NASA Missions Track Interstellar Comet 3I/ATLAS Behind the Sun
NASA’s STEREO (Solar Terrestrial Relations Observatory), the NASA/ESA SOHO (Solar and Heliospheric Observatory), and NASA’s PUNCH (Polarimeter to Unify the Corona and Heliosphere) missions have the unique ability to observe areas of the sky near the Sun, which allowed them to track 3I/ATLAS as it passed behind our sun as seen from Earth.
Traveling through our Solar System at a staggering 209,000 km (130,000 miles) per hour, 3I/ATLAS was made visible by using a series of colorized stacked images from September 11-25, 2025, using the Heliocentric Imager-1 instrument, a visible-light imager on NASA’s STEREO-A spacecraft. Image credit: NASA / Lowell Observatory / Qicheng Zhang.
STEREO observed the interstellar comet 3I/ATLAS from September 11 to October 2, 2025.
Designed to study the Sun’s activity and its influence across the Solar System, the mission is part of a fleet of NASA spacecraft observing the comet, together providing more information about its size, physical properties, and chemical makeup.
At first, comet 3I/ATLAS was expected to be to faint for STEREO’s instruments to see, but detailed image processing and overlaying (or ‘stacking’) telescope images using the Heliospheric Imager-1 instrument, a visible-light telescope, brought 3I/ATLAS into view.
Stacking and aligning multiple exposures ultimately generated several images where the comet appears as a slight brightening in the center.
This image shows comet 3I/ATLAS as a faint brightening in the center.It was created by stacking images from September 25, 2025, using the heliospheric Imager-1 instrument on NASA’s STEREO-A spacecraft. Image credit: NASA / Lowell Observatory / Qicheng Zhang.