TESS spotted 3I/ATLAS two months before it was discovered. It was even active then

One of the advantages of having so many telescopes watching large parts of the sky is that, if astronomers find something interesting, there are probably images of it from before it was officially discovered sitting in the data archives of other satellites that no one thought to look at. That has certainly been the case for our newest interstellar visitor, 3I/ATLAS, which, though discovered in early July, had been visible on other telescopes as early as May.