JWST NIRSpec: 3I/ATLAS Interstellar Probe

• 3I/ATLAS is a unique interstellar object with a large nucleus, hyperbolic orbit, and a distinct red coma indicative of complex volatile processes.
• The JWST NIRSpec campaign employs multi-mode spectroscopy and coordinated observations to characterize coma composition and activity with high precision.
• Early findings reveal an anomalously high CO2/H2O mixing ratio, suggesting formation or irradiation conditions distinct from typical Solar System comets.

3I/ATLAS (C/2025 N1) is the third confirmed interstellar object observed transiting through the Solar System, notable for its large nucleus, hyperbolic orbit, and presence of a distinct coma. The James Webb Space Telescope (JWST) Near-Infrared Spectrograph (NIRSpec) campaign for 3I/ATLAS leverages advanced infrared capabilities to characterize its coma composition, volatile inventory, and activity drivers, yielding insights into the physical and chemical processes that govern interstellar objects. This campaign integrates data from ground- and space-based observatories—including CFHT, IRTF, SOAR, and JWST—to constrain the nature, origin, and evolutionary context of 3I/ATLAS.

1. Orbital and Physical Characteristics of 3I/ATLAS

3I/ATLAS was discovered on 2025 July 1 UT (Seligman et al., 3 Jul 2025). The initial orbital solution yielded the following parameters:

• Eccentricity: $e \sim 6.2$
• Perihelion distance: $q \sim 1.35$ au
• Inclination: $i \sim 175^\circ$
• Hyperbolic excess velocity: $V_\infty \sim 60$ km s$^{-1}$

Such high eccentricity and velocity affirm its interstellar origin. The absolute magnitude ($H_V \sim 12$) and assumed albedo ($p \sim 0.05$, asteroid-like) imply a nuclear radius of approximately $\sim$10–12 km, derived via $R_{3I} = 11.8 \text{ km} \times (0.05/p)^{-1/2}$. This size significantly exceeds those of prior interstellar visitors (1I/‘Oumuamua, 2I/Borisov).

Deep stacked CFHT images revealed a faint coma, and multi-channel photometry established a red visible/near-infrared spectral slope. The object features a nearly constant light curve ($\leq 0.2$ mag variation over 29 h), with faint and steady activity. Observability is constrained: visible until September 2025, unobservable near perihelion due to low solar elongation, returning in November.

2. JWST NIRSpec Instrumentation and Commissioning Readiness

NIRSpec is one of four focal plane instruments on JWST, specifically optimized for faint near-infrared spectroscopy (Böker et al., 2022). The commissioning campaign certified the following capabilities:

• Observing modes: Multi-Object Spectroscopy (MOS), Integral-Field Spectroscopy (IFS), Fixed Slit Spectroscopy (FSS), Bright Object Time Series (BOTS)
• Hardware: Grating Wheel Assembly (GWA), Filter Wheel Assembly (FWA), Re-Focus Mechanism (RMA), Micro-Shutter Assembly (MSA), Hawaii-2RG detectors
• Operating temperature: $42.8 \pm 0.01$ K, stabilized to within 10 mK
• Detector performance: IRS$^2$ mode achieves optimal noise for faint sources; bad pixel fraction $<0.5\%$ (Böker et al., 2023)
• Target acquisition accuracy: 8–25 mas (per axis) for microshutter placements, sufficient for narrow (200 mas) apertures
• Spectral resolution: $\frac{\lambda}{\Delta\lambda} = 100$, $1000$, $2700$

The instrument met or exceeded pre-launch sensitivity and throughput predictions, with improvements majorly attributed to a sharper point spread function (diffraction-limited to $1.1\ \mu$m; Strehl ratio 0.9 at $4\ \mu$m). The microshutter array maintains $>82\%$ usable shutters, supporting multiplexed science in MOS mode.

3. Early Spectrophotometric Constraints: Ground-Based and NIR Observations

Initial visible and NIR reflectance observations from IRTF and SOAR illustrated a red spectral slope at optical wavelengths, transitioning to a neutral or slightly blue slope near $1.1$–$1.5~\mu$m (Kareta et al., 16 Jul 2025, Puzia et al., 4 Aug 2025). The measured $g - i$ color of $1.06 \pm 0.11$ and the spectral slope $S \sim 27.4\%$ per $k\AA$ are comparable to extreme trans-Neptunian objects and some D-type asteroids. No significant variability in the light curve, and an absence of canonical cometary gas emission (CN, C$_3$, C$_2$, CO$^+$, [O i]) at $r_H = 4.4$ AU were detected.

