Interstellar asteroids and comets: what are they and where do they come from?

Synopsis

In 2017, the first asteroid to enter our Solar System from interstellar space was discovered at the PanSTARRS telescope in Hawaii, and has now been named 'Oumuamua, a Hawaiian term which signifies 'Messenger from Afar'. In 2019 a second interstellar visitor, comet Borisov (named after its discoverer) appeared.  Adding to the mystery of their origins are their contrasting properties: 'Oumuamua was rocky (unexpected), relatively slow (unexpected) and small (expected); while Borisov is the reverse: icy, fast and large. I'll briefly summarize what we know about these puzzling visitors, and outline the efforts being made at Western to back-track them to their points of origin, somewhere within our Milky Way galaxy.