| Astrophysics (Index) | About |
3I/ATLAS is an interstellar object discovered in 2025: essentially an interstellar comet en route to pass through the solar system. It is the third such passing interstellar object identified (indicated in the designator by the "3I"), the first being 1I/'Oumuamua in 2017 and the second, 2I/Borisov in 2019. Such objects are identified by their speed and orbital path: the speed is too great for a body orbiting the Sun, i.e., more than the Sun's escape velocity, and its path is hyperbolic rather than elliptical. 3I/ATLAS's eccentricity was 6.13941, substantially greater than 1, the limit on an elliptical orbit. It was spotted 4.5 AU from the Sun, i.e., within Jupiter's orbit. Its closest approach to the Sun was October 29, 2025, roughly 1.4 AU from the Sun. Its route passes fairly close to Mars and Jupiter (passing a fraction of an AU from each), but it gets no closer to Earth than about 1.8 AU, the Earth being on the opposite end of its orbit. It shows comet activity, i.e., a surrounding dust cloud.