1. More guesswork on what it was.

https://arxiv.org/abs/1902.04100

Could 1I/'Oumuamua be an icy fractal aggregate ejected from a protoplanetary disk? A fluffy radiation-pressure-driven scenario

Amaya Moro-Martín (Submitted on 11 Feb 2019)

1I/'Oumuamua was the first interstellar interloper that has been detected and it showed a non-gravitational acceleration (Micheli et al. 2018) that cannot be accounted for by outgassing, given the strict upper limits to outgassing resulting from Spitzer observations, unless the relative abundances of the common volatiles are very different from those in comets (Trilling et al. 2018). As an alternative, Bialy & Loeb (2018) have suggested that its peculiar acceleration is due to radiation pressure, requiring a planar sheet geometry of an unknown natural or artificially origin. Here we assess whether the internal structure of 1I/'Oumuamua, rather than its geometry, could support a radiation-pressure driven scenario. We adopt a mass fractal structure and find that the type of aggregate that could yield the required area-to-mass ratio would have to be extraordinarily porous with a density ∼ 10 −5 g cm −3 . Such porous aggregates can naturally arise from the collisional grow of icy dust particles beyond the snowline of a protoplanetary disk and here we propose that 1I/'Oumuamua might belong to this population. This is a hypothesis worth investigating because, if this were the case, 1I/'Oumuamua could have truly opened a new observation window to study the building blocks of planets around other stars and this could set unprecedented constraints on planet formation models.

2. Why Oumuamua "changed course" and accelerated.

https://arxiv.org/abs/1903.04723

On the Anomalous Acceleration of 1I/2017 U1 Oumuamua

Darryl Seligman, Gregory Laughlin, Konstantin Batygin

(Submitted on 12 Mar 2019)

We show that the P ∼ 8h photometric period and the astrometrically measured Ang∼2.5×10−4cms−2non-gravitational acceleration (at r∼1.4AU) of the interstellar object 1I/2017 (Oumuamua) can be explained by a nozzle-like venting of volatiles whose activity migrated to track the sub-solar location on the object's surface.

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I wonder that such a porous, delicate object, as described in the Moro-Martin paper could survive in the highly elongated form inferred for it. It was wrenched from frigid outer zone of its original star system, yet it was not smashed into a compact shape?
It eventually passed near our Sun. Yet its icy filigree did not perceptibly melt, nor collapse inward on itself?

The exclusively sub-solar venting of a cometary body, described in the Seligman, Laughlin, Batygin paper does not reproduce the distinct light curve observed in Oumuamua. This light curve is highly characteristic of a fully rotating, highly elongated object.

4. Two new papers on Oumuamua, regarding its survival near the Sun and how it got its shape.

https://arxiv.org/abs/1903.06300

1I/Oumuamua and the Problem of Survival of Oort Cloud Comets Near the Sun

Zdenek Sekanina (Submitted on 15 Mar 2019)

A 2000-2017 set of long-period comets with high-quality orbits of perihelion distance <1 AU is used to show that the objects that perish shortly before perihelion are nearly exclusively the Oort Cloud comets, especially those with perihelia within 0.6 AU of the Sun, intrinsically fainter, and dust poor. Their propensity for disintegration is much higher than predicted by Bortle's perihelion survival rule, prompting the author to propose a new synoptic index to be tested in future prognostication efforts. By their susceptibility to demise near the Sun, the nuclei of Oort Cloud comets differ dramatically from the nuclei of other long-period comets that almost always survive. In this scenario, Oumuamua -- discovered after perihelion -- is in all probability a major piece of debris of an interstellar comet that was bound to perish near perihelion if it was similar to, though much fainter than, the known Oort Cloud comets. The nondetection of Oumuamua by the Spitzer Space Telescope is compatible with optical data for pancake shape, but not for cigar shape, with the maximum dimension not exceeding 160 m (at an 0.1 albedo). Although the solar radiation pressure induced nongravitational acceleration requires very high porosity, Oumuamua's estimated mass is orders of magnitude greater than for a cloud of unbound submicron-sized dust grains of equal cross section. The acceleration could have displaced Oumuamua by 250,000 km in 50 days, scattering other potential debris over a large volume of space.

