As the solar system goes around the center of the Galaxy at a galactocentric distance of over 8 kpc, interstellar objects may traverse our planetary neighborhood following hyperbolic paths, just like 1I/2017 U1 ('Oumuamua) and 2I/Borisov did (e.g., Seligman & Moro-Martín 2022; Jewitt & Seligman 2023). In addition, passing stars or the Galactic tide may send Oort cloud comets toward the inner solar system along hyperbolic trajectories (e.g., Torres et al. 2019). Small solar system bodies may also end up in a hyperbolic orbit after a close encounter with the Sun or a giant planet (e.g., de la Fuente Marcos et al. 2018). Comet C/1980 E1 (Bowell), with a current value of the eccentricity of 1.057733 ± 0.000008, reached its present-day path after a close encounter with Jupiter on 1980 December 9, (Buffoni et al. 1982; Branham 2013). Comet A117uUD was found recently. Here, we show that although the hyperbolic excess of its current path is statistically significant, it did not come from interstellar space.

Comet A117uUD was discovered by the Asteroid Terrestrial-impact Last Alert System (ATLAS, Tonry et al. 2018) on 2024 June 14. This early warning system consists of four telescopes, comet A117uUD was found by ATLAS South Africa and immediately included in the Minor Planet Center (MPC, Rudenko 2016) Possible Comet Confirmation Page (PCCP). ³ As of 2024 July 17, there are 145 observations of A117uUD spanning 31.39 days. The orbit determinations available from the MPC argue for an orbital eccentricity barely below 1.0, 0.99998, but the weighted rms residual for their best fitting orbits is 091. However, with a month's worth of modern observations it should be half this value. The Jet Propulsion Laboratory (JPL) Scout ⁴ system (Farnocchia et al. 2016a, 2016b) independently processes the data submitted to the MPC, using systematic ranging for initial orbit determination (Farnocchia et al. 2015), to generate a grid of possible orbits for each object available from the MPC. The grid can be downloaded using the Python package NasaPy ⁵ and consists of 1000 orbits that in the case of A117uUD are based on 142 observations (bad quality observations are discarded). From the grid, the median values and 16th and 84th percentiles of the heliocentric orbital elements are: perihelion distance, q = 3.432 ± 0.003 au, eccentricity, e = 1.037 ± 0.002, inclination, $i=166\buildrel{\circ}\over{.} {5730}_{-0.0012}^{+0.0011}$, ${\rm{\Omega }}=139\buildrel{\circ}\over{.} {166}_{-0.015}^{+0.013}$, and $\omega =290\buildrel{\circ}\over{.} {54}_{-0.08}^{+0.09}$. ⁶ This orbit determination has a normalized rms of 0.48 with a statistical significance of its hyperbolic nature as high as 19.51σ. An independent orbit determination available from Bill Gray's Project Pluto site ⁷ gives: q = 3.432 ± 0.002 au, e = 1.038 ± 0.002, i = 1665732 ± 00009, Ω = 139168 ± 0011, and ω = 29052 ± 007. This orbit determination is fully compatible with Scout's and its accompanying analysis shows a mean residual of 033; the associated ephemerides match the available observations well enough. In other words, the stated uncertainties correctly describe both random and systematic error contributions; therefore, probabilities derived from using such orbits as initial conditions for N-body simulations should lead to reliable conclusions regarding its past and future dynamical evolution. However, the quoted uncertainties are still large enough to require a statistical analysis of the likelihood of an origen in the solar system or a possible arrival from interstellar space (e.g., de la Fuente Marcos et al. 2018). In the following, we used publicly available input data from JPL's solar system Dynamics Group Small-Body Database ⁸ and horizons ⁹ on-line solar system data and ephemeris computation service (Giorgini 2015), which use the new DE440/441 solution (Park et al. 2021).

In order to assess statistically the possible origen of comet A117uUD and its future evolution, we performed integrations backward and forward in time using Scout's grid as control or clone orbits to generate initial positions and velocities (1000 orbits) that were evolved dynamically using a direct N-body code (Aarseth 2003) that implements the Hermite integration scheme described by Makino (1991). ¹⁰ Additional details of the simulations discussed here can be found in de la Fuente Marcos & de la Fuente Marcos (2012, 2019).

Our results for the past evolution of comet A117uUD are summarized in Figure 1. The comet experienced a very close encounter with Saturn in 2022 that makes it difficult to reconstruct the pre-encounter orbit. The top-left panel displays the minimum approach distance (color-coded) to Saturn during the flyby for each orbit—(q, e) values—in the grid. Values under 0.001 au were recorded; the Hill radius of Saturn is 0.412 au. The top-right panel shows the orbital eccentricity (color-coded) of comet A117uUD prior to its flyby with Saturn for the same grid of orbits. The bottom-left panel shows the histogram of calendar dates for this flyby with a median value close to 2022 January 24 00:00. All the orbits in the grid experience this encounter well under the Hill radius of the planet (the largest separation is slightly above 0.01 au). The bottom-right panel shows the histogram of the pre-encounter eccentricities and the median value is 0.876 with relevant percentiles 0.755 (16th) and 1.613 (84th); the probability of having an e prior to the encounter equal to or higher than that of 'Oumuamua, 1.20113, is 0.265, for that of 2I/Borisov, 3.35648, is 0.001. As for its future evolution, all the orbits lead to escaping from the solar system with a velocity close to 3 km s⁻¹.

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Our results show that the case of comet A117uUD is similar to that of C/1980 E1 (Bowell), disfavoring an extrasolar origen for A117uUD. The fact that two ejections after planetary encounter were observed in less than 45 yr suggests that such events are relatively frequent. Its pre-flyby orbit resembles those of retrograde Centaurs or comets.

We thank S. J. Aarseth for providing the code used in this research. We thank A. I. Gómez de Castro for providing access to computing facilities. This work was partially supported by the Spanish "Agencia Estatal de Investigación (Ministerio de Ciencia e Innovación)" under grant PID2020-116726RB-I00 /AEI/10.13039/501100011033.