First Observation of an Interstellar Visitor

[Recusant]

Still showing unusual characteristics . . .

"3I/ATLAS Brightens Dramatically as it Swings Past the Sun" | Universe Today

Comet 3I/ATLAS, only the third known visitor from beyond our Solar System, has been brightening far more rapidly than expected as it approaches perihelion, its closest point to the Sun. From Earth, the comet has been positioned almost directly behind the Sun for the past month, making ground based observations nearly impossible during this crucial period. Instead, the team of astronomers have been watching from space based observatories.

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Between mid September and late October, as 3I/ATLAS closed in from about 2 astronomical units (roughly twice Earth's distance from the Sun) to just 1.36 AU, its brightness surged dramatically. The team calculated that the comet's brightness increased proportionally to the inverse of heliocentric distance to the 7.5 power, a significantly steeper brightening than the earlier rate observed when it was farther out. To put this in perspective, most comets brighten gradually as they approach the Sun and ice turns to gas. This interstellar visitor is brightening at roughly twice that typical rate, suggesting something unusual is happening on its surface.

The observations also revealed that the comet appears distinctly bluer than sunlight, a telltale sign that gases, rather than just dust, are contributing substantially to its visible brightness. Earlier observations had found the comet's dust to be reddish, making this blue shift particularly noteworthy. The researchers suspect emissions from molecules like cyanogen and possibly ammonia are responsible for this unusual colouring.

[Continues . . .]

The paper is open access:

"Rapid Brightening of 3I/ATLAS Ahead of Perihelion" | arXiv (PDF)

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/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.

[Dark Lightning]

I really hope that the solar sail works for those interstellar travelers. 
Sorry, channeling Niven and Pournelle, in the "The Mote in God's Eye". I see that it's the carbonaceous fraction(s) that's glowing bright.

[Recusant]

Yeah the solar sail has a distinct place in science fiction. One of the earlier pieces I read that used the idea prominently was Space War Blues by Lupoff. It's deeply in the vein of late 60s/early 70s New Wave SF and so can be challenging. Some consider it nearly unadulterated nonsense but I enjoyed it when I read it as a youngster.

[Dark Lightning]

I'm reminded of the Lensman Series by EE "Doc" Smith. At least he admitted that it was space opera.