3I/ATLAS shifts from antitail to tail as perihelion nears

An interstellar object known as 3I/ATLAS that has been traversing the inner solar system this year has reversed the direction of its visible jet of material, shifting from a sunward-pointing antitail in July and August to a classic tail pointing away from the Sun by September, astronomers said. The object will reach perihelion, its closest approach to the Sun, on Oct. 29 and presents a rare chance in late October and early November for spacecraft to sample material from its ion tail.

Groundbased and space observations captured the antitail earlier this summer and subsequent changes. The Keck II telescope in Hawaii detected emissions including cyanide and nickel that appeared both toward and away from the Sun, and high-resolution imaging from the Nordic Optical Telescope confirmed that the earlier sunward jet had disappeared and a conventional tail had formed by September. NASA instruments have estimated a mass loss on the order of several hundred pounds per second, dominated by carbon dioxide and carbon monoxide, as the object warms while approaching the Sun.

Researchers offer multiple explanations for the unusual sequence. One model proposed by Harvard astrophysicist Avi Loeb...