Abstract
<jats:title>Abstract</jats:title> <jats:p> We report on the observation and measurement of astrometry, photometry, morphology, and activity of 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 <jats:italic>ugrizy</jats:italic> bands, with a typical precision of ∼70 mas, and briefly describe the reason this is coarser than our measured static source astrometric precision of ∼3 mas in Rubin images. We measure <jats:italic>grizy</jats:italic> magnitudes of 3I/ATLAS photometry at ∼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-sectional ratio of <jats:italic>η</jats:italic> ≳ 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 <jats:italic>g</jats:italic> − <jats:italic>r</jats:italic> = (0.657 ± 0.013) mag, <jats:italic>r</jats:italic> − <jats:italic>i</jats:italic> = (0.235 ± 0.018) mag, <jats:italic>i</jats:italic> − <jats:italic>z</jats:italic> = (0.147 ± 0.042) mag, and <jats:italic>z</jats:italic> − <jats:italic>y</jats:italic> = (0.047 ± 0.052) mag. These data represent the earliest observations of this object by a large (≳8 m class) telescope and illustrate the type of measurements (and discoveries) Rubin’s Legacy Survey of Space and Time will provide after it begins in early 2026. </jats:p>