully additional spectra will reveal some absorption details, maybe even some delayed outgassing if the body has significant thermal inertia.


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Gladstoner
post Nov 2 2017, 12:20 AM
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QUOTE (nprev @ Nov 1 2017, 05:39 PM) *
Going back to the object itself, interesting that it seems to be fairly red. First thing that brings to mind is thiolins, so maybe this thing used to be an Oort Cloud denizen somewhere else. A thick layer of that stuff may have been enough to prevent significant outgassing as well (plus it's definitely not spending much time in the inner Solar System to get really warmed up).

Hopefully additional spectra will reveal some absorption details, maybe even some delayed outgassing if the body has significant thermal inertia.


Would tholins indicate an icy body, or could they also form on carbon-rich material (C-type asteroids)?
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Gerald
post Nov 2 2017, 02:10 AM
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You'll probably need an environment, where light and mostly reduced, hydrogen-rich molecules like CH4 and NH3 don't escape easily, such that they can react under energetic radiation to more complex compounds like the broad class of tholins. Within a low-gravity environment, this will probably require low temperatures.
Carbon taken allone would result in black, soot-like compounds.
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Gladstoner
post Nov 2 2017, 04:12 AM
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Are there asteroids in the inner solar system with a similar reddish color?

It's just odd how a tiny 500-foot object with a KBO-like color comes in from the deep, passes 0.25 AU from the sun, and remains intact with no detectable outgassing. After many years of following comets, my money would have been on a break-up of the object just before perihelion.
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Gerald
post Nov 2 2017, 10:37 AM
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A coating of a few microns of a pigment like tholins should provide the visible color of the object. The interior may well be rocky. The color of tholins will also win over the colors of translucent minerals like silicates in a mineral-tholin mix.
A considerable portion of the material of a stellar system is probably ejected during the first few millions of years after its Jeans collapse,
A rocky body of the inner stellar system may travel through outer parts or interstellar dust clouds, and collect a coating of tholins.
Tholins should survive for some time in an inner stellar system, before they will be degraded by stellar wind or micrometeorites.
That's just one out-of-the-hip scenario. There are certainly many more of varying likelihood.

Amalthea seems to be dark red. It's not in the inner solar system. But it's an example of a small red body with the potential to be ejected by close encounters or collisions. The composition of the interior of a small body depends mostly on its origin. An outer layer may have accreted later, in a new environment.
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JRehling
post Nov 2 2017, 03:18 PM
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A/2017 U1 had no observed coma, so it had little in the way of volatiles, at least near the surface. It could have had volatiles below a darker "crust" that didn't break through, but a dark object passing 0.2 AU from the Sun should have heated up quite a bit. It might be a chip off some exosystem's version of Mars. The surface color might be a thin coating or go all the way down. It's certainly spent a lot of time in interstellar space, which may have caused slow but thorough transformations that nobody's yet imagined.

I'd guess that in future decades we'll be able to spot much smaller versions of this thing which visit the solar system with considerable frequency and we'll start to explore them. There's a lot of untapped potential for huge light-bucket telescopes to monitor the skies and then things that are dimmer and dimmer will be detected earlier. If the size distribution of interstellar objects is like that of the asteroids in our solar system, then at some size threshold, there seemingly must be interstellar interlopers every year. But the process of interstellar travel might introduce some unimagined selection effects. For one, reaching the escape velocity of a star may be extremely violent.
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fredk
post Nov 2 2017, 04:48 PM
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Makes me wonder about capture: some extremely small percentage of these interstellar interlopers will become bound to the sun through an inverse slingshot encounter with Jupiter or other planets. So there must be some now orbiting the sun (or planets). Of course identifying them long after they've left their initial hyperbolic orbit would be insanely hard.
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Gladstoner
post Nov 2 2017, 05:04 PM
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It has been hypothesized that comet 96P/Machholz could be a captured extrasolar object based on its unusual composition and, to a lesser degree, its unusual orbit. However, there are possible solar-system-origin scenarios that could explain these anomalies.

Even a slight possibility that this comet could be interstellar should bring this comet to the top of the list of potential exploration targets.
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