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

************************************************** ************************************************** *****************************

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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"Origin of the First Known Interstellar Object 'Oumuamua"

What is the origin of the famous interstellar object ‘Oumuamua? How was it formed and where did it come from? An article published on April 13 in Nature Astronomy by ZHANG Yun from National Astronomical Observatories of Chinese Academy of Sciences (NAOC) and Douglas N. C. Lin from University of California, Santa Cruz, offers a first comprehensive answer to this mystery, which involves tidal forces like those felt by Earth’s oceans and explains all of the unusual characteristics of this interstellar object.

13. In short, 'Oumuamua was an "active asteroid" from interstellar space, the remnant of a shattered terrestrial planet.

https://arxiv.org/abs/2004.07218
Tidal fragmentation as the origin of 1I/2017 U1 ('Oumuamua)
Yun Zhang, Douglas N. C. Lin
(Submitted on 15 Apr 2020)
The first discovered interstellar object (ISO), Oumuamua (1I/2017 U1) shows a dry and rocky surface, an unusually elongated short-to-long axis ratio, a low velocity relative to the local standard of rest (∼10 km s-1), non-gravitational accelerations, and tumbles on a few hours timescale.... Although some scenarios can cause the ejection of asteroidal ISOs, a unified formation theory has yet to comprehensively link all Oumuamua's puzzling characteristics and to account for the population. Here we show by numerical simulations that Oumuamua-like ISOs can be prolifically produced through extensive tidal fragmentation and ejected during close encounters of their volatile-rich parent bodies with their host stars. Material strength enhanced by the intensive heating during periastron passages enables the emergence of extremely elongated triaxial ISOs with rocky surfaces. Although volatiles with low sublimation temperature (such as CO) are concurrently depleted, H2O buried under surfaces is preserved in these ISOs, providing an outgassing source without measurable cometary activities for Oumuamua's non-gravitational accelerations during its passage through the inner Solar System. We infer that the progenitors of Oumuamua-like ISOs may be km-sized long-period comets from Oort clouds, km-sized residual planetesimals from debris disks, or planet-size bodies at a few AU, orbiting around low-mass main-sequence stars or white dwarfs. These provide abundant reservoirs to account for Oumuamua's occurrence rate.

14. wrong place to post.
Last edited by Roger E. Moore; 2020-Apr-16 at 12:56 AM. Reason: error

15. Was 'Oumuamua made up of molecular hydrogen ice?

https://arxiv.org/abs/2005.12932
Evidence that 1I/2017 U1 (Oumuamua) was composed of molecular hydrogen ice
Darryl Seligman, Gregory Laughlin
[Submitted on 26 May 2020]
Oumuamua (I1 2017) was the first macroscopic (l∼100m) body observed to traverse the inner solar system on an unbound hyperbolic orbit. Its light curve displayed strong periodic variation, and it showed no hint of a coma or emission from molecular outgassing. Astrometric measurements indicate that 'Oumuamua experienced non-gravitational acceleration on its outbound trajectory, but energy balance arguments indicate this acceleration is inconsistent with a water ice sublimation-driven jet of the type exhibited by solar system comets. We show that all of Oumuamua's observed properties can be explained if it contained a significant fraction of molecular hydrogen (H2) ice. H2 sublimation at a rate proportional to the incident solar flux generates a surface-covering jet that reproduces the observed acceleration. Mass wasting from sublimation leads to monotonic increase in the body axis ratio, explaining Oumuamua's shape. Back-tracing Oumuamua's trajectory through the Solar System permits calculation of its mass and aspect ratio prior to encountering the Sun. We show that H2-rich bodies plausibly form in the coldest dense cores of Giant Molecular Clouds, where number densities are of order n∼10^5, and temperatures approach the T=3K background. Post-formation exposure to galactic cosmic rays implies a τ∼100 Myr age, explaining the kinematics of Oumuamua's inbound trajectory.

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Possible problems with molecular H2.
H2 is harder to see but H1 is easy to spot at 21cm. H2 boils at 20K. While it seems reasonable that it could be coated by H2 while traversing an interstellar cloud, its survival into the solar system to inside the orbit of Mercury while maintaining a spin to where enough is left to produce the non-gravitational force that appears constant over observation time might be a problem. Solar radiation should split a trace of H2 to atomic hydrogen, but observations did not show the 1420 MHz signal.
The critical problem H2 faces as does other gases and dust emission models is the lack of evolution of spin. The solar pressure idea of a low density or hollow body or solar sail can address the spin evolution problem as well as the acceleration problem but we do not have the observations of the comet to sort this out or rule it out.

17. Maybe it ISN'T too late to check out 'Oumuamua up close.

https://arxiv.org/abs/2006.03891
Project Lyra: Catching 1I/'Oumuamua -- Using Laser Sailcraft in 2030
[Submitted on 6 Jun 2020]
Discovered in October 2017, the interstellar object designated 1I/'Oumuamua was the first such object to be observed travelling through our solar system. 1I/'Oumuamua has other characteristics never seen before in a celestial body and in-situ observations and measurements would be of extraordinary scientific value. Previous studies have demonstrated the viability of spacecraft trajectories to 'Oumuamua using chemical propulsion with a Solar Oberth burn at a perihelion as low as a few solar radii. In addition to chemical propulsion, there is also the possibility of missions involving light sails accelerated by the radiation pressure of a laser beam from a laser located on Earth. Based on a scaled-down Breakthrough Starshot beaming infrastructure, interplanetary missions and missions to the outer solar system have been proposed using lower sailcraft speeds of 0.001c relaxing the laser power requirements (3-30 GW for 1-100 kg spacecraft) and various other mission constraints. This paper uses the OITS trajectory simulation tool, which assumes an impulsive ΔV increment, to analyze the trajectories which might be followed by a sailcraft to 'Oumuamua, with a launch in the year 2030 and assuming it has already been accelerated to a maximum speed of 300km/s (approx. 0.001c) by the laser. A minimum flight duration of 440 days for a launch in July 2030 is found. The intercept would take place beyond 82 AU. We conclude that the possibility of launching a large number of spacecraft and reaching 1I much faster than chemical propulsion would circumvent several disadvantages of previously proposed mission architectures.

