Astronomers find interstellar comet 3I/ATLAS hiding in images taken before its official discovery

It turns out interstellar comet 3I/ATLAS was almost called 3I/Rubin, after researchers found that the giant survey telescope coincidentally spotted this visitor from the stars over a week before it was officially discovered. 3I/ATLAS was officially identified on July 1, 2025 by the Asteroid Terrestrial-impact Last Alert System (ATLAS), which is a network of robotic telescopes in Hawaii, Chile and South Africa. But ten days before, the Vera C. Rubin Observatory , which is also in Chile, began its science validation phase ahead of entering full operation later that year. The science validation phase was designed to calibrate the 27.6-foot (8.4-meter) telescope and its instruments, to ensure that they were working correctly. Curious as to whether Rubin had seen 3I/ATLAS before its official discovery date, a team led by Colin Orion Chandler of the University of Washington went sifting through the data from Rubin's commissioning phase. Lo and behold, they found that Rubin had imaged 3I/ATLAS on its very first night of taking practice images, on June 20, ten days before ATLAS spotted it. It wasn't an easy task. Today Rubin has a very well planned out routine — called a 'pipeline' — for taking data and processing it for astronomers, but back during the validation phase the pipeline was not in operation. This meant that Chandler and his team had to devise their own custom pipeline to access the data. …