Precovery Observations of 3I/ATLAS from TESS Suggests Possible Distant Activity

Abstract

3I/ATLAS is the third macroscopic interstellar object detected traversing the Solar System. Since its initial discovery on UT 01 July 2025, hundreds of hours on a range of observational facilities have been dedicated to measure the physical properties of this object. These observations have provided astrometry to refine the orbital solution, photometry to measure the color, a rotation period and secular light curve, and spectroscopy to characterize the composition of the coma. Here, we report precovery photometry of 3I/ATLAS as observed with NASA's Transiting Exoplanet Survey Satellite (TESS). 3I/ATLAS was observed nearly continuously by TESS from UT 07 May 2025 to 02 June 2025. We use the shift-stack method to create deep stack images to recover the object. These composite images reveal that 3I/ATLAS has an average TESS magnitude of $T_\textrm{mag} = 20.83 \pm 0.05, 19.28 \pm 0.05$ and an absolute visual magnitude of $H_V = 13.72 \pm 0.35; 12.52 \pm 0.35$, the latter being consistent with magnitudes reported in July 2025. When coupled with recent HST images deriving a nucleus size of R$<$2.8 km (H$>$15.4), our measurements suggest that 3I/ATLAS may have been active out at $\sim 6$ au. Additionally, we extract a $\sim 20$ day light curve and find no statistically significant evidence of a nucleus rotation period. Nevertheless, the data presented here are some of the earliest precovery images of 3I/ATLAS and may be used in conjunction with future observations to constrain the properties of our third interstellar interloper.