3I/ATLAS: A Hyperbolic Interstellar Object

Updated 24 August 2025

Interstellar Object 3I/ATLAS is a hyperbolic interstellar body from the Galactic thick disk, defined by its retrograde orbit, high eccentricity, and excess velocity.
Its active coma, dominated by CO2-driven outgassing and enriched with water ice, reveals a complex mix of sublimation and non-sublimative dust liberation processes.
Coordinated observations and spacecraft flyby opportunities have constrained its nucleus size, spectral properties, and thermal evolution, offering insights into early planetesimal formation.

Interstellar Object 3I/ATLAS, also designated C/2025 N1 (ATLAS), is the third macroscopic interstellar object (ISO) detected traversing the Solar System. Discovered by the Asteroid Terrestrial-impact Last Alert System (ATLAS) on 2025 July 1, it exhibits a suite of orbital and physical properties that distinguish it as unambiguously interstellar, with implications for solar system science, galactic planetesimal populations, volatile processes, and early galactic formation environments.

1. Orbital and Dynamical Properties

3I/ATLAS follows a distinctly hyperbolic trajectory, characterized by an eccentricity of $e \simeq 6.2$ and perihelion $q \simeq 1.35$ au, with an inclination near $175^\circ$ —almost completely retrograde relative to the ecliptic. The object’s hyperbolic excess velocity, $V_\infty \simeq 60$ km s $^{-1}$ , is far in excess of solar system escape speeds and typical Oort cloud comet velocities (Seligman et al., 3 Jul 2025, Bolin et al., 7 Jul 2025, Hopkins et al., 7 Jul 2025, Taylor et al., 10 Jul 2025). Dynamical modeling translates this orbit into a galactocentric velocity vector of $(U, V, W) = (-51.0, -19.2, 18.5)$ km/s, indicating origin not from the Galactic thin disk but rather the thick disk—making 3I/ATLAS the first ISO so assigned (Hopkins et al., 7 Jul 2025, Eubanks et al., 21 Aug 2025). This velocity and radiant place it within the broad, high-dispersion population predicted by chemodynamical ISO models that incorporate Gaia DR3 data and Galactic disk evolution.

2. Physical Size, Brightness, and Coma Activity

Early photometric campaigns established an absolute visual magnitude $H_V \sim 12$ (with robust measurements $H_V = 12.0$ –13.7), implying—for canonical asteroidal geometric albedo $p \simeq 0.05$ —a nominal effective radius $R \sim 10$ km (Seligman et al., 3 Jul 2025, Chandler et al., 17 Jul 2025). However, high-resolution HST observations and dynamical mass-loss arguments constrain the nucleus to be no larger than $r < 2.8$ km (Jewitt et al., 4 Aug 2025). The apparent discrepancy arises because a major fraction of the measured brightness is contributed by active cometary coma, not the solid nucleus; models and observations suggest that over 90% of the optical flux can come from the extended dust and ice enveloping the nucleus, similar to the case for highly active comets (Taylor et al., 10 Jul 2025).

Coma morphology is characterized by a strong sunward emission fan and a sharply weaker, radiation-pressure-swept anti-sunward tail. Surface brightness profiles indicate steady-state outflow within $0.4''$ of the nucleus, turning over to steeper profiles at larger radii due to dust and possible ice grain destruction or sublimation. Dust mass-loss rates are estimated as $dM/dt \sim 6$ --$60$ kg/s for mean grain sizes $a = 1$ -- $100 \mu$ m (Jewitt et al., 4 Aug 2025). The activity level and extended coma were detected even as early as 6.4 au from the Sun in TESS precovery data, implying the object became active at large heliocentric distances, potentially due to hypervolatile-driven activity (Feinstein et al., 29 Jul 2025, Martinez-Palomera et al., 4 Aug 2025).

