Pole, Pericenter, and Nodes of the Interstellar Minor Body A/2017 U1

Interstellar minor bodies can be identified by their positive barycentric energy (i.e., they follow hyperbolic paths as their orbital eccentricities, e, are $\gt 1$) and/or by their unusual chemical composition even if their $e\lt 1$—e.g., the comets C/1996 B2 (Hyakutake) (Mumma et al. 1996) or 96P/Machholz 1 (Langland-Shula & Smith 2007; Schleicher 2008). They can come from deep space or be produced locally, after close encounters with the planets or the Sun—e.g., comet C/1980 E1 (Bowell) was ejected from the solar system with e = 1.0577 after a flyby with Jupiter (Buffoni et al. 1982; Meech et al. 2009; Branham 2013). Captured interstellar minor bodies can be returned to deep space after flybys—e.g., comet 96P/Machholz 1 may be ejected in the relatively near future (de la Fuente Marcos et al. 2015). Interstellar minor bodies represent a rare and unique opportunity to have direct access to material from beyond the solar system and the likelihood of observing them has been studied thoroughly (McGlynn & Chapman 1989; Kresak 1992; Sen & Rama 1993; Moro-Martín et al. 2009; Jura 2011; Cook et al. 2016; Engelhardt et al. 2017).

Here, we explore some peculiar orbital features of the recently discovered asteroid A/2017 U1 (Bacci et al. 2017) ¹ that may help in understanding its nature. The discussion presented here is based on data publicly available from JPL's Small-Body Database, ² but the results are novel and have not appeared elsewhere. Its heliocentric orbit determination (barycentric values in parentheses) as of 2017 October 28 is: perihelion distance, q = 0.254 ± 0.002 au (0.257 au), e = 1.196 ± 0.004 (1.199), inclination, i = 1226 ± 02 (1229), longitude of the ascending node, Ω = 24605 ± 0007 (24791), and argument of perihelion, ω = 2415 ± 03 (2418); ³ this solution is based on 59 observations for a data-arc span of 12 days. The number of known hyperbolic comets stands at 333 and there are five objects with $e\gt 1.01$—C/1947 F1 (Rondanina-Bester), e = 1.0165, C/1980 E1, e = 1.0577, C/1997 P2 (Spacewatch), e = 1.0279, C/1999 U2 (SOHO), e = 1.0162, and C/2008 J4 (McNaught), e = 1.0279. A/2017 U1 is a clear outlier, at the ∼49σ level, when considering the average value of the eccentricity of known hyperbolic comets, 1.002 ± 0.004. Some, perhaps all, the comets with $e\gt 1.01$ have been ejected after close encounters with the Sun or the planets.

Regarding the orientation of its orbit in space, the positions of pericenters—ecliptic coordinates at pericenter, $({l}_{q},{b}_{q})$—and poles—projected pole positions, $({l}_{{\rm{p}}},{b}_{{\rm{p}}})$—of known hyperbolic comets computed as described by de la Fuente Marcos & de la Fuente Marcos (2016) are displayed in figure 1, top and middle panels. The orientation of the orbital plane of A/2017 U1 seems to be away from any obvious clusters (real or induced by observational biases). The closest objects in terms of polar separation, ${\alpha }_{{\rm{p}}}$, are C/1970 N1 (Abe) (479) and C/2014 A4 (SONEAR) (450). When considering both ${\alpha }_{{\rm{p}}}$ and pericenter separations, ${\alpha }_{q}$, the smallest relative angular separations are $\gt 20^\circ$. The distributions of ${\alpha }_{q}$ and ${\alpha }_{{\rm{p}}}$ of all the pairs in the sample are plotted in the bottom panels. The hyperbolic comets with the closest mutual orbital orientation are the pieces of C/1986 P1 (Wilson), C/1996 J1-A and C/1996 J1-B (Evans-Drinkwater), and C/2011 J2 and C/2011 J2-B (LINEAR), all of them fragmented comets. Some of the clustering visible in figure 1, top and middle panels, may be due to dynamically correlated objects, probably the result of break-ups. The descending node of A/2017 U1 is located at 0.355 au and the ascending node at 1.301 au, well away from the paths of the planets of the solar system and the Sun.

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All these orbital properties appear to confirm A/2017 U1 as the first known interstellar asteroid (either truly extrasolar or, far less likely, a returning former solar system minor body that was ejected very long ago).