Rapid Brightening of 3I/ATLAS Ahead of Perihelion

TL;DR

The paper analyzes space-based coronagraphic and heliospheric observations of the interstellar comet 3I/ATLAS during its approach to perihelion. It combines data from STEREO-A SECCHI, SOHO LASCO C3, and GOES-19 CCOR-1, applying astrometric calibration, background subtraction, and comet-centered stacking to extract photometry across multiple bandpasses. The authors find a very steep brightness dependence on heliocentric distance, $n=7.5\pm1.0$, and resolve a $\sim4'$ coma with blue colors that point to gas emission (e.g., $C_2$, $NH_2$) dominating near perihelion, suggesting gas-driven activity. These results fill a critical observational gap during Earth–Sun conjunction and help predict post-perihelion behavior, while highlighting that the rapid brightening may be driven by complex physical processes (e.g., CO$_2$ cooling or nucleus properties) requiring further study.

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 ~4' in diameter. Furthermore, 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.

Figures (3)

• Figure 1: Left: Stack of all CCOR-1 frames of 3I/ATLAS (top), and an equivalent stack centered on a nearby star on the same frames, approximating the PSF (bottom). Right: Similar stacks of all HI1 (top), COR2 (middle), and LASCO C3 Clear (bottom) frames of 3I. All stacks are aligned with north up. The heliocentric velocity ($+v$), and sunward ($\odot$) or antisunward ($-\odot$) directions are labeled for the comet at the midpoint time.
• Figure 2: Light curve of daily (and COR2 2.5 day/1.7 day) photometry, corrected to $\varDelta=1$ au, along with the $r^{-7.5}$ best fit brightness scaling and an $r^{-3.8}$ curve matching the previously reported trend at $r\gtrsim2$ au with an arbitrary vertical placement. Note that the $r^{-7.5}$ curve is specifically scaled for the C3 Clear magnitude of the comet, to which the CCOR-1 points can be converted with a $\sim$0.4 mag shift (i.e., the CCOR-1--Clear color from Table \ref{['tab:color']}), and from which we estimate the HI1/COR2 points may likewise be plausibly offset $\pm0.4$ mag. Note also that the magnitude error bars indicate only the flux measurement uncertainty (i.e., the noise in the S/N of each detection) and do not include the estimated $\lesssim$0.1 mag uncertainties in the photometric calibrations.
• Figure 3: The spectral responses corresponding to the presented observations (upper two panels) compared with a continuum-subtracted emission spectrum of a typical solar system comet, 24P/Schaumasse (bottom panel; courtesy of Q. Ye and C. A. Schmidt), with prominent emission features labeled cochran1992brown1996farnham2000. All panels have arbitrary vertical scaling. While several bandpasses extend past the red end of this spectrum, only a few minor NH$_2$ and CN bands are present there; those are typically dwarfed by the dust continuum, so are not expected to substantially contribute to the observed broadband flux. We caution, however, that this spectrum (1) is not of 3I itself, which may differ substantially in its gas abundances, and (2) only samples emission from a small fraction of the coma, and the relative brightness of different species may not closely match that over a much larger fraction of the coma (i.e., as captured by our large aperture photometry) given the wide variation in scale lengths between species ahearn1995.