Brightness, Motion and a Mission With Side Benefits
TESS measurements place the comet’s brightness at an apparent magnitude of about 11.5, roughly 100 times dimmer than what the human eye can detect, but well within the reach of telescopes.
Although TESS was designed to discover exoplanets by detecting tiny dips in starlight as planets pass in front of distant suns, its wide-angle vision and steady monitoring make it surprisingly effective at tracking nearer objects. Comets and asteroids, it turns out, can also leave detectable fingerprints in its data.
Found Before It Was Known
That capability paid off in a remarkable way. TESS had already observed comet 3I/ATLAS back in May 2025 — nearly two months before astronomers officially identified it. By combing through archived data and stacking multiple observations, scientists were able to isolate the interstellar traveler from background noise and reconstruct its motion.
While the technique cannot pinpoint where the comet came from, it does provide crucial insights into its behavior and structure.
