A Two-month Mini-moon: 2024 PT₅ Captured by Earth from September to November

Earth can regularly capture asteroids from the Near-Earth object (NEO) population and pull them into orbit, making them mini-moons. Sometimes, these temporary captures do not complete one revolution before dropping out of orbit and returning to their regular heliocentric trajectories. Following G. Fedorets et al. (2017), temporarily captured flybys never complete one revolution around Earth, while temporarily captured orbiters complete one or more. Examples of the latter were 2006 RH₁₂₀ that remained gravitationally bound to Earth from 2006 July to 2007 July (T. Kwiatkowski et al. 2009) and 2020 CD₃ that escaped early in 2020 May after being bound to Earth for several years (B. T. Bolin et al. 2020; C. de la Fuente Marcos & R. de la Fuente Marcos 2020; G. Fedorets et al. 2020; S. P. Naidu et al. 2021). As for the temporarily captured flybys, two examples are known: 1991 VG was briefly captured in 1992 February (G. Tancredi 1997; C. de la Fuente Marcos & R. de la Fuente Marcos 2018) and 2022 NX₁ was a short-lived mini-moon of Earth in 1981, 2022, and it will return as such in 2051 (C. de la Fuente Marcos & R. de la Fuente Marcos 2022; R. de la Fuente Marcos et al. 2023). The recently discovered Apollo-class NEO 2024 PT₅ follows a path that resembles that of 2022 NX₁ and may soon become a mini-moon.

Asteroid 2024 PT₅ was discovered on 2024 August 7 by the Asteroid Terrestrial-impact Last Alert System (J. L. Tonry et al. 2018) observing with the instrument located in Sutherland, South Africa (J. Tonry et al. 2024). It has H = 27.6 ± 0.3 mag and, with a size of ∼10 m, is larger than 2020 CD₃, 2006 RH₁₂₀, 1991 VG, and 2022 NX₁. As of 2024 August 30, its heliocentric orbit determination (based on 122 observations spanning 21 days) is: semimajor axis, a = 1.0123051 ± 0.0000002 au, eccentricity, e = 0.02147672 ± 0.00000005, inclination, i = 152051 ± 000004, longitude of the ascending node, Ω = 3055722 ± 00007, and argument of perihelion, ω = 1162485 ± 00002. ³ Such orbital elements are consistent with those of the Arjunas, a sparsely resonant population of small NEOs (see, e.g., C. de la Fuente Marcos & R. de la Fuente Marcos 2013, 2015) in a secondary asteroid belt found surrounding the path followed by the Earth–Moon system (D. L. Rabinowitz et al. 1993).

Figure 1 summarizes our results and provides an early assessment of the current status, and short-term past and future orbital evolution of 2024 PT₅. Our N-body calculations were carried out as detailed in C. de la Fuente Marcos & R. de la Fuente Marcos (2012), using input data from Jet Propulsion Laboratory's Small-Body Database ⁴ and HORIZONS ⁵ on-line solar system data and ephemeris computation service (J. D. Giorgini 2015), which are based on the DE440/441 planetary ephemeris (R. S. Park et al. 2021). Our computations were completed using software developed by S. J. Aarseth (2003) ⁶ that implements the Hermite integration scheme (J. Makino 1991).

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Figure 1, top, shows the evolution of the relevant resonant angle or difference between the mean longitude of the NEO and that of Earth, λ_r (C. D. Murray & S. F. Dermott 1999), whose value is currently oscillating about 180°. Therefore, 2024 PT₅ is a transient co-orbital of the horseshoe type like 1991 VG (C. de la Fuente Marcos & R. de la Fuente Marcos 2018) or 2022 NX₁ (C. de la Fuente Marcos & R. de la Fuente Marcos 2022; R. de la Fuente Marcos et al. 2023). In the figure, we show the evolution of the nominal orbit and those of control orbits with state vectors (Cartesian coordinates and velocities) well away from the nominal one. Figure 1, top, also shows at the bottom the evolution of the geocentric energy, for which negative values signal capture. Figure 1, top-right, is a magnified version of the top-left panels focusing on the time interval (−1,  1) yr. It shows that 2024 PT₅ will become a mini-moon of Earth on 2024 September 29 (20:02), to return to a heliocentric path 56.6 days later, on November 25 (10:33). Figure 1, bottom-left, shows the evolution of the difference between the semimajor axis of 2024 PT₅ and that of Earth, a_r, as a function of λ_r around the epoch of the mini-moon episode. Figure 1, bottom-center and -right panels, shows the geocentric motion during (−1, 1) yr. All the control orbits up to ±9σ from the nominal one confirm that 2024 PT₅ is currently following a horseshoe path and that its geocentric energy will remain negative for 56.6 days due to a temporarily captured flyby. After completing the mini-moon episode, it will approach Earth at ∼1 km s⁻¹, reaching a minimum distance of 0.012 au on 2025 January 9, leaving the neighborhood of Earth shortly afterwards, until its next return in 2055. Differences in the evolution of the control orbits appear beyond 60 yr into the past, but also after 30 yr into the future; close encounters with the Earth–Moon system are the cause (in 1960 and 2055, respectively).

The evolution of all the control orbits indicate that 2024 PT₅ will experience a temporarily captured flyby in 2024, from September 29 until November 25. The object is unlikely to be artificial as its short-term dynamical evolution closely resembles that of 2022 NX₁, a confirmed natural object, see Figure 3 of R. de la Fuente Marcos et al. (2023).

We thank S. J. Aarseth for providing the code used. This work was supported by the Spanish "Agencia Estatal de Investigación (Ministerio de Ciencia e Innovación)" under grant PID2020-116726RB-I00 /AEI/10.13039/501100011033.