Interstellar Comet 3I/ATLAS Shows Rapid Brightening and Gas Emission Near Perihelion
By
bikenaga
Properly proved. Has structure, has flavour, has a point.
Summary
Interstellar comet 3I/ATLAS, approaching its October 29, 2025 perihelion, has been difficult to observe from Earth due to its position opposite the Sun. However, space-based solar instruments including STEREO-A, SOHO, and GOES-19 have captured its final approach, revealing a rapid brightness increase scaling with heliocentric distance as r^(-7.5+/-1.0). The comet appears as an extended source with a ~4' diameter coma and shows distinctly bluer color than the Sun, indicating significant gas emission contributing to its visible brightness near perihelion.
Key quotes
· 4 pulledInterstellar 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.
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 ~4' in diameter.
LASCO 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.
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