Initial characterization of interstellar comet 2I/Borisov

Abstract: Interstellar comets penetrating through the Solar System had been anticipated for decades. The discovery of asteroidal-looking 'Oumuamua was thus a huge surprise and a puzzle. Furthermore, the physical properties of the 'first scout' turned out to be impossible to reconcile with Solar System objects, challenging our view of interstellar minor bodies. Here, we report the identification and early characterization of a new interstellar object, which has an evidently cometary appearance. The body was discovered by Gennady Borisov on 30 August 2019 UT and subsequently identified as hyperbolic by our data mining code in publicly available astrometric data. The initial orbital solution implies a very high hyperbolic excess speed of ~32 km/s, consistent with 'Oumuamua and theoretical predictions. Images taken on 10 and 13 September 2019 UT with the William Herschel Telescope and Gemini North Telescope show an extended coma and a faint, broad tail. We measure a slightly reddish colour with a g'-r' colour index of 0.66 +/- 0.01 mag, compatible with Solar System comets. The observed morphology is also unremarkable and best explained by dust with a power-law size-distribution index of -3.7 +/- 1.8 and a low ejection speed (44 +/- 14 m/s for $\beta$ = 1 particles, where $\beta$ is the ratio of the solar gravitational attraction to the solar radiation pressure). The nucleus is probably ~1 km in radius, again a common value among Solar System comets, and has a negligible chance of experiencing rotational disruption. Based on these early characteristics, and putting its hyperbolic orbit aside, 2I/Borisov appears indistinguishable from the native Solar System comets.

• The paper identifies 2I/Borisov's interstellar origin through its hyperbolic orbit and excess speed of approximately 32 km/s.
• The paper uses telescopic imagery and Monte Carlo modeling to reveal a comet-like morphology and a nucleus radius near 1 km.
• The paper confirms negligible non-gravitational forces, reinforcing 2I/Borisov’s significance as a typical interstellar comet.

Initial Characterization of Interstellar Comet 2I/Borisov: A Detailed Analysis

The paper presents a comprehensive study of the interstellar object 2I/Borisov, detailing its discovery and its distinct characteristics as a cometary body traveling through our Solar System. This analysis reflects a collaborative effort involving institutions across Europe, notably, the Astronomical Observatory of Jagiellonian University and the Netherlands Institute for Space Research.

Discovery and Identification

2I/Borisov was first observed by Gennady Borisov on August 30, 2019, and was identified as an interstellar object due to its hyperbolic trajectory, verified through a data mining code that processed publicly available astrometric data. The orbital calculations indicate a hyperbolic excess speed of approximately 32 km/s, aligning with the velocities anticipated for interstellar objects. This speed, coupled with a significant eccentricity of 3.38 ± 0.02, confirms its origin from beyond the Solar System, as its current trajectory cannot be explained by gravitational influences from Solar System bodies.

Physical Characteristics

The comet exhibits a classic cometary morphology with a visible coma and a broad tail, observed through telescopic imagery from the William Herschel Telescope and Gemini North. Photometric observations reveal a slightly reddish color, with a g'–r' color index of 0.66 ± 0.01 magni, comparable to comets originating within our Solar System. The nucleus is estimated to be around 1 km in radius, a finding derived through two independent methodologies, albeit with inherent uncertainties.

Monte Carlo modeling of the dust environs of 2I/Borisov indicates a power-law size distribution index of -3.7 ± 1.8, with ejected particles possessing a speed of 44 ± 14 m/s for β = 1, where β represents the ratio of solar gravitational forces to solar radiation pressure. These parameters suggest that the ejection of dust from the cometary nucleus is consistent with known cometary activity.

Orbital Dynamics and Non-Gravitational Forces

Despite initial astrometric residuals hinting at significant non-gravitational forces, further unrestricted orbital solutions fit a purely gravitational model, confirming the hyperbolic nature of 2I/Borisov's path. The improbability of these non-gravitational accelerations—which surpass typical values by two to three orders—strongly supports the conclusion that 2I/Borisov is not subject to internal Solar System forces but instead maintains an origin external to it.

Implications and Future Research

The significance of 2I/Borisov lies in its reaffirmation of the existence of interstellar comets, following ‘Oumuamua as the second identified interstellar visitor. Unlike ‘Oumuamua, which defied conventional appearances, 2I/Borisov exhibits attributes akin to Solar System comets, suggesting a potentially prevalent interstellar cometary archetype. Its study not only expands the understanding of interstellar bodies but also serves as an empirical basis for the prediction models estimating one interstellar object detection per annum.

Conclusion

2I/Borisov offers critical insights into the properties of material originating outside our Solar System, providing a point of contrast and comparison with native Solar System bodies. The ongoing observation campaigns enabled by the robust detection of such interstellar objects by upcoming instrumentation, such as the Vera C. Rubin Observatory, will provide further empirical data, reinforcing models of interstellar object frequency and aiding in the broader understanding of comet-like entities in extra-solar space.