1I/`Oumuamua - Probably Too Small To Ever be An Active Comet

Immediately upon its discovery, the first interstellar object 1I/`Oumuamua was thought to be a comet because of its highly eccentric orbit. Soon after, deep images revealed that it did not show the characteristic cometary activity, and thus it was reclassified as an asteroid. However, according to model calculations, many more comets than asteroids were ejected from the early Solar System, making the lack of `Oumuamua's activity very surprising. This, and the familiarly reddish color, suggested it might in fact be a (significantly devolatilized) comet (Fitzsimmons et al. 2017, NatAs 2, 133) - a hypothesis apparently further supported by the detection of non-gravitational forces in the orbital motion of the body (Micheli et al. 2018, Nature 559, 223). However, the hypothesis of cometary nature of `Oumuamua must be confronted with two important evolutionary processes of comets: (1) rotational changes in response to outgassing torques and (2) rotational breakup resulting from those. The former is especially efficient for small nuclei (e.g. Bodewits et al. 2018, Nature 553, 186). We found that for a typical comet having the same dimensions as the expected dimensions of `Oumuamua sublimation of a layer as thin as 1 meter is enough to accelerate the rotation to the point that the nucleus breaks apart, even if it has a non-negligible tensile strength. This fact, combined with the estimated parameters of `Oumuamua, impose strict restrictions on the physical properties of the body. Furthermore, the amount of the sublimated material needed to explain the observed non-gravitational effects seems to be large enough to drive the body to rotational instability during its passage through the Solar System, if only `Oumuamua's physical properties are similar to those of typical active comets. We conclude that: (1) `Oumuamua is supposedly so small that if it were ever an active comet, it would spin up and disrupt before losing a sufficiently deep layer to devolatilize and (2) if `Oumuamua was sublimating while close to the Sun, its physical properties must be far from anything we know from our Solar System.