Interstellar Visitor 3I/ATLAS Unexpectedly Surged in Brightness at Perihelion

3I/ATLAS: A surprising brightening from an interstellar traveler

Since its discovery in early July 2025, 3I/ATLAS — only the third confirmed object ever observed traveling through our Solar System from interstellar space (after 1I/ʻOumuamua and 2I/Borisov) — has captivated astronomers. The visitor, widely thought to be comet-like, reached perihelion on 2025‑10‑29, and during that close passage it brightened far faster than expected.

What was observed

Data from solar observatories recorded the unusual behavior: NASA’s STEREO, ESA’s SOHO, and NOAA’s GOES‑19 all registered a rapid rise in visible light from 3I/ATLAS as it passed near the Sun. Such solar-monitor detections have been crucial because ground-based telescopes could not observe the object while it was hidden behind the Sun.

Why this is surprising

Comets typically brighten as they near the Sun because solar heating causes ices to sublimate, releasing gas and dust that form a reflective coma and tail. But the brightening rate for 3I/ATLAS "far exceeds the brightening rate of most Oort cloud comets at similar radial distance," according to Naval Research Laboratory astrophysicist Karl Battams and Lowell Observatory postdoctoral fellow Qicheng Zhang, in a paper that has not yet completed peer review.

"The reason for 3I’s rapid brightening ... remains unclear," the authors write, while offering several plausible explanations.

Possible explanations

• Extreme speed: Observers estimate the object has been moving at roughly 137,000 mph, substantially faster than the two previously detected interstellar visitors. Higher relative speed can change how material is released and illuminated.
• Nucleus properties: Unusual composition, shape or internal structure — features acquired in its home system or altered during a long interstellar journey — could drive atypical outgassing or fragmentation.
• Gas-dominated emission: The team reports that 3I/ATLAS appears "distinctly bluer than the Sun," which is consistent with strong gas emission contributing a large fraction of the visible light near perihelion.

What remains unknown

Without a confirmed physical explanation, the object’s post‑perihelion behavior is uncertain. The researchers say a brightness plateau, a brief continuation of pre‑perihelion brightening, or rapid fading are all plausible outcomes. Further observations will be needed to distinguish among these possibilities.

When we’ll see it again

Ground-based monitoring is expected to resume when 3I/ATLAS emerges from behind the Sun in early December 2025. The authors note it should return to twilight and then dark-night skies during November–December 2025, providing a window to study the object in far greater detail than the available solar-observatory data allow.

Context: This event offers a rare opportunity to study material from another star system up close. Continued observations — spectral measurements, photometry, and imaging — will help determine whether the brightening was driven by speed, unusual composition, gas emission, fragmentation, or some combination of factors.