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Comet 3I/ATLAS

Cataloguing the journey of comet 3I/ATLAS through the solar system. Because the object comes from outside our solar system, it is just passing through – so we use all the tools at our disposal to observe it before it disappears back into the cosmic dark. A host of NASA missions are coming together to observe this interstellar object, which was first discovered in summer 2025, before it leaves forever. While the comet poses no threat to Earth, NASA’s space telescopes help support the agency's ongoing mission to find, track, and better understand solar system objects.

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NASA’S STEREO Observes Interstellar Comet 3I/ATLAS

NASA’s STEREO (Solar Terrestrial Relations Observatory) observed interstellar comet 3I/ATLAS from Sept. 11 to Oct. 2. The STEREO mission, designed to study the Sun’s activity and its influence across the solar system, is part of a fleet of NASA spacecraft observing this comet, together providing more information about its size, physical properties, and chemical makeup.

At first, comet 3I/ATLAS was expected to be too 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 the comet into view. Stacking and aligning multiple exposures ultimately generated several images where the comet appears as a slight brightening in the center.

The image is made up of vertical streaks of black and various hues of pinks with a hazy white orb at the center, representing the comet 3I/ATLAS. The top left corner has the name of the comet, 3I/ATLAS, and below that, the date range the comet was observed.
This image shows the 3I/ATLAS interstellar comet as a bright, fuzzy orb in the center. Traveling through our solar system at a staggering 130,000 miles (209,000 kilometers) per hour, 3I/ATLAS was made visible by using a series of colorized stacked images from Sept. 11-25, using the Heliocentric Imager-1 instrument, a visible-light imager on the STEREO-A (Solar Terrestrial Relations Observatory) spacecraft. The colorization was applied to differentiate the image from other observing spacecraft images
NASA/Lowell Observatory/Qicheng Zhang

This video shows the 3I/ATLAS interstellar comet as a bright, fuzzy orb in the center. Traveling through our solar system at a staggering 130,000 miles (209,000 kilometers) per hour, 3I/ATLAS was made visible by using a series of stacked images from Sept. 11-25, using the Heliocentric Imager-1 (HI1) instrument, a visible-light imager on the STEREO-A (Solar Terrestrial Relations Observatory) spacecraft. Originally, the comet was expected to be too faint to be observed, but the comet brightened up enough to be visible, allowing the HI1 instrument to see the comet against a noise-dominated background through a combination of frames.
NASA/Lowell Observatory/Qicheng Zhang

NASA’s PUNCH (Polarimeter to Unify the Corona and Heliosphere), the NASA and ESA (European Space Agency) mission SOHO (Solar and Heliospheric Observatory), and other heliophysics and planetary missions have also recently observed the comet. Despite having observed and discovered thousands of comets, this is the first time NASA’s heliophysics missions have knowingly observed an object originating in another solar system.
 
Comet 3I/ATLAS was discovered by the NASA-funded ATLAS (Asteroid Terrestrial-impact Last Alert System) survey telescope in Rio Hurtado, Chile, in July 2025. It is the third known object originating from outside our solar system discovered passing through our solar neighborhood.
 
While the comet poses no threat to Earth, multiple NASA spacecraft are studying it so scientists can learn as much as possible about this interstellar visitor before the comet leaves our solar system and heads back into interstellar space, never to return.

By Desiree Apodaca
NASA’s Goddard Space Flight Center, Greenbelt, Md.