While the interstellar objects (ISOs) represent a unique and unprecedent opportunity to study the physical properties and composition of extrasolar matter in detail, the discovery of the third such kind object has raised interest in a new aspect: orbital dynamics. Unlike its predecessors, 1I/‘Oumuamua and 2I/Borisov, which traversed the Solar System on high-inclination trajectories that kept them largely clear of planetary population fields, 3I/ATLAS (alternative, C/2025 N1) presents a far more complex interaction profile. 3I/ATLAS has provided the first quantitative look at the "encounter environment" for a low-inclination ISO.

Figure 1. Diagram showing the trajectory of comet 3I/ATLAS through inner part of Solar System. Credit: ESA NEOCC

Its retrograde nearly ecliptic orbit (i ≈ 175°) combined with a relatively small perihelion distance (q ≈ 1.36 AU) effectively directs the interloper "against traffic" through the densely populated regions of the inner Solar System. This significantly increases the probability of close encounters and potential interaction (including collision) with native objects, specifically Main Belt and near-Earth asteroids. Researchers from Shanghai Astronomical Observatory, together with colleagues from Shanghai Jiao Tong University and the Main Astronomical Observatory of Ukraine, investigated this distinct dynamical scenario with publishing the results in the Astronomical Journal on January 19, 2026.

A systematic N-body numerical integration covering the period from August 1, 2025, to April 1, 2026 quantifies the risk of direct collisions and assesses the potential impact probabilities. By filtering the trajectories of over 38,000 near-Earth asteroids (NEAs) and 1.4 million Main Belt asteroids (MBAs), the team identified a considerable number of close approaches. The study reveals that 31 NEAs and 736 MBAs come within a physical distance of ≤ 0.03 AU of 3I/ATLAS (Figure 2). This high frequency of encounters highlights the importance of low-inclination ISOs for the detailed study of dynamical interactions between extrasolar and native matter.

While the nominal orbits of the identified asteroids did not predict direct collisions, their orbital uncertainties play a critical role in assessing impact risks. This is demonstrated by the exceptional case of Main Belt Asteroid 2020 BG107. Due to a short observational arc, the asteroid has a highly uncertain orbital solution (Uncertainty Parameter U = 7). Consequently, its 3-sigma "uncertainty ellipsoid" (the region where it might actually be) at the time of the identified close encounter (≈ 0.019 AU) was larger than the nominal approach distance to 3I/ATLAS (≈ 0.0023 AU). Through rigorous Monte Carlo (MC) simulations involving 100,000 orbital clones, the authors estimated a nucleus impact probability of approximately 0.025% (Figure 3). Perhaps more intriguingly, when accounting for the interstellar object’s cometary nature, the probability of the asteroid passing through 3I/ATLAS’s dust coma rose to 2.7%.

Unfortunately, the identification of this specific case was derived post-facto (the actual solution was obtained on August 21, 2025, a few days after the predicted close approach on August 17, 2025), highlighting the importance of timely analysis to strengthen observational efforts to confirm or rule out predicted impacts. Nevertheless, had such an event occurred, would have been a "natural" hypervelocity experiment, offering insights into ISO (and asteroid) internal structure comparable to controlled missions like DART, but at significantly higher energy levels due to the retrograde (in the case of 3I/ATLAS) relative velocity. Overall, the results demonstrate that physical interactions between ISOs and native bodies are statistically possible events.

Furthermore, the research team suggests that implications of this work extend beyond 3I/ATLAS, or ISOs in general. The results can be extrapolated to dynamically new objects (those making their first perihelion passage), particularly those from the Oort Cloud which may share similar low-inclination primordial orbits. As the authors conclude, this research provides a "methodological framework for the proactive analysis of future ISOs and dynamically new comets." With the Vera C. Rubin Observatory (LSST) expected to significantly increase the discovery rate of such objects, the ability to rapidly identify and monitor potential close encounters will be essential. This work demonstrates that the inner Solar System is a busy intersection, and with the right orbital geometry, physical interactions between newcomers and native bodies are a statistically viable phenomenon worthy of dedicated observational campaigns.

The international collaboration developing a Global Observation Network, led by Dr. Mao Yindun's research team of Shanghai Astronomical Observatory, has been engaged in long-term research on NEAs, with a special focus on high-precision astrometry of fast-moving objects and collision assessments for potentially hazardous asteroids (PHAs). Over the past several years, the research group performed specialized observations with originally developed methods to solve one of the biggest challenges: accurately tracking fast-moving objects during close approaches to Earth. This capability is vital for newly discovered NEAs; without immediate, high-quality astrometry to extend the observational arc, these objects risk becoming 'lost' before their orbits can be reliably determined for future apparitions. The team’s results demonstrate that their specialized techniques allow even small-aperture telescopes to effectively compensate for fast apparent motion, yielding the precise data needed to secure these orbits.

link: https://iopscience.iop.org/article/10.3847/1538-3881/ae2ea6

Scientific Contact Person:

Yindun Mao, dundun@shao.ac.cn

Anton Pomazan, antpomaz@shao.ac.cn