One unusual thing cited about Oumuamua is its shape. We have been repeatedly told that the object was cigar-shaped, and press coverage has repeatedly shown a visual of a cigar-shaped object. But that visual is not an actual photo. It is a speculative “artist's visualization” thing. No actual photographs have been taken of Oumuamua. In a recent Scientific American article, Loeb says this:
We do not have a photo of ‘Oumuamua, but its brightness owing to reflected sunlight varied by a factor of 10 as it rotated periodically every eight hours. This implies that ‘Oumuamua has an extreme elongated shape with its length at least five to 10 times larger than its projected width.
So Oumuamua may be only five times longer than its width, which would make it merely pickle-shaped rather than cigar-shaped. A pickle-shaped object is not particularly strange, and many asteroids and comets have such a shape. As for the statement that “its brightness owing to reflected sunlight varied by a factor of 10 as it rotated periodically every eight hours,” that indicates the kind of rotation that you would have if a bottle-shaped object were to be continually spinning around like a bottle that was spun around on the floor in the teenager's game “spin the bottle.” That type of rotation (which the wikipedia.org article on Oumuamua describes as a "tumbling" motion) is what we would expect from a natural object, but we would not expect it from an extraterrestrial probe. An extraterrestrial probe would almost certainly keep pointing in the same direction, so that its propulsion system would stay pointed in a single direction. When the US sends space probes to Mars, they keep pointing in the same direction so that their rockets can keep thrusting in a particular direction. Such probes don't tumble or spin around like a bottle spinning on the floor.
In his Scientific American article, Loeb tries to make Oumuamua sound as strange as he can make it sound. He refers to some paper he co-authored years ago calculating that the chance of a comet from another solar system being detected by a modern space telescope was between .001% and 1%. But that's not a very compelling consideration, as the calculations in the paper were extremely complicated and largely speculative, and could have gone wrong in any number of places. Moreover, something isn't terribly improbable if it has a likelihood of 1%.
Loeb also tells us, “The trajectory of ‘Oumuamua deviated from that expected based on the sun’s gravity alone.” But in the next sentence we learn that this deviation was only very tiny, for Loeb says that the deviation was only a tenth of a percent. Such a tiny discrepancy is nowhere near sufficient to imply that the craft was some extraterrestrial probe. We would hardly be persuaded if someone were to argue, “I think that the light in the sky I saw was an alien spaceship, because I expected a jet to be traveling at 200 miles per hour, but the object was traveling at 201 miles per hour.”
The small discrepancy involving Oumuamua's trajectory can easily be explained as a measurement error, an interpretation error or a calculation error. Such things occur all the time in the world of science. Another explanation is that there was some outgassing from Oumuamua, such as a water vapor emission caused by it moving closer to the sun. Loeb attempts to argue that if Oumuamua was ourgassing, it would have had a cometary tail that was not observed. But this is not a strong argument, as the object could have had enough outgassing to explain the slight trajectory anomaly, but not enough outgassing for the object to have produced a cometary tail big enough to be detected. And how can Loeb be arguing from a non-detection of a comet tail, after he has admitted that a photo of Oumuamua does not exist?
There is a powerful reason for rejecting the idea that Oumuamua was an extraterrestrial probe: the fact that the object came nowhere close to Earth. It seems likely that any deliberately designed extraterrestrial probe would have sufficient technical skill to move towards any Earth-like planet in a solar system it was entering. Building a probe capable of traveling between stars would require a technology far beyond ours, and for a technology so advanced it would be easy to program a probe so that it could move in for a closeup look on any Earth-like planet that it found in a solar system it entered. But there is no sign that Oumuamua changed its path to get a closer look at our planet, and Oumuamua came no closer than 15 million miles to Earth. So both because of its "spin the bottle" spinning motion and because of its failure to move closer to Earth, it seems that Oumuamua did not act the way an extraterrestrial probe would have acted.
Astronomers made very sensitive checks of radio signals from Oumuamua, and found none. A probe from another planet would presumably be sending such signals to report back to its home planet what it found.
Path of Oumuamua (from wikipedia.org)
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