Dynamical simulation of the Interstellar Comet 3I/ATLAS

TL;DR

This study uses REBOUND N-body simulations with the IAS15 integrator to explore the past and future dynamical evolution of interstellar comet 3I/ATLAS, including gravitational perturbations from Mars and Jupiter and plausible non-gravitational forces from outgassing. A 500-clone ensemble drawn from the initial covariance assesses uncertainties, while high-resolution 1-hour steps are used near planetary encounters. Results show Jupiter’s close approach dominates perturbations, Mars being comparatively minor, and substantial sensitivity of the trajectory to non-gravitational accelerations, especially for $A_1$ and $A_2$ roughly in the $10^{-5}$–$10^{-6}$ au day$^{-2}$ range. The long-term outlook places the object on a path toward Gemini after a past origin in the Galactic disk, with NGFs potentially altering future predictions; the work highlights the need for NGF constraints and identifies a targeted Juno-observation window in March 2026 to capture critical dynamical signatures.

Abstract

Comet 3I/ATLAS, also known as C/2025 N1, was discovered on 2025 July 1 UT by NASA Asteroid Terrestrial-impact Last Alert System (ATLAS), with a v$_{\infty}$ $\sim$ 58 kms$^{-1}$. This is the fastest among the three interstellar objects discovered so far. In this work, we study the interaction of the 3I/ATLAS with Mars, pre-perihelion, and Jupiter post-perihelion. We also present the results of the dynamical simulations of the orbital evolution of the comet for a hundred years in the past and future. For our analysis, we have used REBOUND, an N-Body simulation package, to study these situations. We use the adaptive size mathematical integrator \textsc{Ias15}, with a 1-day time step for long-term integration, and a 1-hour time step to study the effect of the planets on this body during the close encounters. We have seen an effect of Jupiter on the orbital parameters of the comet, which affects its post-perijove trajectory significantly. The impact of Mars on this comet is minimal compared to the effect of Jupiter. This is consistent with the point that the comet moves well past Mars's Hill radius but very close to Jupiter's Hill radius at the respective close approaches. However, the effect of non-gravitational forces will alter the results. Since the non-gravitational forces are not yet known, we predict the variation of the orbital parameters considering a range of possible magnitudes of the non-gravitational acceleration.

Figures (6)

• Figure 1: Left: Close encounters between interstellar comet 3I/ATLAS with Mars at Epoch JD 2460951.5 (2025 October 03). Right: Close encounters between interstellar comet 3I/ATLAS with Jupiter at Epoch JD 2461115.5 (2026 March 16). For clarity, we show only the planets' orbits(up to Jupiter), whereas, in the simulation, we have also considered the three other planets (Saturn, Uranus, Neptune), the four big asteroids (Ceres, Pallas, Juno, Vesta) in the asteroid belt, and five KBOs (Pluto, Makemake, Haumea, Quaoar, Sedna).
• Figure 2: Variation of Orbital Parameters and the Cartesian velocity in z-direction. In each panel, the gray curves are for the simulation without considering interactions with Mars and Jupiter, whereas the blue curves include these planets. The vertical red and black dashed lines represent the date at which the comet is close to Mars and Jupiter. The vertical dotted lines in indigo represent the time of perihelion. Here, we plotted the behavior of 50 clones to keep the image size manageable.
• Figure 3: The distance from comet to Juno spacecraft, $d_{Juno}$, and Jupiter $d_{Jupiter}$, are plotted in blue and red curves respectively in log scale. The black dashed line is the time of the interaction with Jupiter. The masked region is for the optimal observation when the comet 3I will be at 0.4 au or below from the Juno spacecraft with a solar elongation between 28$^{\circ}$ to 100$^{\circ}$.
• Figure 4: Variation of Orbital Parameters and the Cartesian velocity in z-direction. In each panel, the orange curves represent non - gravitational accelerations of 10$^{-5}$ auday$^{-2}$, the green, brown and navy blue curves represent non - gravitational accelerations of 10$^{-6}$ auday$^{-2}$, 10$^{-7}$ auday$^{-2}$, and 10$^{-8}$ auday$^{-2}$. Here, we plotted the behavior of 50 clones to keep the image size manageable.
• Figure 5: Histogram of 500 cometary clones in the presence of different forces.