Investigation of the dynamics and origin of the NEA pair 2021 PH27 and 2025 GN1

TL;DR

This study investigates the origin and past dynamics of the near-Earth asteroid pair 2021 PH27 and 2025 GN1. By combining independent X-type spectral confirmation with backward dynamical integrations that include relativistic corrections, it tests several separation scenarios. The results disfavour tidal fragmentation from Venus or the Sun and favour a rotational-disintegration/ex-binary pathway, with a recent separation during a low-perihelion phase and a probable accompanying meteoroid stream that could generate a Venus atmospheric meteor shower. The findings illustrate how YORP-driven binary formation and subsequent dissolution can produce NEA pairs with closely matching orbits and physical properties, and they highlight observational opportunities to search for related meteoroid material on Venus’s orbit.

Abstract

We analyse the association between the NEAs 2021 PH27 and 2025 GN1, which share similar heliocentric Keplerian elements and the same taxonomic classification. First, we confirm the spectral similarity by getting independent colours measurements of 2025 GN1 and confirming that they are both X-type. From numerical integration of the orbits up to 100 kyr in the past, taking into account relativistic corrections, we found that the two asteroids experienced five similar flybys with Venus, but none of them were closer than the Roche limit. The perihelion distance also reached values between 0.1 and 0.08 au about 17/21 kyr and 45/48 kyr ago, but still well outside the Roche limit with the Sun. So, the origin of the pair by tidal disruption of a progenitor rubble-pile asteroid appears unlikely. On the other hand, we found periods lasting several thousand years where the perihelion was below 0.1 au, and this can lead to thermal fracturing of the surface. We found that the rotation period of the primary and the mass ratio secondary/primary make the pair indistinguishable from the binary systems known among the NEAs, and the YORP effect can double the rotation period of 2021 PH27 in $150 \pm 50$ kyr. So it is plausible that the pair was formed by the rotational disintegration of a rubble-pile asteroid due to anisotropic gas emission or the YORP effect, which formed a binary system that later dissolved due to the internal dynamics of the pair. We are unable to give a value for the separation age; we can only say that it occurred more than 10.5 kyr ago and may have occurred between 17/21 kyr ago during the last and longer phase of lower perihelion distance. In this scenario, little meteoroids released in space due to the fragmentation event are still near the pair's orbit and can generate a meteor shower in Venus' atmosphere.

Figures (7)

• Figure 1: Plot of the primary rotation periods (when known) as a function of the mass ratio for the NEA pairs listed in Table \ref{['tab:NEA_physical_data']}. The uncertainty of the mass ratio is computed from Eq. \ref{['eq:mass_ratio']} assuming a 0.1 mag uncertainty over the absolute magnitude given by the Minor Planet Center. The continuous curve, which is purely indicative and is not a best fit, is an example of the predictions of the post-separation model fission from Pravec2010 with asteroid's parameters shape $a_1=1$, $b_1=0.8$, $c_1=0.2 b_1$; mean density $\rho = 1000~\text{kg}~\text{m}^{-3}$ and starting orbit semimajor-axis of the satellite $A_{ini}=2$. The rotation period value of 2015 EE7 is uncertain.
• Figure 2: Colours of 2025 GN1 obtained from observations of the Gemini South 8.1 m telescope on 16 April 2025. Template spectra for B-, C-, X-, S-, and V-type asteroids are also reported for comparison, and for each reflectance spectrum, the variation within the taxonomic class is indicated DeMeo2013.
• Figure 3: Visual magnitude (blue curve) and solar elongation (orange curve) of 2021 PH27 (top panel) and 2025 GN1 (bottom panel) until 1 January 2028. The green shaded areas correspond to visibility windows, identified by visual magnitude smaller than 22 and solar elongation larger than 30 deg in absolute value.
• Figure 4: Orbital elements of clones of 2021 PH27 (black), with nominal orbit in green, and of 2025 GN1 (grey), with nominal orbit in blue.
• Figure 5: On the left panel, distance of the recorded close encounters with Venus, as a function of time. Blue dots are close encounters registered for 2021 PH27, while the red dots are those for 2025 GN1. On the right panel, a scatter plot of close encounters between 2021 PH27 and 2025 GN1, in the plane of close approach year and close approach distance. The colour is proportional to the relative velocity of the close encounter.