Pre-discovery TESS Observations of Interstellar Object 3I/ATLAS

Abstract

3I/ATLAS, also known as C2025 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 2025-05-07 and 2025-06-02, 55 days prior to the discovery date (2025-07-01). We retrieve the TESS pixel data, perform a robust background correction and use a data-driven approach to compute the objects position on the TESS detectors. We find a consistent offset between the targets observed and predicted positions which is dominated by uncertainty in the TESS World Coordinate System (WCS) rather than ephemeris errors. 3I/ATLAS is too faint to be detected in the individual 200 second 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 $\text{SNR}>3$, 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 $T_{\text{mag}}=20.9\pm0.29$ to $19.57\pm0.15$. This is expected as 3I/ATLAS approaches the inner Solar System. Its absolute magnitude decreases from $H_{V}=14.3\pm0.4$ to $13.7\pm0.3$ and shows signs of faint activity consistent with other observations. This paper highlights the power of using TESS for Solar System science by increasing the number of pre-discovery observations, in an otherwise sparsely populated region of the light curve, the long-term behavior of 3I/ATLAS can be investigated.