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A curious coincidence in the comet paper was the factor of 10^-6 which also shows up in Anderson's flyby paper where they suggested an empirical formula that fitted some of the earlier flyby results but did not work for some latter flybys.

2. Interesting TED Talk about Oumuamua.

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Looking at some numbers

Wondering if there is some connection between the A_1 of the comet paper's fitted free parameter of 5.01X10^(-6) and Anderson's K of 3.099X10^(-6) which is derived from a formula giving a dimensionless value:
K=2 w_earth X R_earth/c where w is the angular rotational velocity in rads/sec and R is Radius of earth and c is the speed of light.

It is assumed that radius is measured from the core to the surface of earth and not to the top of the atmosphere.

A caveat is that we have no accurate similar measure of the comet prior to solar closest approach. It seems probable that it had at least one prior hyperbolic encounter in its home stellar system in order to escape to visit our system. Unknown if it had other encounters before arriving in our system. So it is unknown if it had some anomaly before arriving here. A measurement of anomaly prior to solar encounter would allow a better measurement of gain or loss of anomaly due to post solar encounter.

So consider the extreme case where no anomaly was present before solar encounter and solve for the effective solar radius.

The Sun rotates about 27 times slower than the Earth. A_1/K_earth=1.617 which suggests that the effective radius for the sun is around 278,400 kilometers which is about .4 of the total solar radius within the radiative zone. This is where the density is a bit above the density of water but less than gold. ( https://solarscience.msfc.nasa.gov/interior.shtml )

The authors suggestion that there might be material coming off the comet that is below current detection levels to account for the apparent non-gravitational acceleration might require IN SITU observations to either confirm or rule out their hypothesis. Considering the possible shape of the comet being up to 10:1 allows the suggestion that it might be a surface shard from a larger high rotation body that got too close to the Roche limit of a larger body getting a large gravity assist. A surface shard would not have suffered subduction which could leave microfossils, if present, still intact. A sample return mission would give an idea of what constitutes the visitor from another system. If there is no evidence to support comet like activity that could be seen upon approach, then a lab can test for a possible violation of UFF by comparing the fall of an earth rock to a sample from the anomalous comet.

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A new paper today casting doubt on outgassing for the non-gravitational force. Spin evolution that would accompany it appears to be too small to be plausible:

SPIN EVOLUTION AND COMETARY INTERPRETATION OF THE INTERSTELLAR MINOR OBJECT 1I/2017 ’OUMUAMUA

https://arxiv.org/pdf/1809.06389.pdf

They do not forward an alternative interpretation to explain the force.

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Interstellar object ‘Oumuamua is an asteroid, not a comet argues astrophysicist
https://physicsworld.com/a/interstel...strophysicist/

This is about the above arxiv paper on spin evolution.

The mystery deepens a bit when applying the Anderson formula to Oumuamua. The value between the ingoing and outgoing can only produce values between the extremes of 2 to -2. The angle was estimated to be around 66 degrees. So if the Earth flyby model is applied, which is similar to a one time change near perigee, like an impulse, the amount of Delta v anomaly accrued would only be around 10% of the total anomaly observed. The authors of the comet like paper also noted that a one time impulse was improbable when compared to either r^(-2) or r^(-1) force gleaned from observations made over different times. Solar pressure seems unlikely unless the comet was hollow.

Oumuamua may be a more pure and less complicated form of the flyby anomaly. The sun is the only relevant body and the comet is clearly hyperbolic during the entire event. This may suggest that there is another parameter with regard to Earth flyby anomalies that has not been well considered if hyperbolic speed is required: sphere of influence. None of the Earth flybys increased the overall speed to beyond 42km/sec@earth needed to attain solar escape speed. There are three relevant bodies: Earth, the Sun and the Moon. The anomaly at earth would be truncated once the Sun's sphere of influence begins to dominate because gravity assist was insufficient to bring the craft to solar hyperbolic speed. Perhaps by taking this parameter into consideration might account for some prediction failures of the more favored models.

