NASA-led observations have revealed that interstellar comet 3I/ATLAS is travelling through the solar system at about 210,000 kilometres per hour, the highest speed ever recorded for a visitor from another star. The object, first detected on 1 July 2025, is only the third confirmed interstellar body—after 1I/ʻOumuamua in 2017 and 2I/Borisov in 2019—and is believed to be roughly eight billion years old, making it almost twice as old as the solar system itself. High-resolution images from the Hubble Space Telescope indicate that the nucleus of 3I/ATLAS is far smaller than early ground-based estimates suggested, measuring between 320 metres and 5.6 kilometres across. Despite its compact size, the comet is already shedding dust and water vapour, behaviour typical of solar-system comets, even though it remains hundreds of millions of kilometres from the Sun. Astronomers note that the comet’s unexpectedly intense activity at great distance could provide rare insights into the chemistry of planetary systems that formed long before our own. "It’s like glimpsing a rifle bullet for a millisecond—you see it, but tracing its origin is almost impossible," said University of California, Los Angeles researcher David Jewitt, who is leading part of the observing campaign. 3I/ATLAS will reach perihelion in late October 2025, passing about 210 million kilometres inside Mars’s orbit before being ejected back into interstellar space. NASA, the James Webb Space Telescope and several ground-based observatories have scheduled additional observations in the coming months to catalogue the comet’s gas emissions and refine models of its trajectory. Scientists say the data could advance understanding of interstellar objects and the early conditions that shaped the Milky Way.