11/11/2025
31/ATLAS, only the third confirmed interstellar comet, is about to emerge from behind the Sun. In four months of observation, it has already challenged assumptions about comet behavior.
Discovered July 1, 2025 by the ATLAS survey in Chile, it moves on a hyperbolic path with eccentricity ~6.1 and a speed near 57 km/s — clear evidence it originated outside the solar system. At perihelion it will pass 1.36 AU from the Sun and remain 1.8 AU from Earth.
Observations from Kottamia, Palomar, and Apache Point show a small icy nucleus under ~1 km surrounded by a dust coma. Dust ejection rates near 1 kg/s resemble typical comets, but its unusually red spectrum and unique chemical signatures suggest non-solar-system material.
31/ATLAS blends familiar comet physics with unfamiliar composition and tail behavior. After its brief passage, it will exit the solar system permanently, offering a short window to study the chemistry and origins of matter from beyond our planetary system.