It has been noted that the Earth flyby anomalies appear to be not conservative. The Oumuamua anomaly which appears to reveal either a r^(-2) or r^(-1) force might allow a restoration of conservation. This would be similar to gravity assist where a spacecraft gains or loses speed while the planets loses or gains an imperceptible mount of speed. The example that comes to mind is in Mahdavi's A520 paper where there was noted a darkened core of dark matter and a dimmer core of dark matter in galaxy cluster 5 that seemed to balance each other. While all galaxy clusters were ram stripped, the other galaxy dark cores appeared not to be dimmed. Mainly cluster 5 was at a different angle from the others. Total dark matter effect seems to be conserved while there appears a darkening/dimming but conserving effect happened between 5 and 3 (the dark core of A520). Within the dark matter particle paradigm,where DM is independent of baryons, there has been considered self interacting dark matter and within this there is a recent resonant self interacting DM that helps the core/cusp and too-big-to-fail problems. These, however, are large scale only attempts that would not work at the level of a comet and star. The reverse is not ruled out. The flyby anomalies could occur at the large scale as well as small scale.

There has been a try at a dark matter interpretation of the flyby anomaly that centered on an earthbound two fluid dark matter interacting with a craft without regard to a hyperbolic requirement but orbiting spacecraft did not see the anomaly and small anomalies seen in the more eccentric orbits seem to cancel out.

There have been modified classical inertia theories but they have not always been successful.

A dark matter intensity conservation interpretation would require that dark matter inertia would be inherent in baryons but in addition to its classical inertia.

In Oumuamua's case, the angle allowed an interaction where the comet's dark matter was dimmed while the Sun's dark matter became imperceptibly darker. This would allow the comet to escape faster than it should as observed. Rather than invoking an additional r^(-2) force, part of the dark inertia has been altered. The possibility of either darkening or dimming can explain the independence from the classical gravity assist.

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So it is more like a pancake,, flat and not spindle shaped?

7. Originally Posted by publiusr
So it is more like a pancake,, flat and not spindle shaped?
I didn't notice anyone saying that. Can you point out where you think that was implied?

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The possibility of a more pancake was brought up in the above link to the TED talk. Before posting the link to the spin evolution paper, it was important to read it in light of pancake shape. They gave 4 independent reasons for their conclusions and only 1 was with regard to shape.

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It might be a long shot considering dispersion also due to solar pressure and Yarkovsky effect, but comet paper's authors stated it might be below detection levels to see a tail. A possible search out of the ecliptic might be done by the Next Generation VLA looking for the faint trailL
Science with an ngVLA: High-resolution imaging of comets
https://arxiv.org/abs/1810.07867

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Project Lyra looks at the challenges of a mission to Oumuamua:
Project Lyra: Sending a Spacecraft to 1I/'Oumuamua (former A/2017 U1), the Interstellar Asteroid
https://arxiv.org/abs/1711.03155
Trying to bring back a sample would be especially challenging. After landing and getting a sample having used fuel to arrive, slow down, and land, you are going out of the solar system at 26km/sec. Even with initial fuel to start back, we might need one or two more craft to relay the sample to perigee near Earth.

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The solar pressure hypothesis for a non-gravitational force is reviewed. The possibility was mentioned in the earlier comet paper. Featured is the thickness of between 0.3-0.9 mm There is a discussion about whether rotation or tidal forces would disallow the thin shell. Tensile strength is discussed. Solar pressure would slightly slow the craft prior to closest approach. A constraint not considered is the possible effect of heat upon the tensile strength. Coming in at near 0.25 AU inside the orbit of Mercury might soften, melt, or vaporize some compounds. Could there be an oven effect if the surface is closed? Would there be some modulation in the spin evident in the observations during the closer points to the sun if the object melted and deformed the shape before freezing out at further distances?

COULD SOLAR RADIATION PRESSURE EXPLAIN ’OUMUAMUA’S PECULIAR ACCELERATION?

https://arxiv.org/abs/1810.11490

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Originally Posted by borman
The solar pressure hypothesis for a non-gravitational force is reviewed. The possibility was mentioned in the earlier comet paper. Featured is the thickness of between 0.3-0.9 mm There is a discussion about whether rotation or tidal forces would disallow the thin shell. Tensile strength is discussed. Solar pressure would slightly slow the craft prior to closest approach. A constraint not considered is the possible effect of heat upon the tensile strength. Coming in at near 0.25 AU inside the orbit of Mercury might soften, melt, or vaporize some compounds. Could there be an oven effect if the surface is closed? Would there be some modulation in the spin evident in the observations during the closer points to the sun if the object melted and deformed the shape before freezing out at further distances?

