Mars Orbit Anomaly of Interstellar Object 3I/ATLAS between 9 October and 25 November 2025

Draft version December 19, 2025 Typeset using L A T EXdefault style in AASTeX7.0.1 Mars Orbit Anomaly of Interstellar Object 3I/ATLAS between 9 October and 25 November 2025 Andreas Martin Lisewski 1 1School of Science, Constructor University, 28759 Bremen, Germany ABSTRACT Interstellar cometary object 3I/ATLAS moved into the inner solar system where, after a planetary passage near Mars at a distance of 0.193 AU, it had two distinct approaches to the Martian osculating orbit on the 9 October 2025 and the 25 November 2025, with distances 0.129 AU and 0.018 AU, respectively. Here, by analyzing JPL/Horizons astrometric data, it is shown that between these two dates 3I/ATLAS was on an anomalous trajectory with exceptional proximity, radial speed, and alignment to the planetary orbit of Mars. Conditional on the 9th October passage, these data consistently result in a probability of less than 2 in a million that 3I/ATLAS was on a random course from an isotropic distribution before the tight orbital approach at 0.018 AU. This estimate suggests a recurrence time of interstellar objects with a comparable Mars orbit anomaly of the order of 100 million years, and that the timing of 3I/ATLAS discovery might not had been accidental. Keywords: Interstellar objects (52) —Exocomets (2368) —Comet dynamics (2213) 1. INTRODUCTION Object 3I/ATLAS (C/2025 N1) was discovered on 1 July 2025 by the Asteroid Terrestrial-impact Last Alert System (ATLAS) survey as a high velocity cometary body on an inbound trajectory toward the inner solar system (D. Z. Seligman et al. 2025). It rapidly moved toward the solar system’s habitable zone between the orbits of Mars and Earth with perihelion on the 30 October 2025 at 1.36 AU. Observational data put 3I/ATLAS on a hyperbolic, retrograde orbit, with an escape velocity of v∞≈58 km/s and a remarkably small inclination at 4.9°, and therefore close to 3°in complement to the Mars orbital plane. Its rise in activity and change in morphology toward perihelion was not unlike that of other comets (T. M. Eubanks et al. 2025), but considerable uncertainties and questions remain regarding its galactic origin, tail and coma structure, chemical composition, as well as basic physical parameters such as size and mass (A. D. Feinstein et al. 2025;A. Loeb 2025; E. Keto & A. Loeb 2025). 3I/ATLAS movement, structure, and evolution have therefore been a recent research focus both in solar system astronomy and in the context of planetary defense. Here, by computational analysis of JPL/Horizons (J. D. Giorgini 2015) ephemeris data2, it is shown that between 9 October and 25 November 2025, i.e. following Mars passage and around three weeks before and after perihelion, 3I/ATLAS was on an anomalous trajectory. The precise physical nature of the anomaly is presented and its quantitative extent estimated. 2. 3I/ATLAS MARS ORBIT ANOMALY Mars passage of 3I/ATLAS occurred during 3 October 2025 (UTC) at a minimum distance of 0.290 ×108km to the planet and with a negative radial speed of −85 km/s relative to 3I/ATLAS during its post-detection Mars approach (Figure 1A and B). On 9 October 2025, a different fly-by occurred near the Mars orbit at d1= 0.193 ×108km between 3I/ATLAS and the closest point on Mars’ osculating orbit (M1 in Figure 1A). Point M1 represented the beginning of a ∆t12 = 47 Email: [email protected]ersity 2Throughout, JPL/Horizons astrometric data were accessed through its application programming interface (API) available through protocol requests of the type https://ssd.jpl.nasa.gov/api/horizons.api; see A. M. Lisewski (2025) for data and software. 2 0 1 0.027 2 3 4 5 Distance (10 km) Distance 3I/ATLAS - Mars orbit Distance 3I/ATLAS - Mars 50 0 50 Radial speed (km/s) [Dist. 3I/ATLAS-Mars orbit] / 2025-08-01 2025-09-01 2025-10-01 2025-11-01 2025-12-01 Date (UTC) 0 20 40 60 Velocity (km/s) M1 2025-10-09 2025-10-30 2025-11-01 2025-11-25 M2 M0 P0 0.4 0.5 0.6 0.7 0.8 0.9 1.0 Ratio 3I/ATLAS heliocentric velocity v 3I/ATLAS - Mars orbit component vII II t [Dist. 3I/ATLAS-Mars] / t Ratio v / v 8 1.5 1.0 0.5 0.0 0.5 1.0 1.5 X (AU, ecliptic J2000) 3 2 1 0 1 Y (AU, ecliptic J2000) M1 (2025-10-09) M2 (2025-11-25) M0 (2025-11-01) P0 (2025-10-30) Mars osculating orbit Mars trajectory 3I/ATLAS trajectory Mars 2025-08-01 Mars 2025-12-01 3I/ATLAS 2025-08-01 3I/ATLAS 2025-12-01 Sun 0.193 A B c c Figure 1. (A). Ecliptic plane (X,Y/J2000) coordinates of 3I/ATLAS and Mars positions between 1 August 2025 and 1 December 2025; Mars full osculating orbit as reference. M1 and M2 indicate closest 3I/ATLAS orbital approaches near perihelion (P0) and along local maximum M0. (B). Astrometric distances, radial speeds, and heliocentric velocities with (UTC) time points of