3. Spectroscopic Characterization and Coma Composition

Initial spectroscopy with VLT/MUSE, Palomar, Apache Point, and SOAR established the continuum emission of the coma as characteristically “red” in the visible, with spectral slopes of $(18 \pm 4)\%$ per 1000 Å (VLT/MUSE), $18.9\%/100$ nm (Palomar), and similar values consistent with complex refractory organics or irradiated carbonaceous material typical of D-type asteroids or certain trans-Neptunian objects (Opitom et al., 7 Jul 2025, Belyakov et al., 15 Jul 2025, Puzia et al., 4 Aug 2025, Bolin et al., 7 Jul 2025). Near-infrared spectroscopy (IRTF SpeX, Gemini-S) revealed a flattened continuum beyond 1 $\mu$ m and a distinct broad absorption near 2.0 $\mu$ m, diagnostic of water ice (Yang et al., 20 Jul 2025, Lisse et al., 21 Aug 2025). Mixing models support a coma composed of approximately 70% Tagish Lake analog meteorite (carbonaceous chondrite) and 30% water ice, with the latter likely present as $10 \mu$ m-sized grains.

No emission bands from canonical cometary gases (C $_2$ , CN, [OI], CO $^+$ ) were detected at $r_h \sim 4.5$ au, consistent with suppressed volatile outgassing at such distances (Opitom et al., 7 Jul 2025, Belyakov et al., 15 Jul 2025, Puzia et al., 4 Aug 2025). However, ultraviolet spectrophotometry with the Neil Gehrels-Swift Observatory yielded OH emission and a water production rate of $(1.35 \pm 0.27) \times 10^{27}$ molecules/s ( $\sim$ 40 kg/s) at 3.51 au, placing 3I/ATLAS among the few objects with OH emission beyond 3 au (Xing et al., 6 Aug 2025). SPHEREx imaging in August 2025 detected a strong 4.3 $\mu$ m CO $_2$ emission band, with a gas production rate $Q_{CO_2} = 9.4 \times 10^{26}$ molecules/s and a symmetric extended ( $\sim3'$ ) CO $_2$ coma (Lisse et al., 21 Aug 2025). Water ice absorption dominates the $\geq2.5~\mu$ m spectrum, implying that most water remains in solid phase, likely suppressed from outgassing by efficient CO $_2$ -driven activity and evaporative cooling.

The absence of CN emission and late detection of OH, in combination with the dominance of CO $_2$ -driven coma, suggest the volatile inventory of 3I/ATLAS differs appreciably from both Solar System comets and prior ISOs, with implications for its formative environment (Xing et al., 6 Aug 2025).

4. Origin, Population Context, and Mass Budget Constraints

Chemodynamic models and kinematic analyses robustly assign 3I/ATLAS to the Galactic thick disk, a population with lower metallicity and an origin in the Galaxy’s “cosmic noon” period (9–13 Gyr ago) of intense star formation (Hopkins et al., 7 Jul 2025, Eubanks et al., 21 Aug 2025, Taylor et al., 10 Jul 2025). Statistical arguments using the age–velocity dispersion relation infer a median kinematic age of 7 Gyr with 68% confidence between 3–11 Gyr (Taylor et al., 10 Jul 2025). This is considerably older—and formed in a more metal-poor environment—than 1I/‘Oumuamua or 2I/Borisov.

Population statistics and mass budget calculations indicate a tension between observed luminosity and the number density of large interstellar objects. Interpreting 3I/ATLAS’s brightness as indicating a 10 km solid nucleus leads to an implied galactic mass density ( $\rho_\mathrm{ATLAS}\sim0.01~M_\odot~\mathrm{pc}^{-3}$ ) that is orders of magnitude above the expected contribution from planetesimal ejection models. To resolve this, either (i) the object is a comet with a small (sub-kilometer) nucleus and a bright, extended coma, or (ii) large ISOs are extremely rare, and detection is aided by a bias toward plunging trajectories into the inner Solar System (Loeb, 8 Jul 2025, Taylor et al., 10 Jul 2025). In either case, the unusual orbital orientation of 3I/ATLAS (e.g., rare argument of perihelion, high inclination) supports selection effects at play.