COULD SOLAR RADIATION PRESSURE EXPLAIN ’OUMUAMUA’S PECULIAR ACCELERATION?

https://arxiv.org/abs/1810.11490
If it is a discarded or escaped piece of space engineering it would be designed to withstand these things.

Also it has been calculated that such objects would survive many kpc of journey distance through the galaxy.

Is it likely that an object this size with a thickness of a fraction of a millimetre could be of natural origin ?

Here we have our first sight of ET space junk ?
Last edited by kzb; 2018-Oct-31 at 12:19 PM.

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An extraterrestrial artifact? It's not out of the question. If a comet shed enough material to affect its motion through space, to the extent observed in Oumuamua, its rate of rotation should apparently have been visibly changed. This object was closely watched, but the rotation rate was reportedly unchanged.

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Originally Posted by Ross 54
An extraterrestrial artifact? It's not out of the question. If a comet shed enough material to affect its motion through space, to the extent observed in Oumuamua, its rate of rotation should apparently have been visibly changed. This object was closely watched, but the rotation rate was reportedly unchanged.
Apparently, no outgassing was detected. Given that, we only have radiation pressure to change its course.

For radiation pressure to have achieved the magnitude of deviation observed, the object can only be a fraction of a millimetre thick.

It's reportedly several hundred metres in other dimensions. You have to ask what natural explanation could there be for such a large thin structure. Also it would require a certain degree of physical strength to survive tidal forces, which again points to non-natural origin.

Of course the fraction of millimetre thickness only comes from the course change, and assuming it all comes from radiation pressure, but perhaps that is all mistaken.

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Astronomers from Harvard University have suggested it might a UFO.

https://edition.cnn.com/2018/11/06/h...ntl/index.html

A mysterious cigar-shaped object spotted tumbling through our solar system last year may have been an alien spacecraft sent to investigate Earth, astronomers from Harvard University have suggested.

The object, nicknamed 'Oumuamua, meaning "a messenger that reaches out from the distant past" in Hawaiian, was first discovered in October 2017 by the Pan-STARRS 1 telescope in Hawaii.

Since its discovery, scientists have been at odds to explain its unusual features and precise origins, with researchers first calling it a comet and then an asteroid, before finally deeming it the first of its kind: a new class of "interstellar objects."

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Originally Posted by selvaarchi
Astronomers from Harvard University have suggested it might a UFO.

https://edition.cnn.com/2018/11/06/h...ntl/index.html
If it was a laser sail or similar, surely it would be going a lot faster?

At the speed it was going, the journey time from even one of the closer stars would be millennia.

Personally I think it is more likely space junk. Some kind of stellar shield off a space colony. Apparently such things could survive intact in interstellar space for many millions of years.

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Originally Posted by kzb
If it was a laser sail or similar, surely it would be going a lot faster?
Not necessarily. Remember that Robert Forward's sail was staged:
https://space.stackexchange.com/ques...n-a-solar-sail

A thought experiment:

If a breakthough starshot mini-probe were fired backwards from 'Oumuamua --towards Earth, say--what would be visible is the parent object accelerating, while the mini probe might not be visible. Or if a thin tether were backspun to release a microprobe so it wouldn't fly out of the solar system.

This would all result in an acceleration of 'Oumuamua, which seems the case.

Straining at gnats here, mind you. Very big IFS

Mini-probe has laser signal sent to sail--that bounces back to homeworld.

Cue optical SETI?
Last edited by publiusr; 2018-Nov-09 at 06:40 PM.

https://phys.org/news/2018-11-nasa-i...uamua.html#jCp

NASA Learns More About Interstellar Visitor 'Oumuamua
November 14, 2018 by Calla Cofield, NASA

The fact that 'Oumuamua was too faint for Spitzer to detect sets a limit on the object's total surface area. However, since the non-detection can't be used to infer shape, the size limits are presented as what 'Oumuamua's diameter would be if it were spherical. Using three separate models that make slightly different assumptions about the object's composition, Spitzer's non-detection limited 'Oumuamua's "spherical diameter" to 1,440 feet (440 meters), 460 feet (140 meters) or perhaps as little as 320 feet (100 meters). The wide range of results stems from the assumptions about 'Oumuamua's composition, which influences how visible (or faint) it would appear to Spitzer were it a particular size.