===

https://arxiv.org/abs/1903.06373

Collisional Elongation: Possible Origin of Extremely Elongated Shape of 1I/Oumuamua

Keisuke Sugiura, Hiroshi Kobayashi, Shu-ichiro Inutsuka (Submitted on 15 Mar 2019)

Light curve observations of a recently discovered interstellar object 1I/Oumuamua suggest that this object has an extremely elongated shape with the axis ratio 0.3 or smaller. Planetesimal collisions can produce irregular shapes including elongated shapes. In this paper, we suggest that the extremely elongated shape of 1I/Oumuamua may be the result of such an impact. To find detailed impact conditions to form the extremely elongated objects, we conduct numerical simulations of planetesimal collisions using Smoothed Particle Hydrodynamics method for elastic dynamics with self-gravity and interparticle friction. Impacts into strengthless target planetesimals with radius 50 m are conducted with various ratios of impactor mass to target mass q, friction angles phi_d, impact velocities v_imp, and impact angles theta_imp. We find that impacts with q \geq 0.5, phi_d \geq 40 degrees, v_imp \leq 40 degrees, and theta_imp \leq 30 degrees produce remnants with the ratio of intermediate to major axis length less than 0.3. This impact condition suggests that the parent protoplanetary disk in the planetesimal collision stage was weakly turbulent (alpha < 10^{-4} for the inner disk) and composed of planetesimals smaller than ~ 7 km to ensure small impact velocity.

5. "Doubt" on the nature of 'Oumuamua. The argument continues.

https://arxiv.org/abs/1904.02218

Evidence against non-gravitational acceleration of 1I/2017 U1 Oumuamua

J. I. Katz (Submitted on 29 Mar 2019)

Micheli, et al. (2018) reported that a seven-parameter fit to the orbit of 1I/2017 U1 Oumuamua indicated a non-gravitational acceleration in the anti-Solar direction, and attributed it to recoil from comet-like outgassing. The implied gas to dust ratio is at least 100 times greater than that of known Solar System comets. The reported collapse of the scatter of nearly contemporaneous coordinate residuals upon inclusion of the non-gravitational term in the orbital fits is difficult to understand. There are grounds for skepticism.

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The "doubt" is not so much about whether or not there is a source for non-gravitational acceleration, but whether it is caused by out gassing. They point to the ratio of gas to fine dust being off by a few orders of magnitude compared to our solar system comets. They have not empirically ruled out the gas theory because of the possibility of larger than fine dust aggregates falling off but the situation appears more stringently constrained,

7. Was 'Oumuamua a lightweight interstellar comet after all?

https://arxiv.org/abs/1905.00935

Outgassing As Trigger of 1I/Oumuamua's Nongravitational Acceleration: Could This Hypothesis Work at All?

Zdenek Sekanina (Submitted on 2 May 2019)

The question of what triggered the nongravitational acceleration of 1I/Oumuamua continues to attract researchers' attention. The absence of any signs of activity notwithstanding, the prevailing notion is that the acceleration of the stellar, cigar-like object was prompted by outgassing. However, the Spitzer Space Telescope's failure to detect Oumuamua not only ruled out the CO_2 and/or CO driven activity (Trilling et al. 2018), but made the cigar shape incompatible with the optical observations. Choice of water ice as the source of outgassing is shown to be flawed as well: (i) the water sublimation law is demonstrably inconsistent with the observed variations in the nongravitational acceleration derived by Micheli et al. (2018), the point that should have been assertively highlighted; and (ii) an upper limit of the production rate of water is estimated at as low as 4 x 10^(23) molecules s^(-1), requiring that, at most, only a small area of the surface be active, in which case the conservation of momentum law is satisfied only when Oumuamua's bulk density is extremely low, <0.001 g cm^(-3), reminiscent of the formerly proposed scenario with Oumuamua as a fragment of a dwarf interstellar comet disintegrating near perihelion, with the acceleration driven by solar radiation pressure (Sekanina 2019a) and no need for activity at all. This conclusion is possible thanks to the high quality of astrometry and Micheli et al.'s orbital analysis, whose results were confirmed by the computations of other authors.

