Observable tests for the light-sail scenario of interstellar objects

¹ Université Côte d’Azur, Observatoire de la Côte d’Azur, CNRS, Laboratoire Lagrange, 06304 Nice, France
e-mail: wenhan.zhou@oca.eu
² Origin Space Technology Co. Ltd., Apartment D4, Hongfeng Science Park, Nanjing, Jiangsu 212415, PR China
e-mail: liushangfei@mail.sysu.edu.cn
³ School of Physics and Astronomy, Sun Yat-sen University, 2 Daxue Road, Zhuhai 519082, Guangdong Province, PR China
⁴ CSST Science Center for the Guangdong-Hong Kong-Macau Greater Bay Area, Sun Yat-sen University, 2 Daxue Road, Zhuhai 519082, Guangdong Province, PR China
⁵ Department of Aerospace Engineering, University of Maryland, College Park, MD 20742, USA
⁶ Department of Astronomy and Astrophysics, University of California, Santa Cruz, CA 95064, USA
⁷ Institute for Advanced Studies, Tsinghua University, Beijing 100086, PR China

Received: 25 May 2022
Accepted: 24 August 2022

Abstract

Context. Enigmatic dynamical and spectral properties of the first interstellar object (ISO), 1I/2017 U1 (Oumuamua), led to many hypotheses, including a suggestion that it may be an “artificial” spacecraft with a thin radiation-pressure-driven light sail. Since similar discoveries by forthcoming instruments, such as the Vera C. Rubin telescope and the Chinese Space Station Telescope (CSST), are anticipated, a critical identification of key observable tests is warranted for the quantitative distinctions between various scenarios.

Aims. We scrutinize the light-sail scenario by making comparisons between physical models and observational constraints. These analyses can be generalized for future surveys of ‘Oumuamua-like objects.

Methods. The light sail goes through a drift in interstellar space due to the magnetic field and gas atoms, which poses challenges to the navigation system. When the light sail enters the inner Solar System, the sideways radiation pressure leads to a considerable non-radial displacement. The immensely high dimensional ratio and the tumbling motion could cause a light curve with an extremely large amplitude and could even make the light sail invisible from time to time. These observational features allow us to examine the light-sail scenario of interstellar objects.

Results. The drift of the freely rotating light sail in the interstellar medium is ~100 au even if the travel distance is only 1 pc. The probability of the expected brightness modulation of the light sail matching with ‘Oumuamua’s observed variation amplitude (~2.5–3) is <1.5%. In addition, the probability of the tumbling light sail being visible (brighter than V = 27) in all 55 observations spread over two months after discovery is 0.4%. Radiation pressure could cause a larger displacement normal to the orbital plane for a light sail than that for ‘Oumuamua. Also, the ratio of antisolar to sideways acceleration of ‘Oumuamua deviates from that of the light sail by ~1.5 σ.

Conclusions. We suggest that ‘Oumuamua is unlikely to be a light sail. The dynamics of an intruding light sail, if it exists, has distinct observational signatures, which can be quantitatively identified and analyzed with our methods in future surveys.