1I/‘Oumuamua was detected when it was already on its way out of the Solar System, following its close perihelion passage interior to the orbit of Mercury at 0.26 au9. It was relatively small upon detection, at most a few hundred metres in extent (Trilling et al, 2018; Jackson and Desch, 2021). High-amplitude variation in its lightcurve suggested a shape then considered unusual: the best-fit shape was a flattened disc with a 6:6:1 aspect ratio (Mashchenko, 2019), similar to the TNO (486958) Arrokoth (Spencer et al, 2020) but more extreme (6:1 aspect ratio compared to 2:1). It had a red reflection spectrum, similar to the D-type asteroids of the outer Main Belt and Jupiter Trojan regions (Fitzsimmons et al, 2018).
The nature of 1I/‘Oumuamua initially caused an extensive discussion (’Oumuamua ISSI Team et al, 2019), as it lacked detected activity but showed a non-gravitational acceleration (Micheli et al, 2018; Trilling et al, 2018; ’Oumuamua ISSI Team et al, 2019; Seligman et al, 2019). Non-gravitational acceleration of ISOs implies the presence of volatiles, which may inform our understanding of where in their disc they formed (Seligman et al, 2022). Given the lack of detected gas, however, the volatile species responsible for 1I/‘Oumuamua’s acceleration has been debated. One possibility would be that 1I/‘Oumuamua was a collisional fragment rich in N2 ice from a
9Orbital elements for these and other bodies are available at the JPL Small-Body Database: https: //ssd.jpl.nasa.gov/tools/sbdb lookup.html#/ parent body like Pluto, which well reproduces the observed non-gravitational acceleration and the lack of observed outgassing or released dust (Jackson and Desch, 2021), a scenario also proposed by Biver et al (2018)

for the Solar System comet C/2016 R2 (PanSTARRS). H2 ice has also been proposed for 1I/‘Oumuamua (Seligman and Laughlin, 2020; Bergner and Seligman, 2023), although it is physically challenging to form and neither Lyman α emission nor water (a potential progenitor) were detected (Trilling et al, 2018; Sabat and Drahus, 2023). These unusual compositions and ISOs’ long travel times through the interstellar medium have motivated several studies of space weathering from Galactic cosmic rays on these bodies (Fitzsimmons et al, 2018; Seligman and Laughlin, 2020; Jackson and Desch, 2021; Bergner and Seligman, 2023). It is likely that 1I/’Oumuamua decreased in size due to sublimation as it passed perihelion, and especially with an N2 composition it may also have shrunk during its journey through interstellar space due to Galactic cosmic rays, and such erosion can explain its unusual aspect ratio (Jackson and Desch, 2021; Desch and Jackson, 2021).
(Desch and Jackson, 2021) predict that collisional fragments of differentiated bodies should dominate ISOs in the size range 50 m ≲D ≲1, km. The two ISOs discovered after 1I/‘Oumuamua have both more clearly resembled Solar System comets, but these are considerably easier to detect as their larger size and outgassing vastly increases their magnitude. Hopefully, the advent of the Vera C. Rubin Observatory’s Legacy Survey of Space and Time, and its anticipated ISO discoveries, will allow such origin hypotheses to be tested.