8. Analysis of the most likely shapes of our recent interstellar visitor, 'Oumuamua.

https://arxiv.org/abs/1906.03696

Modeling the light curve of Oumuamua: evidence for torque and disc-like shape

Sergey Mashchenko (Submitted on 9 Jun 2019)

We present the first attempt to fit the light curve of the interstellar visitor Oumuamua using a physical model which includes optional torque. We consider both conventional (Lommel-Seeliger triaxial ellipsoid) and alternative ("black-and-white ball", "solar sail") brightness models. With all the brightness models, some torque is required to explain the timings of the most conspicuous features -- deep minima -- of the asteroid's light curve. Our best-fitting models are a thin disc (aspect ratio 1:6) and a thin cigar (aspect ratio 1:8) which are very close to being axially symmetric. Both models are tumbling and require some torque which has the same amplitude in relation to Oumuamua's linear non-gravitational acceleration as in Solar System comets which dynamics is affected by outgassing. Assuming random orientation of the angular momentum vector, we compute probabilities for our best-fitting models. We show that cigar-shaped models suffer from a fine-tuning problem and have only 16 per cent probability to produce light curve minima as deep as the ones present in Oumuamua's light curve. Disc-shaped models, on the other hand, are very likely (at 91 per cent) to produce minima of the required depth. From our analysis, the most likely model for Oumuamua is a thin disc (slab) experiencing moderate torque from outgassing.

9. Using future 'Oumuamua's to discover what exoplanetary systems are made of.

https://arxiv.org/abs/1906.03270

Probing Extrasolar Planetary Systems with Interstellar Meteors

Amir Siraj, Abraham Loeb (Submitted on 7 Jun 2019)

The first interstellar object, `Oumuamua, was discovered in the Solar System by Pan-STARRS in 2017, allowing for a calibration of the impact rate of interstellar meteors of its size ∼100 m. The discovery of CNEOS 2014-01-08 allowed for a calibration of the impact rate of interstellar meteors of its size ∼1 m. Analysis of interstellar dust grains have allowed for calibrations of the impact rate of smaller interstellar meteors down to the size ∼10 −8 m. We analyze the size distribution of interstellar meteors, finding that for smooth power-law fits of the form N(r)∝r −q , the possible values of q are in the range 3.41±0.17 . We then consider the possibility of analyzing interstellar meteors to learn about their parent planetary systems. We propose a strategy for determining the orbits and chemical compositions of interstellar meteors, using a network of ∼600 all-sky camera systems to track and conduct remote spectroscopy on meteors larger than ∼5 cm once every few years. It should also be possible to retrieve meteorites from the impact sites, providing the first samples of materials from other planetary systems.

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From the discussion on page 19 of the Mashchenko paper:

“As a side note, we suggest here one mechanism which by design will only produce linear non-gravitational acceleration (or rather an appearance of such), with zero torque: if ‘Oumuamua happens to be made of some sort of exotic matter for which the gravity law deviates slightly from the canonical form. Indeed, if the gravity constant G for ‘Oumuamua were only 0.0008 of fractional units smaller than the standard value, this would completely reproduce the effect discovered by Micheli et al. (2018): the appearance of an additional force which is radially directed away from the Sun, scales as r −2 and has the right magnitude. As this is not a real force, there would be zero torque by design.”

F=G M_Sun X m_'Oumuamua/r^2

If G were to alter as though there were a Gdot this would also affect all other bodies. If there was a significant alteration of the Solar mass, this also would affect all other bodies. If there were an effect on distance, this also would be apparent elsewhere.

This would leave open the question whether the response of some portion of the mass of 'Oumuamua to the gravity field was different. With respect to the flyby anomaly the possibility of exotic composition does not hold. The Earth spacecraft that suffered the anomaly were not made of exotic composition. While Rosetta I showed an anomaly, it was absent in both Rosetta II and Rosetta III. Rosetta was unlikely to have altered its composition while in orbit.

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Originally Posted by Ross 54
I wonder that such a porous, delicate object, as described in the Moro-Martin paper could survive in the highly elongated form inferred for it.
The recent find on Ceres bubble mountains (and the look of https://en.wikipedia.org/wiki/Hyperion_(moon) made me think of something:
http://forum.cosmoquest.org/showthre...t=#post2487263

Remember, Ceres impactors don't seem very violent--no really big craters:
https://www.nature.com/articles/ncomms12257

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How to make a low density space body.....

You have a large Ceres-like object with a level of gravity--such that there is a "down" This allows for differentiation, where low density slurries can rise and bubble to the surface in a cluster, and freeze hard.

But the gravity is still low enough that such "foam structures can be spalled off into space with low velocity impacts. These foam asteroids are then their own bodies--their own asteroids.

They look a bit like pumice--and in a way--that is what they are.

The vesicles burst, and release gas--making it look like a craft dong a "burn."

Maybe foam ships can be a thing. https://memory-beta.fandom.com/wiki/Erehwon
Last edited by publiusr; 2019-Jun-22 at 08:08 PM.

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