Astronomers have unveiled a remarkable precovery: archival data from NASA’s TESS (Transiting Exoplanet Survey Satellite) captured the interstellar object 3I/ATLAS as early as 7 May 2025, nearly two months before its formal discovery in early July. Even at that time, the object was already clearly active, offering scientists an unprecedented early glimpse at a visitor from beyond our solar system.
TESS, primarily designed to detect exoplanets by monitoring stellar brightness fluctuations, isn’t optimized for detecting faint, fast-moving objects like interstellar comets. Yet researchers Adina Feinstein, Darryl Seligman, and John Noonan harnessed a clever technique known as “shift-stacking” to pinpoint 3I/ATLAS in a crowded sky. By predicting its location in each snapshot and aligning those frames before stacking them, the team amplified the faint signal of the object — otherwise undetectable in individual images.
Hidden in Plain Sight
During the observational window through early June, 3I/ATLAS significantly brightened by a factor of approximately 5× while its distance from the Sun decreased from around 6.35 AU to 5.47 AU. Notably, the brightness surge cannot be fully attributed to proximity alone, which should account for only a 1.5× increase. Instead, researchers suggest that the object underwent vigorous outgassing of hypervolatile compounds such as carbon dioxide and carbon monoxide—materials that typically sublimate at greater distances than water ice and are more common in interstellar materials.
Attempts to determine the rotational period of 3I/ATLAS proved inconclusive, primarily because a surrounding coma likely obscured any surface features, masking periodic brightness changes.
This precovery not only enriches the history of 3I/ATLAS but also showcases how revisiting archival data can yield hidden discoveries. As astronomers continue to mine existing datasets from telescopes like TESS, Vera Rubin, and others, more insights into interstellar visitors—both past and future—may emerge.
Based on a study by A. Feinstein, J Noonan, & D. Seligman – Precovery Observations of 3I/ATLAS from TESS Suggests Possible Distant Activity.
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