article: http://iopscience.iop.org/article/10...81/aae88f/meta

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While the published version can be found in Astronomical Journal,156, 261 (2018), a preprint became available in the arXiv:

Spitzer Observations of Interstellar Object 1I/`Oumuamua

https://arxiv.org/abs/1811.08072

It places constraints on the size, shape, and albedo. It also constrains what may have been emitted to produce the mystery of the non-gravitational force implied by the trajectory. Not seeing it in infrared seems to imply that it is cold and so reflects light rather than absorb it suggesting an icy exterior. While dust, CO and CO2 are not seen. H2O is not ruled out. Long term exposure like space weathering tends to make water ice amorphous while fresh ice tends to be more crystalline. Coming into the solar system from the long trip from interstellar space, its ices would rate to be amorphous. So crystal ice would have to be made in the flyby of the sun. If the signal of the ices can be sorted out with regard to Oumuamua it may give an opportunity to solving a piece of the puzzle.

20. Originally Posted by kzb
Of course the fraction of millimetre thickness only comes from the course change, and assuming it all comes from radiation pressure, but perhaps that is all mistaken.
It's interesting that there is a natural explanation, but I haven't heard it mentioned really, namely that this is another example of the flyby or Pioneer anomaly.

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Originally Posted by Jens
It's interesting that there is a natural explanation, but I haven't heard it mentioned really, namely that this is another example of the flyby or Pioneer anomaly.
That's a good point Jens. It will be if it is actually a similar phenomenon to the Pioneer anomaly anyhow.

I do not know enough about it to say if they are similar or different. Can anyone more knowledgeable comment on this?

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The flyby anomaly can add on the order of 10 millimeters per second to the velocity of a very low mass object. If I'm not mistaken, that's very inadequate in explaining the fact that Oumuamua was found 100,000 kilometers from where it was expected to be.

The Pioneer anomaly is based on heat from the power source inside inside a spacecraft. Radiating selectively in one direction, it provides a very slight push. This applies to artificial, not natural objects.

23. Spitzer speaks on the topic. of NOT detecting Oumuamua, which had a lot of consequences. It's smaller than once thought.

Spitzer Observations of Interstellar Object 1I/`Oumuamua

DE Trilling, et al. (Submitted on 20 Nov 2018)

1I/`Oumuamua is the first confirmed interstellar body in our Solar System. Here we report on observations of `Oumuamua made with the Spitzer Space Telescope on 2017 November 21--22 (UT). We integrated for 30.2~hours at 4.5 micron (IRAC channel 2). We did not detect the object and place an upper limit on the flux of 0.3 uJy (3sigma). This implies an effective spherical diameter less than [98, 140, 440] meters and albedo greater than [0.2, 0.1, 0.01] under the assumption of low, middle, or high thermal beaming parameter eta, respectively. With an aspect ratio for `Oumuamua of 6:1, these results correspond to dimensions of [240:40, 341:57, 1080:180] meters, respectively. We place upper limits on the amount of dust, CO, and CO2 coming from this object that are lower than previous results; we are unable to constrain the production of other gas species. Both our size and outgassing limits are important because `Oumuamua's trajectory shows non-gravitational accelerations that are sensitive to size and mass and presumably caused by gas emission. We suggest that `Oumuamua may have experienced low-level post-perihelion volatile emission that produced a fresh, bright, icy mantle. This model is consistent with the expected eta value and implied high albedo value for this solution, but, given our strict limits on CO and CO2, requires another gas species --- probably H2O --- to explain the observed non-gravitational acceleration. Our results extend the mystery of `Oumuamua's origin and evolution.

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A summary listing leading questions and references that are linked for quick access:

https://arxiv.org/abs/1811.08832

A question with the shiny artificial thin shell hypothesis centers on the apparent observation of tumbling: How is spin evolution evaded if there is already tumbling in the solar pressure approach? How is momentum change different for solar pressure from momentum change due to out gassing?