5. Thermal Evolution and Dust Liberation Mechanisms

Thermal-diffusion models indicate that at heliocentric distances $\gtrsim$ 4 au, water ice remains stable only at depths $\gtrsim$ 15–20 cm. Sublimation thresholds for volatiles (H $_2$ O, CO $_2$ , NH $_3$ , CO) are only reached at shallower layers as 3I/ATLAS nears perihelion (Puzia et al., 4 Aug 2025). The presence of a significant coma without volatile gas emission at large distances suggests non-sublimative dust liberation processes, including solar wind sputtering, UV photo-desorption, radioactivity, exothermic phase transitions, or electrostatic lofting; an irradiation mantle accumulated during Gyr of interstellar travel may also suppress immediate volatile release. The appearance of grain-driven water vapor and large icy grains in the coma indicates that dust and ice aggregate processes—possibly involving heterogenous grain size and porosity distributions—dominate pre-perihelion outgassing (Xing et al., 6 Aug 2025, Kareta et al., 16 Jul 2025, Puzia et al., 4 Aug 2025).

6. Observational Campaigns and Spacecraft Encounters

Observational coverage of 3I/ATLAS is complicated by its geometry: perihelion (2025-10-29) occurs at a solar elongation near $13^\circ$ , making observations from Earth and near-Earth facilities unfeasible at peak activity (Seligman et al., 3 Jul 2025, Eubanks et al., 21 Aug 2025). Pre- and post-perihelion windows (July–September and November–March) are critical for constraining onset/cessation of activity, volatile composition, and dynamical evolution.

The close passage of 3I/ATLAS to Psyche, Mars-adjacent spacecraft, Juice, Europa Clipper, Hera, and Lucy opens prospects for in situ or remote spacecraft studies (Eubanks et al., 21 Aug 2025). Mission analyses demonstrate that flybys are possible with existing spacecraft given Mars-based or Jupiter-based launches (e.g., Juno), with $\Delta V$ requirements substantially lower than from Earth launches (Yaginuma et al., 21 Jul 2025, Loeb et al., 29 Jul 2025). Such encounters can provide high-resolution multi-wavelength spectroscopy, direct dynamical measurements, and unique perspectives on dust/grain environments otherwise inaccessible from Earth (Loeb et al., 29 Jul 2025, Eubanks et al., 21 Aug 2025). LSST-class survey telescopes, TESS, and HST have together provided the deepest and earliest imaging constraints, enabling improved orbit determination, coma characterization, and detection efficiency assessment (Chandler et al., 17 Jul 2025, Feinstein et al., 29 Jul 2025, Martinez-Palomera et al., 4 Aug 2025, Jewitt et al., 4 Aug 2025).

7. Comparative Assessment and Broader Implications

Relative to 1I/‘Oumuamua (a small, highly elongated, inactive object with anomalous non-gravitational acceleration and no confidently detected activity) and 2I/Borisov (a sub-kilometer active comet with volatile- and dust-dominated coma), 3I/ATLAS occupies a unique regime. Its inferred nucleus is larger ( $r<2.8$ km), its activity is driven primarily by CO $_2$ (rather than H $_2$ O at current distances), and it presents a highly red continuum more akin to D-type asteroids and some TNOs, but with clear water ice signatures as the object approaches the Sun (Seligman et al., 3 Jul 2025, Opitom et al., 7 Jul 2025, Yang et al., 20 Jul 2025, Lisse et al., 21 Aug 2025).

Chemodynamical and population modeling frames 3I/ATLAS as an ancient thick-disk relic, potentially formed under the high FUV, low metallicity, and different C/O environments of the Galaxy’s early star-forming phase (Hopkins et al., 7 Jul 2025, Taylor et al., 10 Jul 2025, Eubanks et al., 21 Aug 2025). The presence of substantial water ice in such an object provides direct evidence that planetesimal formation (and ejection) in low-metallicity or thick-disk conditions produced volatile-rich bodies, offering constraints on early protoplanetary disk evolution, ejection efficiency, and volatile retention.

In summary, 3I/ATLAS is a hyperbolic, retrograde, thick-disk interstellar object, physically characterized by a sizable active coma, red-sloped and water-ice-rich spectroscopic continuum, CO $_2$ -dominated outgassing, and a complex thermal and evolutionary history. It is accessible to both large-aperture survey telescopes and spacecraft flyby opportunities, promising unprecedented insights into the composition, physical evolution, and galactic origins of extrasolar objects traversing the planetary region. The coordinated paper of 3I/ATLAS sets a new observational and theoretical benchmark for interstellar object science, constraining the efficiency of planetesimal formation at cosmic noon and the physical and compositional diversity of the ISO population throughout the Milky Way.