On October 19, 2017, the first interstellar object in the Solar System, ‘Oumuamua (1I/2017 U1) was discovered by the PAN-STARRS1 survey. It has a highly hyperbolic trajectory (with eccentricity e = 1.1956±0.0006) and pre-entry velocity of v∞ ≈ 26 km s−1 (Meech et al. 2017). Based on the survey properties and the single detection, Do et al. (2018) estimated the interstellar density of objects like ‘Oumuamua or larger to be n ≈ 2 × 1015 pc−3, 2-8 orders of magnitude larger than expected by previous theoretical models (Moro-Martin et al.
2009). The large variations in its apparent magnitude and the non-trivial periodicity of the lightcurve, suggest that ‘Oumuamua is rotating in an excited spin state (tumbling motion), and has an extreme aspect ratio of at least 5 : 1 (Fraser et al. 2018; Drahus et al. 2018), an unprecedented value for previously known asteroids and comets in the Solar System. Belton et al. (2018) have shown that if ‘Oumuamua rotates in its highest rotational energy state, it should be extremely oblate (pancake-like).
Recently, Micheli et al. (2018) reported the detection of non-gravitational acceleration in the motion of ‘Oumuamua, at a statistical significance of 30σ. Their best-fit to the data is obtained for a model with a non-constant excess acceleration which scales with distance from the Sun, r, as ∆a ∝ r −2, but other power-law index values are also possible. They concluded that the observed acceleration is most likely the result of a cometary activity. Yet, despite its close Solar approach of r = 0.25 AU, ‘Oumuamua shows no signs of a any cometary activity, no cometary tail, nor gas emission/absorption lines were observed (Meech et al. 2017; Knight et al. 2017; Jewitt et al. 2017; Ye et al. 2017; Fitzsimmons et al. 2017). From a theoretical point of view, Rafikov (2018) has shown that if outgassing was responsible for the acceleration (as originally proposed by Micheli et al. 2018), then the associated outgassing torques would have driven a rapid evolution in ‘Oumuamua’s spin, incompatible with observations.
If not cometary activity, what can drive the nongravitational acceleration observed? In this Letter we explore the possibility of ‘Oumuamua being a thin object accelerated by Solar radiation pressure, which would naturally result in an excess acceleration ∆a ∝ r −2. 1 However, for radiation pressure to be effective, the mass-to-area ratio must be very small. In §2 we derive the required mass-to-area ratio and find (m/A) ≈ 0.1 g cm−2, corresponding to an effective thin sheet of thickness w ≈ 0.3−0.9 mm. We explore the ability of such an unusually thin object to survive interstellar travel, considering collisions with interstellar dust and gas (§3), as well as to withstand the tensile stresses caused by rotation and tidal forces (§4). Finally, in §5 we discuss the possible implications of the unusual requirements on the shape of ‘Oumuamua.'