Spectral modeling reveals an anomalously steep dust coma grain size distribution (power-law index $n \sim 5.5$–$8.8$), and an upper limit on pure water ice area fraction $f_\mathrm{ice} < 7\%$, indicating dominating small dust grains or complex grain compositions. Thermal-evolution modeling suggests that sublimation of volatile ices is suppressed at large heliocentric distances, allowing non-sublimation dust-liberation mechanisms (solar wind sputtering, UV desorption, crust formation) to produce the observed early coma.

4. JWST NIRSpec Spectroscopy: Coma Composition and Volatile Inventory

Spectroscopic imaging at $r_H=3.32$ AU using the JWST NIRSpec campaign produced the first direct, spatially resolved detection of gas and ice in the coma of 3I/ATLAS (Cordiner et al., 25 Aug 2025). Key findings from the 0.6–5.3 $\mu$m data cube include:

• Dominant CO$_2$ emission (4.3 $\mu$m), with secondary detection of H$_2$O (2.7 $\mu$m), CO (4.7 $\mu$m), OCS (4.85 $\mu$m)
• Water ice detected via broad absorption feature at 3.0 $\mu$m
• Dust continuum strongest near 1.2 $\mu$m, showing sunward plume-like enhancements

Quantitatively, the measured CO$_2$/H$_2$O mixing ratio was $8.0\pm1.0$, among the highest recorded for a comet, and 6.1$\sigma$ above trends observed in long-period and Jupiter-family comets. This value strongly implies that CO$_2$ outgassing dominates solar-driven activity, differing from Solar System comets where H$_2$O sublimation becomes significant at comparable heliocentric distances.

The lower H$_2$O abundance may result from inhibited heat penetration into the nucleus—possibly due to a more insulating crust or high-albedo layers—suppressing H$_2$O release relative to CO$_2$ and CO. Alternatively, this composition could emerge from formation near the CO$_2$ ice line in the parent protoplanetary disk, or due to long-term exposure to high-energy radiation in the interstellar medium.

5. Comparative Context: 3I/ATLAS and Other Interstellar Objects

The behavior and composition of 3I/ATLAS diverges from previous interstellar visitors:

• 1I/‘Oumuamua: Moderately red spectrum, highly variable light curve, no observed coma or gas activity (Seligman et al., 3 Jul 2025)
• 2I/Borisov: Classical cometary outgassing, red spectral continuum, detections of water and other volatiles (Seligman et al., 3 Jul 2025)
• 3I/ATLAS: Large, nearly constant light curve, distinctly red spectral slope, faint activity at large distances, and CO$_2$-dominated outgassing at inbound distances

A plausible implication is that 3I/ATLAS represents a population of larger, CO$_2$-rich nuclei, undergoing unique surface and compositional evolution during interstellar transit.

6. JWST NIRSpec Campaign Methodology and Operational Challenges

NIRSpec science operations for 3I/ATLAS utilized IFU and MOS modes, optimized for moving target tracking and spatially resolved spectroscopy (Böker et al., 2022, Böker et al., 2023). Key methodologies included:

• Astrometric positioning via small angle maneuvers and reference star field calibration, yielding placement accuracies $\sim8$ mas—critical for faint-object spectroscopy using narrow microshutters
• Spaxel-based mapping of species emission and continuum in the data cube
• “Q-curve” method for volatile production rate analysis, mitigating optical depth effects
• Adjustment of exposure times and readout modes to counter higher-than-expected photon conversion efficiency (PCE), preventing saturation
• Coordinated temporal monitoring to capture potential changes in outgassing, morphology, and volatile inventory near perihelion

Limitations included constrained observability near perihelion and systematic uncertainties in calibration pipeline steps such as S-flat normalization and extraction aperture centering (Böker et al., 2023).

7. Implications and Future Directions

The JWST NIRSpec campaign identified 3I/ATLAS as a unique outlier among observed comets, with a highly CO$_2$-enriched coma and suppressed H$_2$O outgassing. This suggests the nucleus may have undergone either formation or subsequent irradiation conditions unlike those experienced in our Solar System. Ongoing and future observations—especially at reduced heliocentric distances as the thermal wave penetrates more deeply—are expected to further elucidate the volatile inventory, test the predictions of thermal-evolution models, and constrain isotopic ratios for formation environment diagnostics.

Strategic coordination of ground-based (e.g., Rubin Observatory), space-based (JWST), and multi-wavelength (optical, NIR, sub-mm) spectroscopy will be required to resolve outstanding questions regarding dust-liberation mechanisms, surface processing, and active compositional transitions in interstellar objects. The NIRSpec campaign on 3I/ATLAS provides an essential reference for understanding the diversity, origin, and evolutionary trajectories of interstellar material entering the Solar System.