Non-gravitational acceleration in the trajectory of 1I/2017 U1 (‘Oumuamua)

Abstract

'Oumuamua (1I/2017 U1) is the first known object of interstellar origin to have entered the Solar System on an unbound and hyperbolic trajectory with respect to the Sun. Various physical observations collected during its visit to the Solar System showed that it has an unusually elongated shape and a tumbling rotation state and that the physical properties of its surface resemble those of cometary nuclei, even though it showed no evidence of cometary activity. The motion of all celestial bodies is governed mostly by gravity, but the trajectories of comets can also be affected by non-gravitational forces due to cometary outgassing. Because non-gravitational accelerations are at least three to four orders of magnitude weaker than gravitational acceleration, the detection of any deviation from a purely gravity-driven trajectory requires high-quality astrometry over a long arc. As a result, non-gravitational effects have been measured on only a limited subset of the small-body population. Here we report the detection, at 30σ significance, of non-gravitational acceleration in the motion of 'Oumuamua. We analyse imaging data from extensive observations by ground-based and orbiting facilities. This analysis rules out systematic biases and shows that all astrometric data can be described once a non-gravitational component representing a heliocentric radial acceleration proportional to r or r (where r is the heliocentric distance) is included in the model. After ruling out solar-radiation pressure, drag- and friction-like forces, interaction with solar wind for a highly magnetized object, and geometric effects originating from 'Oumuamua potentially being composed of several spatially separated bodies or having a pronounced offset between its photocentre and centre of mass, we find comet-like outgassing to be a physically viable explanation, provided that 'Oumuamua has thermal properties similar to comets.

Pre-discovery Activity of New Interstellar Comet 2I/Borisov beyond 5 au

“…At this stage it is not entirely clear whether this behavior is caused by systematic errors in the bulk of the astrometric data, which were taken at low solar elongation and therefore at high airmass, or by non-gravitational accelerations. We note that non-gravitational accelerations were detected in the motion of 'Oumuamua, despite the lack of visible outgassing (Micheli et al 2018). Table 1 reports JPL solution 37, which fits all the precovery observations and uses non-gravitational forces assuming CO as the primary driver ( § 2.1), more consistent than H 2 O with the photometric data.…”

Capture of interstellar objects: a source of long-period comets

“…The discovery of our first known interstellar visitor 1I/'Oumuamua (Meech et al 2017;'Oumuamua ISSI Team et al 2019) raised the intriguing possibility that we might one day be able to understand the planet formation environment around alien stars by studying asteroids or comets ejected from them. The nature of 'Oumuamua was immediately questioned with some studies suggesting that the object displayed minor levels of cometary activity (Micheli et al 2018), some suggesting it did not (Rafikov 2018) and some setting relatively low upper limits on any potential activity (Trilling et al 2018). Unfortunately, the transient nature of 'Oumuamua means that it is impossible to gather further data, and curious parties were forced to await the chance detection of another Interstellar Object (ISO).…”

New Insights into Interstellar Object 1I/2017 U1 (‘Oumuamua) from SOHO/STEREO Nondetections

“…With the ephemeris calculated by JPL Horizons based on the nongravitational solution by Micheli et al (2018), we did not detect 'Oumuamua in any of the individual SOHO or STEREO images. The 3σ positional uncertainties are small, corresponding to 5 pixels (∼1.…”

“…To see how this would affect the uncertainty region, we assumed the nongravitational force of 'Oumuamua to be a step function of time: 'Oumuamua did not exhibit the nongravitational effect until some specific epoch t NG . We tried three different epochs: 2017 September 10.0, 27.0 and October 14.0 (in units of the internal Barycentric Dynamical Time, i.e., TDB), all of which are earlier than the earliest astrometry fit by Micheli et al (2018) and could therefore fit the available astrometric observations. The largest positional deviation from the nominal orbit is from the orbit with t NG = TDB 2017 October 14.0 (∼ −4 in RA, ∼ +3 in Decl.…”

“…The motion of 'Oumuamua has been identified to show a strong nongravitational effect (Micheli et al 2018). The leading candidate causing the nongravitational acceleration is likely due to the mass-loss activity rather than the solar radiation pressure.…”