VLT and Keck recovery and characterisation of 2025 SC$_{79}$: linking 'Ayló'chaxnim and Atira asteroids

Abstract

Asteroids near the boundary between the Atira and 'Ayló'chaxnim classes can trace dynamical links between the two populations, but their small solar elongations make recovery and physical characterisation difficult. We recover 2025 SC$_{79}$, measure its optical colours, and investigate its likely source region and transitions between these populations. We combine Keck/LRIS astrometry and VLT/FORS2 four-filter photometry with Bus-DeMeo taxonomic comparisons, published source-model probabilities, and integrations of 10,000 clones drawn from the covariance of JPL solution #2 and followed for 5 Myr in each direction. The SDSS-equivalent colours are $g-r=0.8254_{-0.1339}^{+0.2136}$, $r-i=0.1764_{-0.0540}^{+0.0523}$ and $i-z=-0.4975_{-0.1162}^{+0.1055}$ mag. The V-type class mean gives the closest colour match. At the V-type median albedo $p_V=0.362$, $H=18.83\pm0.15$ mag implies a diameter of $0.379^{+0.027}_{-0.025}$ km. The source model assigns a combined probability of 95.4% to delivery channels associated with the inner Main Belt. The ensemble-mean 'Ayló'chaxnim/Atira residence fractions are 30.2%/69.8% over the preceding 100 kyr and 3.6%/96.0% over the following 100 kyr; the remaining 0.4% of future ensemble time follows collision removal. Clones spend an average of 231 kyr in the 'Ayló'chaxnim class over the preceding 500 kyr and 120 kyr over the following 500 kyr. In the 5-Myr forward integrations, 14.56% of clones collide with Venus, 3.77% with Mercury and 0.22% with Earth. The V-like colours and source-model probabilities favour an origin in the inner Main Belt for 2025 SC$_{79}$. Its clones move between the 'Ayló'chaxnim and Atira populations.