A coronal mass ejection (CME) launched from the Sun on September 19 is forecast by NASA’s ENLIL model to strike interstellar Comet 3I/ATLAS on September 24 or 25, an extremely rare event that comes as the comet brightens 40-fold and prepares to disappear into the Sun’s glare.
CMEs striking Solar System comets have been observed before, often bending or disconnecting their plasma tails. But no such interaction has ever been documented with an interstellar comet.
“What happens to an interstellar comet is anyone’s guess,” said Dr. Tony Phillips of SpaceWeather.com.
The potential impact comes during a phase of intense activity, giving astronomers only a short window to record changes to its tail and brightness.
Since early September, the comet has brightened from magnitude +16 to +12 — a roughly 40-fold increase in brightness. On September 17, Gerald Rhemann and Michael Jäger captured images confirming Thomas Lehmann’s earlier report of the surge.
Much of the increase is concentrated in green light. This color comes from diatomic carbon (C2), a molecule that fluoresces when exposed to solar ultraviolet radiation. The strong green emission indicates a sharp rise in gas production as the comet approaches the Sun.
Marshall Eubanks of Space Initiatives, Inc. modeled the comet’s brightness, separating the contributions of dust and gas. His analysis suggests the surge is dominated by gas release, with dust playing a smaller role. The result is a comet that is not only brighter but also highly dynamic in its emission.
The comet’s activity comes just ahead of an early October flyby of Mars. During that time, several orbiting spacecraft will have a chance to observe the comet from close range. This will provide a second rare opportunity to study an interstellar object interacting with both the solar wind and a planetary environment.
Comet 3I/Atlans was discovered on July 1, 2025 and is only the third known object from outside of our solar system. The comet will soon be lost in the Sun’s glare, limiting opportunities for observation.
References:
1 A CME is heading for interstellar comet Atlas – SpaceWeather – Accessed on September 24, 2025