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When the mysterious extrasolar object, named Oumuamua, was detected and characterized, the leading analysis of its signal strongly suggested that it was a long, cylindrical or cigar-shaped object. And so there were various theories as to what might shape a comet or chunk of asteroid into such a shape and deplete it of readily evaporable volatiles.

The possibility that it might be of alien origin was suggested by some scientists, and predictably latched onto by the press, which exaggerated the xenotechnology hypothesis far beyond any apparent evidence.

The odd thing is that the main paper that defends the possibility of a xenotechnology origin for Oumuamua makes an "opposite" assumption: that the object was a derelict solar sail, which was tumbling, and would also match the detected signature of Oumuamua.

This seems... a bit odd to me, considering that "long, narrow, roughly-cylindrical metal-heavy object" is a dead ringer for many designs for a space station module, tank, or spent booster.

Why is a solar sail more likely to be a good fit than something like a large cylindrical derelict such as a booster, drop tank, or habitat module? Is this based entirely upon the assumption that acceleration of Oumuamua due to light pressure was accurately measured?

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tl;dr: In order to maintain plausibility they chose something plausible.

The abstract includes the following:

The observed trajectory is best explained by an excess radial acceleration $\Delta a ∝ r^-2$, where r is the distance of ‘Oumuamua from the Sun. Such an acceleration is naturally expected for comets, driven by the evaporating material. However, recent observational and theoretical studies imply that ‘Oumuamua is not an active comet. We explore the possibility that the excess acceleration results from Solar radiation pressure. The required mass-to-area ratio is (m/A) ≈ 0.1 g cm-2. For a thin sheet this requires a thickness of ≈ 0.3−0.9 mm. We find that although extremely thin, such an object would survive an interstellar travel over Galactic distances of ∼ 5 kpc, withstanding collisions with gas and dust-grains as well as stresses from rotation and tidal forces.

That would only be 0.4 millimeters thick total if it were aluminum for example, traditional human-built rocket bodies would have a much thicker. While a theory of xenotechnological origin would not require the use of current materials, there's probably nothing even theorized that could be a launch vehicle.

Between requirements for strength and and meteor survivability, they probably didn't want to explore unknown technologies or currently unimagined materials.

However something we do know of materials that can be used that match this density range and would be useful in interstellar missions when they leave or enter star systems is the solar sail. So in order to maintain plausibility they chose something plausible.

Related:

Less related:

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  • $\begingroup$ So basically you're saying that it's heavily based on the assumption that they measured acceleration due to light pressure on Oumuamua, and it isn't supported if Oumuamua is not accelerated primarily by light pressure? What do you mean by "probably nothing even theorized"-- if it really is cigar-shaped, then it probably isn't low density, is it? $\endgroup$ – ikrase Aug 1 at 5:48
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    $\begingroup$ Not an assumption; they said that they simply "explore the possibility that the excess acceleration results from Solar radiation pressure." By "nothing theorized" I suggest there are no theoretically plausible materials could be used to build launch vehicles. I don't rule out space stations with that particular sentence. $\endgroup$ – uhoh Aug 1 at 6:04

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