'Oumuamua is a small interstellar object, estimated to be between 100-1,000m (300-3,000ft) in length and between 35-167m (115-548ft) wide. It is red in colour, similar to other objects in the outer Solar System. It is notable for being the first interstellar object to be detected passing through our Solar System. It was originally designated 1I/2017 U1, and was discovered by Robert Weryk using the Pan-STARRS telescope at Haleakalā Observatory, Hawaii, on October 19, 2017. The name 'Oumuamua is the Hawaiian term for scout.
'Oumuamua's origin is extrasolar, which means that it comes from outside our Solar System, and it is moving fast enough that it has not become bound into a solar orbit around our Sun as it moves through the Solar System. Therefore, it will eventually travel beyond our Solar System and into interstellar space again. How old it is and where it came from are unknown at this time, although scientists have developed some theories.
The "most convincing model so far", according to astronomer Marco Micheli from the European Space Agency, is that 'Oumuamua is a comet, albeit one with a different makeup to other comets we've encountered in the past. A team of researchers at the University of California, Berkeley studied 'Oumuamua and developed a model that suggested that it began life as a water-rich comet that was ejected from its original orbit around a star. The high-energy cosmic rays that are found throughout the galaxy then might have turned up to 30% of the water ice that made 'Oumuamua up into hydrogen, which in turn then became trapped in the ice as 'Oumuamua continued to travel through space.
As 'Oumuamua got closer to the Sun, the heat would have caused the trapped hydrogen to be released, giving it a speed boost. Furthermore, molecular hydrogen would have not had the momentum to pull dust after it in its wake, as it is "less massive" than carbon monoxide or carbon dioxide, the gases that are released in more typical comets. This could potentially explain why 'Oumuamua does not have the coma (the nebulous envelope that forms around the nucleus of a comet as it passes close to the Sun) or tail that other comets have.
Previous theories about 'Oumuamua's origins had suggested that it might have been an asteroid or a shard of hydrogen ice that was formed in a cold interstellar cloud of dust and gas. Another suggestion, from theoretical physicist Avi Loeb and others, is that 'Oumuamua is actually an alien spacecraft of some sort, or possibly an alien probe firing a thruster as it flies past the Sun, but there is little to no evidence to substantiate this hypothesis at this time.
'Oumuamua has now passed beyond Neptune's orbit as it continues on its path out of our Solar System, which is sadly well beyond the range of any of our telescopes. However, in January 2022 Project Lyra, which was created in 2017 to investigate the feasibility of studying interstellar objects like 'Oumuamua and 2I/Borisov, suggested that it might be possible to send a spacecraft, similar to the Rosetta spacecraft that studied and eventually landed on 67P/Churyumov-Gerasimenko, to catch up to and land on 'Oumuamua. Depending on when it was launched, such a spacecraft could take anything from 5-26 years to complete its mission.
Since 'Oumuamua's discovery, several other interstellar objects have been discovered traveling through our Solar System. These include 2I/Borisov, a rogue comet believed to have come from the Cassiopeia constellation; and 514107 Ka'epaoka'awela ("Bee-Zed"), a small asteroid (3km in diameter) that might have originally been an interstellar asteroid caught into an orbit around the sun 4.5 billion years ago. With each new discovery, we learn more about these interstellar objects and about the universe in general.
Sources:
'Oumuamua (Wikipedia)
"Mystery of our first interstellar visitor may be solved" (Science)
"Acceleration of 1I/‘Oumuamua from radiolytically produced H2 in H2O ice" (Nature)
"A Possible Link between 'Oumuamua and Unidentified Aerial Phenomena" (Scientific American