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New research has indicated, there are potentially 4 interstellar asteroids orbiting the sun and possibly hundreds more.

https://www.popularmechanics.com/spa...-solar-system/

The new study, published on the ArXiv and awaiting peer review, claims the discovery of an additional four interstellar asteroids, with the possibility of discovering hundreds more in the future.

First, the astronomers wanted to determine what an interstellar asteroid in our solar system would even look like. They developed a simulation of what would happen if a rogue asteroid happened to drift too close to our solar system. They found that the Sun and Jupiter have a complex interaction that tends to trap interstellar asteroids in the solar system. The natural next step was to try and find some of those asteroids in real life.

To discover these cosmic interlopers, the Harvard researchers turned to a database of asteroids, comets, and other small solar system objects maintained by the International Astronomical Union. They combed that database for any objects with similar orbits as the ones produced by their simulation. With that filter, they successfully found four possible candidates.

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What it would take to reach the object
https://www.nextbigfuture.com/2018/1...a-clipper.html

The SLS launched Interstellar Explorer would have a speed of 63 km/sec, well above 'Oumuamua's at 26.33 km/s
https://studylib.net/doc/18163873/sls-mission-booklet (page 25)

27. How the interstellar visitor got its weird cigar shape.

https://arxiv.org/abs/1812.11334

Dust bombardment can explain the extremely elongated shape of 1I/'Oumuamua and the lack of interstellar objects

Dmitrii E. Vavilov, Yurii D. Medvedev (Submitted on 29 Dec 2018)

Asteroid 1I/'Oumuamua is the first observed interstellar object. Its light-curve amplitude indicates that the object is highly elongated with an axis ratio of at least 5:1. The absence of such elongated asteroids in the Solar system, as well as the apparent lack of observed interstellar objects, are intriguing problems. Here we show that 'Oumuamua may have originated as a slightly-elongated asteroid about 500×300 meters in size. Surface erosion, caused by interstellar dust bombardment, subsequently increased the axis ratio. Simply traveling through the interstellar medium for 0.03 to 2 Gyrs would have sufficed to give 1I its present shape. Passing through a 10 pc dust cloud with a grain density of 10 −23 g/cm 3 at 50 km/s would have had a similar effect on 'Oumuamua's form. Smaller objects of around 100 meters in diameter can travel the Galactic disk for merely 30 Myrs before they are disrupted. This could explain the small number of interstellar objects observed to date.

28. Deeper look into interstellar objects flying through the solar system.

https://arxiv.org/abs/1901.00508

Turning up the heat on `Oumuamua

John C. Forbes, Abraham Loeb (Submitted on 2 Jan 2019)

We explore what may be learned by close encounters between extrasolar minor bodies like `Oumuamua and the Sun. These encounters may yield strong constraints on the bulk composition and possible origin of `Oumuamua-like objects. We find that such objects collide with the Sun once every 30 years, while about 2 pass within the orbit of Mercury each year. We identify preferred orientations for the orbits of extrasolar objects and point out known Solar System bodies with these orientations. We conclude using a simple Bayesian analysis that about one of these objects is extrasolar in origin, even if we cannot tell which.

29. Paper suggesting that Oumuamua was ejected from a star cluster not far (in galactic terms) from us.

https://arxiv.org/abs/1901.02465

The fate of planetesimal discs in young open clusters: implications for 1I/'Oumuamua, the Kuiper belt, the Oort cloud and more

Thomas Oliver Hands, Walter Dehnen, Amery Gration, Joachim Stadel, Ben Moore (Submitted on 8 Jan 2019)

We perform N -body simulations of the early phases of open cluster evolution including a large population of planetesimals, initially arranged in Kuiper-belt like discs around each star. We evolve the whole system under the stellar gravity, i.e. treating planetesimals as test particles, and consider two types of initial cluster models, similar to IC348 and the Hyades, respectively. In both cases, planetesimals can be dynamically excited, transferred between stars or liberated to become free-floating (such as A/2017 U1 or 'Oumuamua) during the early cluster evolution. We find that planetesimals captured from another star are not necessarily dynamically distinct from those native to a star. After an encounter both native and captured planetesimals can exhibit aligned periastrons, qualitatively similar to that seen in the Solar system and commonly thought to be the signature of Planet 9. We discuss the implications of our results for both our Solar system and exoplanetary systems.

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