Interstellar Comet 3I/ATLAS Through the Eyes of Rubin Observatory

The NSF–DOE Vera C. Rubin Observatory has yet to begin its 10-year Legacy Survey of Space and Time, but it’s already keeping an eye on the sky. Astronomers recently described Rubin’s serendipitous spotting of 3I/ATLAS days before the interstellar object’s discovery.

Welcome, 3I/ATLAS

Our solar system has been playing host to a rare interstellar tourist: 3I/ATLAS. Just the third known interstellar object to have passed through our star system, 3I/ATLAS was discovered on 1 July 2025 by the robotic telescope network of the Asteroid Terrestrial-impact Last Alert System (ATLAS). After making its closest approach to the Sun in October 2025, 3I/ATLAS is now journeying out of the solar system and is currently located beyond the orbit of Jupiter.

While astronomers would likely grumble about hosting a human guest for nearly a year, they’ve been thrilled with 3I/ATLAS’s lengthy visit and have turned countless telescopes toward the rare interstellar object. They’ve also turned to the archives, searching for pre-discovery sightings of the object — and finding them in the commissioning data from one of the most highly anticipated observatories on the planet: Rubin Observatory.

A Serendipitous Sight

In the months since the release of Rubin’s exceptional first images, its 8.4-meter telescope and array of instruments have undergone rigorous testing to ensure they can meet the high standards required for the upcoming survey.

For most of 2025, Rubin was in its commissioning phase, carrying out science validation observations — a critical period during which instruments, procedures, and pipelines are fine tuned, and the quality and quantity of data collection can vary. On 20 June 2025, the very first night of its science validation survey, Rubin happened to turn toward the small patch of sky containing the as-yet undiscovered 3I/ATLAS.

More to Come

In a recent article, Colin Orion Chandler (LSST Interdisciplinary Network for Collaboration and Computing Frameworks; University of Washington) and collaborators described how they tracked down 3I/ATLAS in Rubin’s commissioning data after the object’s discovery was announced. In total, the observatory serendipitously spotted 3I/ATLAS nine times between 21 June and 2 July, then several more times — some on purpose, some by chance — between 3 and 20 July.

Analyzing Rubin’s observations of 3I/ATLAS required the development of custom techniques, since the observatory’s pipeline for handling observations of rapidly moving solar system objects was not in operation at that time. Despite these challenges, Chandler’s team successfully made astrometric measurements, collected photometry, determined 3I/ATLAS’s orbit, and studied the comet’s structure and nucleus. Among other findings, these data clearly show the presence of the object’s fuzzy coma, providing the earliest high-resolution evidence of cometary activity.

In their analysis of Rubin’s observations of 3I/ATLAS, Chandler and collaborators found that had the science validation survey started sooner, and had the analysis pipelines been operational, Rubin might have been the first to welcome humanity’s third interstellar visitor. Findings like these heighten the anticipation for the start of the Legacy Survey of Space and Time, during which Rubin will surely build upon its already impressive record of interstellar object observations.

Citation

“NSF-DOE Vera C. Rubin Observatory Observations of Interstellar Comet 3I/ATLAS (C/2025 N1),” Colin Orion Chandler et al 2026 ApJL 1001 L35. doi:10.3847/2041-8213/ae4b3a