Rapid Brightening of 3I/ATLAS Ahead of Perihelion

Abstract Interstellar comet 3I/ATLAS has been approaching its 2025 October 29 perihelion while opposite the Sun from Earth, hindering ground-based optical observations over the preceding month. However, this geometry placed the comet within the fields of view of several space-based solar coronagraphs and heliospheric imagers, enabling its continued observation during its final approach toward perihelion. We report photometry from STEREO-A’s SECCHI HI1 and COR2, SOHO’s LASCO C3, and GOES-19’s CCOR-1 instruments in 2025 September–October, which show a rapid rise in the comet’s brightness scaling with heliocentric distance r as r −7.5±1.0 . CCOR-1 also resolves the comet as an extended source with an apparent coma of ∼ 4 ′ in diameter. Furthermore, LASCO/CCOR-1 color photometry shows the comet to be distinctly bluer than the Sun, consistent with gas emission contributing a substantial fraction of the visible brightness near perihelion.