18. Project Lyra technology would also enable missions to the Sun's Gravitational Focus in a reasonable amount of time.

19. Maybe NOT an interstellar iceberg, as Oumuamua would have disintegrated long ago if it were an H2 berg.

https://arxiv.org/abs/2006.08088
[Submitted on 15 Jun 2020]
Destruction of molecular hydrogen ice and Implications for 1I/2017 U1 (Oumuamua)
Thiem Hoang, Abraham Loeb
The first interstellar object observed in our solar system, 1I/2017 U1 (Oumuamua), exhibited a number of peculiar properties, including extreme elongation, tumbling, and acceleration excess. Recently, Seligman \& Laughlin (2020) proposed that the object was made out of molecular hydrogen (H2) ice. The question is whether H2 objects could survive their travel from the birth sites to the solar system. Here we study destruction processes of icy H2 objects through their journey from giant molecular clouds (GMCs) to the interstellar medium (ISM) and the solar system, owing to interstellar radiation, gas and dust, and cosmic rays. We find that thermal sublimation due to heating by starlight can destroy Oumuamua-size objects in less than 10 Myr. Thermal sublimation by collisional heating in GMCs could destroy H2 objects before their escape into the ISM. Most importantly, the formation of icy grains rich in H2 is unlikely to occur in dense environments because collisional heating raises the temperature of the icy grains, so that thermal sublimation rapidly destroys the H2 mantle before grain growth.

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The need for 'Oumuamua to have been some kind of remarkably low-density object, in order to reasonably allow the sort of anomalous accelerate observed as it departed from the Sun, appears to cause similar difficulties for all such hypotheses. Such a thing might very well have been too fragile to have survived the trip.

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"'Oumuamua isn't made from molecular hydrogen ice after all"

https://www.spacedaily.com/reports/O...r_all_999.html

The debate over the origins and molecular structure of 'Oumuamua continues with an announcement in the Astrophysical Journal Letters that despite earlier promising claims, the interstellar object is not made of molecular hydrogen ice after all.

The earlier study, published by Seligman and Laughlin in 2020 - after observations by the Spitzer Space Telescope set tight limits on the outgassing of carbon-based molecules - suggested that if 'Oumuamua were a hydrogen iceberg, then the pure hydrogen gas that gives it its rocket-like push would have escaped detection. But scientists at the Center for Astrophysics | Harvard and Smithsonian (CfA) and the Korea Astronomy and Space Science Institute (KASI) were curious whether a hydrogen-based object could actually have made the journey from interstellar space to our solar system.

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A look at various missions including a sample return mission.
Interstellar Now! Missions to and Sample Returns from Nearby Interstellar Objects
https://arxiv.org/abs/2008.07647

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"Interstellar visitor 'Oumuamua could still be alien technology, new study hints"

https://www.space.com/oumuamua-inter...or-aliens.html

One explanation? The object was propelled by an alien machine, such as a lightsail — a wide, millimeter-thin machine that accelerates as it's pushed by solar radiation. The main proponent of this argument was Avi Loeb, a Harvard University astrophysicist.

Most scientists, however, think 'Oumuamua's wonky acceleration was likely due to a natural phenomenon. In June, a research team proposed that solid hydrogen was blasting invisibly off the interstellar object's surface and causing it to speed up.

Now, in a new paper published Monday (Aug. 17) in The Astrophysical Journal Letters, Loeb and Thiem Hoang, an astrophysicist at the Korea Astronomy and Space Science Institute, argue that the hydrogen hypothesis couldn't work in the real world — which would mean that there is still hope that our neck of space was once visited by advanced aliens — and that we actually spotted their presence at the time.

24. Extra ordinary claims require extra ordinary evidence.

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A couple weeks earlier about reaching 1I with Nuclear Thermal Rockets
Project Lyra: Catching 1I/'Oumuamua Using Nuclear Thermal Rockets
https://arxiv.org/abs/2008.05435

A recent paper on the “Dust Bunny” origin that could be subject to solar pressure.

'Oumuamua as a Cometary Fractal Aggregate: the "Dust Bunny" Model

https://arxiv.org/abs/2008.10083

We did not get a good look at 1I to rule out solar pressure models to account for the acceleration whether a fluff or a spacecraft or discarded rocket stage. A number of constraints would have to be imposed to avoid spin evolution. No Yarkovsky effect to cause torque. Sample return via scooping debris from a 4-6 km/sec impactor sent ahead might destroy a dust bunny or do serious damage to a craft. If it shedded material like a comet, then how it was released would be of interest. A core sample return might reveal if a honeycomb or clathrate structure could source shedded material.

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