Abstract 3I/ATLAS, also known as C/2025 N1 (ATLAS), is the third known macroscopic interstellar object to pass through our solar system. We report serendipitous Transiting Exoplanet Survey Satellite (TESS) observations of 3I/ATLAS taken between 2025 May 7 and 2025 June 2, 55 days prior to the discovery date (2025 July 1). We retrieve the TESS pixel data, perform a robust background correction, and use a data-driven approach to compute the object’s position on the TESS detectors. We find a consistent offset between the target’s observed and predicted positions, which is dominated by uncertainty in the TESS World Coordinate System rather than ephemeris errors. 3I/ATLAS is too faint to be detected in the individual 200 s TESS integrations, so we stack images to improve detectability. We perform aperture and pixel response function (PRF) photometry on the stacked images to create two light curves. Each light curve consists of 15 measurements with a signal-to-noise ratio > 3, collected across two different TESS cameras during the 26 days that the object was observed. The PRF light curve, which is more robust against image noise, in the TESS bandpass shows a gradual increase in brightness from T mag = 20.9 ± 0.29 to 19.57 ± 0.15. This is expected as 3I/ATLAS approaches the inner solar system. Its absolute magnitude decreases from H V = 14.3 ± 0.4 to 13.7 ± 0.3 and shows signs of faint activity consistent with other observations. This Letter highlights the power of using TESS for solar system science; by increasing the number of prediscovery observations, in an otherwise sparsely populated region of the light curve, the long-term behavior of 3I/ATLAS can be investigated.
Barclay et al. (2025) studied this question.