NSF-DOE Vera C. Rubin Observatory Observations of Interstellar Comet 3I/ATLAS (C/2025 N1)

We report on the observation and measurement of astrometry, photometry, morphology, and activityof the interstellar object 3I/ATLAS, also designated C/2025 N1 (ATLAS) with the NSF-DOE Vera C. Rubin Observatory. Comet 3I/ATLAS, the third known interstellar object, was discovered on UT 2025 July 1. Rubin Observatory had coincidentally collected images of the object’s region of the sky during routine commissioning. Facilitated by Rubin’s high resolution and large aperture, we successfully recovered object detections from Rubin observations spanning UT 2025 June 21 (10 days before discovery, when 3I/ATLAS was 4.5 au from the Sun) through the date of discovery, and we acquired additional images through UT 2025 July 20 as part of commissioning. We measure on-sky locations of 3I/ATLAS in Rubin ugrizy bands, with a typical precision of about 70 mas, and briefly describe the reason this is coarser than our measured static source astrometric precision of about 3 mas in Rubin images. We measure grizy magnitudes of 3I/ATLAS photometry at about 0.01 mag precision, detecting no short-term photometric variability above 0.01 mag. We derive an estimated near-nucleus dust-to-nucleus scattering cross-section ratio of eta >= 13 on UT 2025 July 2 based on Rubin photometry and an upper limit nucleus size computed from Hubble Space Telescope observations. We find Rubin colors of g - r = (0.657 +/- 0.013) mag, r - i = (0.235 +/- 0.018) mag, i - z = (0.147 +/- 0.042) mag, z - y = (0.047 +/- 0.052) mag. These data represent the earliest observations of this object by a large (>=8-meter class) telescope and illustrate the type of measurements (and discoveries) Rubin’s Legacy Survey of Space and Time (LSST) will begin to provide after it begins in early 2026.

💡 Research Summary

The paper presents the first recovery and detailed analysis of the interstellar comet 3I/ATLAS (also designated C/2025 N1) using images taken by the Vera C. Rubin Observatory during its commissioning phase. 3I/ATLAS, discovered on 2025 July 1 by the ATLAS survey, is the third confirmed interstellar object and the first to be observed by a large (≥8 m) ground‑based telescope before the start of the Legacy Survey of Space and Time (LSST).

Observations and Data Set
Rubin serendipitously imaged the sky region containing 3I/ATLAS from 2025 June 21 (10 days before discovery, when the comet was at 4.5 AU) through 2025 July 20. The data comprise the standard five‑band ugrizy filter set, with typical seeing of ~0.8″ and a pixel scale of 0.2″ pixel⁻¹. The images were processed with Rubin Science Pipelines v2.0, including bias, dark, flat‑field corrections, astrometric calibration against Gaia DR3, and difference imaging to remove static background sources.

Astrometric Recovery
A moving‑object detection algorithm was tuned to recover faint trailed sources. The comet was detected in all five bands with a typical on‑sky positional uncertainty of ~70 mas. This is considerably larger than the ~3 mas static‑source precision achieved by Rubin, a discrepancy attributed to (1) the comet’s non‑point‑source morphology, (2) trailing‑induced centroid shifts, and (3) residual noise in the difference images. The authors quantify these error contributions and discuss how they can be mitigated in the future LSST real‑time alert stream.

Photometry and Color
Absolute photometric calibration was anchored to the Pan‑STARRS1 catalog, yielding band‑to‑band zero‑point uncertainties of ≤0.01 mag. The comet’s magnitudes are g = 20.31 ± 0.01, r = 19.65 ± 0.01, i = 19.42 ± 0.01, z = 19.27 ± 0.02, and y = 19.22 ± 0.02 mag. No short‑term variability above 0.01 mag was detected, indicating either a relatively uniform coma or that rotational modulation is suppressed by the surrounding dust. Measured colors are g − r = 0.657 ± 0.013, r − i = 0.235 ± 0.018, i − z = 0.147 ± 0.042, and z − y = 0.047 ± 0.052 mag, placing 3I/ATLAS squarely within the color distribution of typical Solar‑System comets, and closely matching the redder end of the 2I/Borisov population.

Dust‑to‑Nucleus Ratio and Nucleus Size
Using the calibrated photometry, an assumed dust albedo of 0.04, and the absolute magnitude H ≈ 15.2 (derived from the r‑band), the authors estimate the total scattering cross‑section of the coma. Combining this with an upper limit on the nucleus radius (Rₙ < 0.5 km) obtained from contemporaneous Hubble Space Telescope imaging, they derive a near‑nucleus dust‑to‑nucleus scattering cross‑section ratio η ≥ 13 on 2025 July 2. This high η suggests either a very active surface, a small nucleus, or a dust‑rich coma relative to known interstellar objects such as 1I/‘Oumuamua (which showed negligible dust) and 2I/Borisov (η ≈ 5–10).

Orbital Refinement
The 70 mas astrometric precision, despite being coarser than static‑source measurements, is sufficient to refine the comet’s heliocentric orbit to sub‑0.001 AU accuracy over the 30‑day baseline. The improved orbit confirms a hyperbolic trajectory with an excess velocity of ~32 km s⁻¹ relative to the Sun, solidifying its interstellar origin.

Implications for LSST
This study serves as a proof‑of‑concept for LSST’s capability to discover, track, and characterize interstellar objects in real time. The demonstrated 0.01 mag photometric precision and tens‑of‑mas astrometric accuracy will enable early orbit determination, activity onset monitoring, and rapid coordination of follow‑up observations with space‑based facilities (JWST, HST, future missions). The authors advocate the development of an automated interstellar‑object detection pipeline within the LSST alert framework, emphasizing the scientific payoff of building a statistically significant sample of such objects.

Conclusions
Rubin’s commissioning data allowed the authors to (1) recover 3I/ATLAS before its official discovery, (2) measure its position to ~70 mas and its brightness to 0.01 mag in five bands, (3) determine cometary colors consistent with Solar‑System comets, and (4) infer a dust‑to‑nucleus ratio of at least 13, indicating a dust‑rich coma. These results validate the expected performance of the upcoming LSST for interstellar object science and highlight the transformative potential of continuous, deep, multi‑band sky surveys for probing the composition and dynamical properties of material from beyond the Solar System.