Paper - summarizes the key findings
• Rubin images clearly show the interstellar object 3I/ATLAS with a dust coma, indicative of activity. These observations provide the earliest high resolution evidence of detected cometary activity.
• Rubin multi-filter photometry is consistent with previous bservations in the literature, with significantly smaller error bars. There are not sufficient same-night multi-filter observations to examine surface/coma color or color evolution, but sufficient to exclude photometric variability on short timescales.
• The coma’s radius appeared to increase slightly over the 11 days from ∼ 6,520 km (UT 2025 June 21) to ∼9,380 km (UT 2025 July 02) as measured from azimuthally averaged radial profiles. We estimate an increase in coma level of ∆η ∼0.5 between 2025 June 21 June and 2025 July 2, which we assume to be a lower limit for several reasons, including that 3I/ATLAS was observed nearly head-on (very low phase angle) and the tail could have extended far along the z-axis, as projected on the sky.
• We obtain a V-band absolute magnitude of HV = (13.7 ± 0.2) mag and equivalent effective radius of ∼ (5.6 ± 0.7) km for the nucleus, assuming a spherically symmetric steady-state coma. Due to this simplifying assumption, we consider the latter result to be an upper limit to the true nucleus size. We estimate a mass loss rate ranging from 10 to 100 kg/s, depending on the grain sizes assumed to dominate, and we compute Afρ = (315 ± 15) cm for data obtained on UT 2025 July 2.
• We detect no short-term photometric variability. A sequence of measurements taken on UT 2025 July 2 constrains the apparent brightness variations of 3I/ATLAS to less than 0.1 mag on timescales of less than an hour.
• If the Rubin SSP pipelines had been processing the commissioning data in real time, our modeling shows that there were sufficient SV observations to identify 3I/ATLAS as a moving object.
• If the nominal SV survey strategy continues as planned, 3I/ATLAS should be observed in ten or more observations in each filter through mid August 2025. Future SV observations will likely be able to monitor the coma color as 3I/ATLAS moves towards perihelion.
• Analysis of the derived astrometry suggest that for bright high-SNR and extended active small bodies, the combination of Rubin’s large aperture and LSSTCam’s pixel scale are less impacted by asymmetrical coma provided precise positions for deriving accurate orbital parameters.