A trio of observational platforms – NASA’s STEREO (Solar Terrestrial Relations Observatory), the joint NASA/ESA SOHO (Solar and Heliospheric Observatory), and NASA’s PUNCH (Polarimeter to Unify the Corona and Heliosphere) missions – possess the distinctive capability to monitor regions of the celestial sphere adjacent to the Sun. This unique vantage point facilitated their tracking of the interstellar comet 3I/ATLAS as it transited behind our Sun from Earth’s perspective.
Navigating our Solar System at an impressive velocity of 209,000 kilometers (130,000 miles) per hour, the interstellar object 3I/ATLAS was rendered visible through the compilation of multiple colorized stacked images captured between September 11 and 25, 2025. These observations were acquired utilizing the Heliocentric Imager-1 instrument, a visible-light sensor aboard NASA’s STEREO-A spacecraft. Image credit: NASA / Lowell Observatory / Qicheng Zhang.
The STEREO mission provided observations of the interstellar comet 3I/ATLAS spanning the period from September 11 to October 2, 2025.
Developed with the objective of investigating solar activity and its pervasive influence throughout the Solar System, this mission is integral to a constellation of NASA spacecraft tasked with scrutinizing the comet. Collectively, these instruments are contributing to a more comprehensive understanding of the comet’s dimensions, physical characteristics, and elemental composition.
Initially, it was anticipated that comet 3I/ATLAS would be too ephemeral for STEREO’s sensitive instruments to detect. However, through sophisticated image processing techniques, including the overlaying or ‘stacking’ of numerous telescope images via the Heliospheric Imager-1 instrument—a visible-light telescope—the presence of 3I/ATLAS was successfully discerned.
The resultant composite images, created by aligning and merging multiple exposures, depict the comet as a subtle intensification of light located at the image center.
This visual representation of 3I/ATLAS integrates observational data gathered by the NASA/ESA SOHO mission during the interval of October 15 to 26, 2025. Image credit: NASA / ESA / Lowell Observatory / Qicheng Zhang.
The SOHO spacecraft managed to capture a fleeting image of 3I/ATLAS between October 15 and 26, 2025.
During this observation window, SOHO’s Large Angle and Spectrometric Coronagraph (LASCO) instrument suite registered the comet as it traversed its field of view, at an approximate distance of 358 million kilometers (222 million miles) – a distance exceeding twice that of Earth from the Sun.
SOHO maintains an orbit around the Sun-Earth Lagrange point 1, a gravitationally stable region situated approximately 1.5 million kilometers (one million miles) closer to the Sun along the axis connecting the Earth and the Sun.
The team members responsible for SOHO also employed image stacking methodologies to generate the captured image of 3I/ATLAS.
Within this composite image, derived from an amalgamation of observations by NASA’s PUNCH mission between September 20 and October 3, 2025, 3I/ATLAS is discernible as a luminous point near the central area. Image credit: NASA / Southwest Research Institute.
The PUNCH mission documented 3I/ATLAS from September 20 to October 3, 2025.
These observational data revealed the comet’s tail, presenting as a subtle elongation extending towards the lower right quadrant.
The comet exhibited considerable faintness during these observational periods, leading to initial uncertainty within the PUNCH team regarding the spacecraft’s capacity to obtain a clear view. This uncertainty stemmed from the spacecraft’s design, which is primarily intended for the study of the Sun’s atmosphere and the solar wind, rather than cometary bodies.
Nonetheless, the cumulative effect of stacking multiple PUNCH observations successfully brought 3I/ATLAS and its attendant tail into clearer view.
“We are truly operating at the extreme capabilities of the system,” commented Dr. Kevin Walsh, a planetary scientist affiliated with the Southwest Research Institute, who spearheaded PUNCH’s efforts in observing the comet.
