Evidence for ET?

[Anomalocaris]

Perhaps outgassing.

That's possible; it's a question of F = ma.

Again, comets frequently show major deviations from keplarian orbits due to jetting effects of volatile sublimation.   Why are we so surprised this object should do the same thing just because it came in from outside the solar system?

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It is on a hyperbolic orbit, which, per Newtonian mechanics, should be completely predictable.  Why the deviation from the hyperbola?

That is the question.

Impact with another object is ridiculously improbable and would have produced debris. It's also been accelerating over a fair bit of time rather than a one-off bump.

Outgassing of volatiles wasn't observed. It may be too distant to make out the gasses, though that seems unlikely.

Both of the above should have altered the rate of its tumble, but that wasn't observed either.

It's possible that it's a light sail or fragment of a light sail, though the reddish colour makes that unlikely. The better answer is our old stand-by, "We don't know." It has made Oumuamua a bit more interesting though. Regardless of what it is, it's unlikely that our first detection of a large extra-solar object is wildy anomalous, so we'll likely be visiting something similar with a probe in the next decade or two.

Thrust of outgassing would not appreciably effect the rate of tumble if the thrust vector directs through the elongated object near the middle.  Since the expelled gas is not attached to the object, the departure of the gas would also have no effect on the angular momentum of the remaining mass of the object.

It would be interesting to determine how much density and optical depth would the plume of expelled gas need to have to be detectable from earth, and how large must the plume be to be resolvable separately from object itself as a coma.   Couple that with reasonable estimate of the velocity of the outgas would put a constaint on the amount of material that can be outgased without being detected from earth.  That in turn would provide a constraint on the maximum amount of orbital energy change outgassing can cause without the outgas being detectable from earth.

A solar sail able to appreciably affect the orbit of a solid object of reasonable density a good fraction of a km long would be enormous and at least severa Times larger in dimension than the object itself.   Unless the sail is a near perfect, non-scattering mirror reflector from IR to UV, we would have been able to detect the sail more easily than the object itself???