M1, M2, M0, and P0. ∆t= 1d; see A. M. Lisewski (2025) for deposited data and program code. day period which ended at point M2, on 25 November 2025, with a second passage near the Mars osculating orbit at a distance d2of only 2.7 million kilometers, or 0.018 AU (Figure 1B). During this time interval, another distinct feature (Figure 1B) was the broadly extended maximum around d0= 0.426 ×108km at point M0, on 1 November 2025 and 2 days past perihelion P0, which lead to a remarkably gradual approach with average radial speed s12 = (d2−d1)/∆t12 ≈ −4 km/s. At M0, 3I/ATLAS precisely aligned its heliocentric velocity vector with the retrograde Mars orbit tangent at a ratio of v||/vc= 0.997; here, vc≈68 km/s is the total heliocentric velocity (vector norm) and v|| its component along the retrograde tangent unit vector at the closest point on the Mars osculating orbit (Figure 1B). This retrograde alignment did not coincide with the crossing of the Mars orbital plane which occurred on the 19 November 2025. As the direct distance between 3I/ATLAS positions M1 and M2 was D12 = 2.748 ×108km ≈1.840 AU, a disk with a radius d2at a distance D12 > d2encircles a solid angle of Ω=2π 1−s1−d2 D12 2 ≈3.1×10−4sr These data allowed a provisional estimate of the orbital anomaly with a (null model) test particle initially at the same M1 position as 3I/ATLAS, but with a randomly oriented, from an isotropic distribution, and constant magnitude velocity equal to vc= 68 km/s. The test particle traverses D12 in ∆t12, with a line-of-sight approximation for the nearly linear arc of the hyperbolic orbit (see, Figure 1A.) Its orbit crossing velocity parallel to the orbital plane is the projection vccos θat an acute angle 0 ≤θ≤π/2. With the observed 3I/ATLAS value of 3◦≈0.05 rad for θ, this results in a probability p′ a=2θ π 1−s1−d2 D12 2 ≈1.6×10−6 3 of a second Mars orbit approach within target distance d2and at θ < 0.05 rad. To cross-validate p′ awith an alternative method, a Monte Carlo statistical sampling method (A. M. Lisewski 2025) produced test values (D12, d2,s12) from 107randomly oriented test trajectories based on time-resolved Mars orbit and 3I/ATLAS M1 positional data. To obtain a probability estimate pa, this procedure recorded the number of trajectories that simultaneously met the three observed Mars orbit conditions from 3I/ATLAS, (1) d2/D12 <0.01,(2) −0.06 < s12/vc≤0,and (3) θ < 0.05 rad, which resulted in a fraction pa= 19/107= 1.9×10−6≈p′ a. This relative numerical consistency between both probabilities paand p′ arepresents a statistical anomaly: the null hypothesis—that from the first (M1) Mars osculating orbit passage to the second (M2) orbital approach 3I/ATLAS was on a randomly set course—is rejected at 99.9998% confidence level. 3. CONCLUSIONS 3I/ATLAS astrometric data make highly improbable, with an estimated probability of pa≈10−6, the hypothesis that after its first Mars fly-by 3I/ATLAS had been on a randomly oriented trajectory, i.e. a local trajectory drawn from an isotropic distribution. For instance, such null model isotropy would mirror long-period comets which are considered isotropic with orbits inclined at random angles to the ecliptic plane. Theoretically, interstellar objects might develop anisotropy after ∼100 million years of random motion (J. C. Forbes et al. 2025), but thus far the observed sample of such objects has been not large enough to test this prediction. It is noted that pais a conservative estimate on the anomaly in the sense that it is already conditional on the first and close orbital passage on 9 October 2025. As such, additional constraints (conditionals or priors) may further lower the resulting probability and strengthen the anomaly. These could include the high level of retrograde alignment (0.997 ratio) around 1 November 2025 of 3I/ATLAS velocity tangential to the Mars orbit. Together with the keyhole radius of 0.018 AU of the second approach on 25 November, these unusual features cannot be explained by 3I/ATLAS orbital plane alignment alone. An orbital anomaly tightly associated with Mars raises questions regarding the timing of 3I/ATLAS discovery. For example, the observational window in which the necessary technology existed to observe 3I/ATLAS at sufficient physical detail has been available for about ∼102years. The fact that 3I/ATLAS is the first of its kind observed with this type of Mars anomaly implies that similar interstellar objects should arrive every 100/pa∼108years. It is then remarkable that 3I/ATLAS was discovered precisely during an extremely narrow and ongoing epoch, when for the first time Mars itself appeared within the range of advanced space travel of a relatively young terrestrial civilization. REFERENCES Eubanks, T. M., DeForest, C. E., Walsh, K. 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