The first interstellar object, October 2017: a tumbling sliver that accelerated away from the Sun with no visible exhaust and left before anyone could ask twice.
Documented
Six weeks on record
Pan-STARRS1 found it on October 19, 2017 (Meech et al., Nature 2017). Its light swung about 3.5 magnitudes, implying an extreme shape; no coma, no tail, no detected gas. Micheli et al. (Nature 2018) then measured a small but real non-gravitational acceleration in its outbound path. After that it was too faint to follow.
Assessment: Documented: the discovery paper and the acceleration measurement are both in Nature.
Stories
The lightsail
Bialy and Loeb (ApJL 2018) showed radiation pressure could produce the push if the object were a thin sheet; Loeb's book Extraterrestrial (2021) carried the case to the public. The field's objection was not outrage but absence: no signature of artificial origin beyond the acceleration itself.
Assessment: Stories: a serious, published hypothesis that most specialists decline to buy.
Interesting claims
The invisible exhaust
Seligman and Laughlin (2020) proposed a hydrogen iceberg, which would outgas nothing a telescope could catch; Bergner and Seligman (Nature 2023) refined it: cosmic rays rework water ice into amorphous ice laced with molecular hydrogen, released on warming, acceleration with no coma. The anomaly may be solved by exhaust no instrument could have seen.
Assessment: Interesting claims: the most mundane answer requires a kind of object never seen anywhere else.
Sources and further reading
Meech et al., A brief visit from a red and extremely elongated interstellar asteroid, Nature (2017)
Micheli et al., Non-gravitational acceleration in the trajectory of 1I/2017 U1 (ʻOumuamua), Nature (2018)
Bialy and Loeb, Could Solar Radiation Pressure Explain ʻOumuamua's Peculiar Acceleration?, ApJL (2018)
Seligman and Laughlin, Evidence that 1I/2017 U1 (ʻOumuamua) was composed of molecular hydrogen ice, ApJL (2020)
Bergner and Seligman, Acceleration of 1I/ʻOumuamua from radiolytically produced H2 in H2O ice, Nature (2023)
Loeb, Extraterrestrial: The First Sign of Intelligent Life Beyond Earth (2021)
Worth asking
The first visitor broke one rule of celestial mechanics and never came back; what does science do with an experiment that refuses to repeat?