ESO
Interstellar object
Something passing through the solar system that was never bound to the Sun.
An object on a hyperbolic orbit is not returning; it came from another star and is leaving for good. Two are confirmed: ʻOumuamua in 2017, oddly elongated and behaving strangely, and 2I/Borisov in 2019, which looked like an ordinary comet from somewhere else. Both were found on the way out, with weeks of observing time available. Vera Rubin Observatory is expected to find them regularly enough that a mission to intercept one becomes worth building in advance.
Where it has come up lately
Related terms
Near-Earth object
An asteroid or comet whose orbit brings it inside 1.3 astronomical units of the Sun.
Asteroid and comet
Rock that stays rock, and ice that grows a tail when it gets warm.
Coma and tail
A comet's atmosphere, and the two separate tails the Sun pushes off it.
Oort cloud
A shell of icy bodies at the edge of the Sun's influence, inferred rather than observed.
Questions
What is Interstellar object?
Something passing through the solar system that was never bound to the Sun.
Why does it matter?
Both were found on the way out, with weeks of observing time available. Vera Rubin Observatory is expected to find them regularly enough that a mission to intercept one becomes worth building in advance.