Outgassing As Trigger of 1I/`Oumuamua's Nongravitational Acceleration: Could This Hypothesis Work at All?

Abstract

The question of what triggered the nongravitational acceleration of 1I/`Oumuamua continues to attract researchers' attention. The absence of any signs of activity notwithstanding, the prevailing notion is that the acceleration of the stellar, cigar-like object was prompted by outgassing. However, the Spitzer Space Telescope's failure to detect `Oumuamua not only ruled out the CO_2 and/or CO driven activity (Trilling et al. 2018), but made the cigar shape incompatible with the optical observations. Choice of water ice as the source of outgassing is shown to be flawed as well: (i) the water sublimation law is demonstrably inconsistent with the observed variations in the nongravitational acceleration derived by Micheli et al. (2018), the point that should have been assertively highlighted; and (ii) an upper limit of the production rate of water is estimated at as low as 4 x 10^(23) molecules s^(-1), requiring that, at most, only a small area of the surface be active. In this case the conservation of momentum law is satisfied only when `Oumuamua's bulk density is extremely low, <0.001 g cm^(-3), reminiscent of the formerly proposed scenario with `Oumuamua as a fragment of a dwarf interstellar comet, possibly an embryo planetesimal, disintegrating near perihelion, with the acceleration driven by solar radiation pressure (Sekanina 2019a) and no need for activity at all. High quality of astrometry and Micheli et al.'s orbital analysis, whose results were confirmed by the computations of other authors, is acknowledged.