NASA's STEREO (Solar-Terrestrial Link Observatory), NASA/ESA SOHO (Solar and Heliospheric Observatory) and NASA PUNCH (Polarimeter for Corona-Heliosphere Unification) missions have the unique ability to observe regions of the sky near the Sun, which has allowed them to track 3I/ATLAS when it passed behind our Sun as seen from Earth.
Traveling through our solar system at 209,000 km (130,000 miles) per hour, 3I/ATLAS became visible through a series of color stacked images from September 11 to 25, 2025, using Heliocentric Imager-1, a visible light imager on NASA's STEREO-A spacecraft. Image credit: NASA/Lowell Observatory/Qicheng Zhang.
STEREO observed interstellar comet 3I/ATLAS from September 11 to October 2, 2025.
The mission, designed to study the sun's activity and its impact on the solar system, is part of NASA's fleet of spacecraft observing the comet, which together are providing additional information about its size, physical properties and chemical composition.
Comet 3I/ATLAS was initially expected to be too dim to be seen by the STEREO instruments, but detailed image processing and overlaying (or “stacking”) of the telescope images with the Heliogenic Imager-1 instrument, a visible light telescope, allowed 3I/ATLAS to be seen.
Combining and combining multiple exposures ultimately resulted in several images in which the comet appears as a small clearing in the center.
This 3I/ATLAS image combines observations from the NASA/ESA SOHO mission between October 15 and October 26, 2025. Image credit: NASA/ESA/Lowell Observatory/Qicheng Zhang.
The SOHO spacecraft captured the 3I/ATLAS image on October 15-26, 2025.
During this time period, SOHO's High Angle Spectrometric Coronagraph (LASCO) instrument cluster detected the comet crossing its field of view at a distance of approximately 358 million kilometers (222 million miles), more than twice the Earth's distance from the Sun.
SOHO orbits the Sun-Earth Lagrange Point 1, a region of gravitational balance located about 1/6 million km (one million miles) closer to the Sun along the Sun-Earth line.
SOHO team members also used the stacking technique to create the 3I/ATLAS image.
3I/ATLAS appears as a bright object near the center of this image, a composite of observations from NASA's PUNCH mission taken from September 20 to October 3, 2025. Image courtesy of NASA/Southwest Research Institute.
The PUNCH mission observed 3I/ATLAS from September 20 to October 3, 2025.
The observations revealed the comet's tail, visible as a slight extension in the lower right corner.
During these observations, the comet was very faint, so the PUNCH team was unsure if the spacecraft would be able to see it clearly, since the spacecraft is designed to study the Sun's atmosphere and solar wind, not comets.
However, the summation of several PUNCH observations revealed 3I/ATLAS and its tail.
“We're really pushing the boundaries of the system,” said Dr. Kevin Walsh, a planetary scientist at the Southwest Research Institute who led PUNCH's observations of the comet.
