Meet Arjuna 2025 PN₇, the Newest Quasi-satellite of Earth

Finding a new class of objects always prompts diverse and sometimes speculative hypotheses that may or may not be confirmed later on when more data become available. The discovery of the first asteroid in an Earth-like orbit, 1991 VG, in 1991 December by the Spacewatch Project was no exception. The fact that it moved close to our planet, in a similar path, was then used to argue that it was a candidate interstellar space probe (see, e.g., discussion in C. de la Fuente Marcos & R. de la Fuente Marcos 2018). Over three decades later, it is now widely accepted that such objects are natural and constitute a secondary asteroid belt that occupies the region in which the Earth–Moon system orbits around the Sun, defining the Arjuna dynamical class (see, e.g., D. L. Rabinowitz et al. 1993; C. de la Fuente Marcos & R. de la Fuente Marcos 2013). The Arjunas with the most Earth-like orbits can experience temporary captures as mini-moons of our planet (see, e.g., T. Kwiatkowski et al. 2009; C. de la Fuente Marcos & R. de la Fuente Marcos 2020; G. Fedorets et al. 2020; R. de la Fuente Marcos et al. 2023). The quasi-satellites are objects which are capable of longer engagements with Earth, trapped in a 1:1 mean-motion resonance as their mean longitudes relative to Earth, λ_r = λ − λ_⊕ oscillate around the value 0°—where λ and λ_⊕ are the mean longitudes of the object and Earth, respectively, λ = M + Ω + ω, where M is the mean anomaly, Ω is the longitude of the ascending node, and ω is the argument of perihelion (see, e.g., C. D. Murray & S. F. Dermott 1999). Quasi-satellites are in a resonant orbit but are not gravitationally bound to Earth, allowing for more sustained, though unbound, proximity; while mini-moons are characterized by temporary gravitational captures by Earth, meaning they are gravitationally bound, albeit for a limited time. The list of known current quasi-satellites of Earth includes 164207 Cardea (2004 GU₉), 469219 Kamo‘oalewa (2016 HO₃), 277810 (2006 FV₃₅), 2013 LX₂₈, 2014 OL₃₃₉, and 2023 FW₁₃ (see, e.g., T. Wiegert et al. 2005; P. Wajer 2010; M. Connors 2014; C. de la Fuente Marcos & R. de la Fuente Marcos 2014, 2016; S. Hu et al. 2023). Here, we present the newest member of this class, 2025 PN₇.

Apollo asteroid 2025 PN₇ was discovered on 2025 August 2 by Pan-STARRS 1, Haleakala.³ It has H = 26.4 ± 0.3 mag and, as of 2025 August 31, its heliocentric orbit determination (based on 27 observations spanning 4279 days) is: semimajor axis, a = 1.003019631 ± 0.000000013 au, eccentricity, e = 0.10750419 ± 0.00000014, inclination, $i=1\mathop{.}\limits^{^\circ }979552\pm 0\mathop{.}\limits^{^\circ }000003$, longitude of the ascending node, ${\rm{\Omega }}=112\mathop{.}\limits^{^\circ }58072\pm 0\mathop{.}\limits^{^\circ }00006$, and argument of perihelion, $\omega =81\mathop{.}\limits^{^\circ }04210\pm 0\mathop{.}\limits^{^\circ }00004$. Such orbital elements are consistent with those of the Arjunas. The Arjunas are a group of near-Earth asteroids with orbits that are Earth-like that have (i) a ∈ (0.985, 1.013) au (very close to Earth’s) (ii) Low eccentricity (e < 0.1, nearly circular orbits), and (iii) Low inclination i < 8° (close to Earth’s orbital plane).

In our analysis, we use data computed by Jet Propulsion Laboratory’s (JPL) Horizons on-line solar system data and ephemeris computation service⁴  (J. D. Giorgini 2015). Data queries were made via the Python package Astroquery (A. Ginsburg et al. 2019). Our input data sample was retrieved from JPL’s Small-Body Database (SBDB).⁵ It includes 402 objects with semimajor axis a ∈ (0.985, 1.015) au and eccentricity e < 0.5 (see Figure 1, bottom panel), 207 have e < 0.2. Statistical analyses were carried out using NumPy (C. R. Harris et al. 2020) and visualized using the Matplotlib library (J. D. Hunter 2007).

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The top panels in Figure 1 show the short-term evolution of λ_r for the six previously known quasi-satellites of Earth and 2025 PN₇, and confirm its resonant nature. Its evolution is comparable to the one of Kamo‘oalewa but its time of residence in the quasi-satellite state is shorter, 128 yr versus 381 yr. Only 2013 LX₂₈, 2014 OL₃₃₉, and 2023 FW₁₃ are not affected by the Yarkovsky effect (W. F. Bottke et al. 2006). The bottom panel in Figure 1 shows the median value of λ_r in the year range 2000–2050 (${\lambda }_{{\rm{r}}}^{* }$) as a function of the equivalent value for the semimajor axis (a^*). Known quasi-satellites reside inside the innermost section of the Arjuna belt, 2025 PN₇ appears close to Cardea in Figure 1, bottom panel. True Arjunas have short error bars in a^*.

The available data indicate that 2025 PN₇ is a relatively short-lived quasi-satellite of Earth and its short-term evolution resembles the one of Kamo‘oalewa although it is found close to Cardea in the (a^*, ${\lambda }_{{\rm{r}}}^{* }$) diagram.