'Oumuamua

'Oumuamua (Hawaiian for "scout"), object catalog 1I/2017 U1, which at least doesn't sound so weird, is the first known interstellar object discovered while crossing our Solar System.

A few otherwise well-qualified individuals have suggested it might be an alien spacecraft or some other kind of alien artifact, based on its trajectory and measurements suggesting it may be accelerating — although none of the evidence is anywhere near conclusive, and there are alternative natural explanations.

Discovery and trajectory
'Oumuamua was discovered in October 2017 with the Pan-STARSS telescope after its closest approach to the Sun. After having classified it as yet another unremarkable asteroid or comet, it was recognized as a body coming from outside the Solar System the following month once earlier images of the same object were found from digging through archives of images taken with other telescopes. It was concluded that, unlike standard comets or asteroids, there was no way that a Solar System minor body could have such trajectory and relative speed.

Its trajectory brought it from the direction of the Lyra constellation and after approaching the Sun closer than the orbit of Mercury and receiving a boost on its speed, it's on course towards the Pegasus constellation. It is also estimated that it will leave our Solar System within 20,000 years.

Properties
Due to its faintness and small size, very little is known of 'Oumuamua. It has been suggested it could have come from a stellar association of young stars located 300 light-years from us, or be either a small chunk of a planet that got shredded by a star, or an asteroid ejected from a dying star system, but for all intents and purposes its origin is unknown and it could have well roamed the Milky Way for countless millions of years.

Measurements of 'Oumuamua's spectrum suggests it's a very red object similar to other outer Solar System bodies such as certain asteroids or comets. Its size is not well determined but it's estimated to be roughly a battleship-size turd highly elongated in the range of around 200 meters in chaotic rotation with a period of around 8 hours. More recent findings suggested 'Oumuamua could be not an asteroid but a mildly active (still interstellar) comet instead, since it appears that it's material after passing close the Sun and it giving a very small acceleration. . Later studies, however, dispute that claim because if it worked like Solar System comets, it would have broken apart (so is not a comet, neither is it an asteroid).

It has been proposed to send a probe to 'Oumuamua, but sadly even if we had technology to move a spacecraft so fast ('Oumuamua's relative speed gives a run for their money to our current spacecraft propulsion technologies), given the way things work with space agencies — read: limited budgets and lots of bureaucracy — it's unlikely to happen.

Woo
Funny and harmless comparisons have been made with the starship "Rama", which appears in Arthur C. Clarke's novel Rendevous with Rama, including that both "Rama" and 'Oumuamua are highly elongated.

'Oumuamua has also been linked to Nibiru/Planet X by pseudoastronomers, which has been incorrectly predicted to arrive in the Solar System multiple times, including some predictions that were at least in the same year as 'Oumuamua (never mind that it did none of the things Nibiru were predicted to do).

Not Woo
Some astronomers have proposed 'Oumuamua could actually be a (read: a spaceship) attempting to explain how it's accelerating outward as mentioned above. However others have contested that, as data is pretty much insufficient and as commented above the probability of a spacecraft being sent to study it pretty much nil.