5 Minutes
New high-resolution images from NOIRLab’s Gemini South telescope reveal that interstellar comet 3I/ATLAS is developing a distinct tail as it moves inward toward the Sun. The emerging tail is an important marker of changing activity on this visitor from beyond our Solar System and offers astronomers a chance to study its dust and gas in greater detail before the object returns to interstellar space.
Recent observations and imaging strategy
Gemini South captured the comet using multiple color filters while tracking the object so it remained centered in the frame. Gemini’s tracking method makes the background stars appear as colored streaks because the telescope follows the comet’s motion across the sky. Gemini South used different color filters to snap the comet, keeping it fixed in the center of its field of view. The stars in the background appear to move in relation to the comet, appearing therefore as colored streaks in the sky.
These observations were designed to measure the comet’s color profile — a proxy for dust-particle composition and size distribution — and to obtain spectra that can directly identify chemical species in the coma. Early analysis indicates a visible extension of material forming a tail, and preliminary spectral data have already hinted at compositional shifts compared with earlier images.
What makes 3I/ATLAS unusual
3I/ATLAS differs from the two previously confirmed interstellar visitors, 1I/’Oumuamua and 2I/Borisov, and it also shows properties unlike many native Solar System comets. Notably, its coma began to expand while the object was still well beyond Jupiter’s orbit — much farther from the Sun than where most comets become active.
Astronomers attribute that early activation to the comet’s ice inventory, which appears to be unusually rich in carbon dioxide (CO2). Carbon dioxide ice sublimates at lower temperatures than water ice, so CO2-driven outgassing can start farther from the Sun. That early sublimation releases dust and gas to form a coma, and as solar radiation and the solar wind act on that coma, a tail forms. Radiation pressure tends to push dust outward, producing the broad dust tail, while the solar wind ionizes gaseous molecules and forms a narrower ion tail; both are typically pointed away from the Sun.
The comet’s calculated perihelion — its closest approach to the Sun — is around 29 October 2025 and will lie just inside Mars’ orbit. Its path does not cross Earth’s orbit, and Earth’s closest approach will occur after perihelion, providing additional months for ground- and space-based observatories to monitor evolving activity.
Scientific goals and implications
Ongoing spectroscopy and photometry aim to quantify the coma’s chemistry (including volatile abundances such as CO2, CO, and water), measure dust-particle sizes, and compare 3I/ATLAS’s properties with both interstellar and Solar System comets. Understanding the relative abundances of ices and refractory material in an interstellar object gives insight into formation conditions in other stellar systems and the diversity of small bodies that traverse our galaxy.
Expert Insight
Dr. Elena Park, an astrophysicist specializing in small bodies, comments: “3I/ATLAS presents a rare opportunity to sample material formed around another star. If Gemini’s spectra confirm elevated CO2 and unexpected organics or dust-size distributions, it will reshape our models of comet formation outside the Solar System. Coordinated observations with infrared facilities and large synoptic surveys will be crucial to map how the activity evolves through perihelion.”
Conclusion
Gemini South’s images showing a growing tail on 3I/ATLAS mark a key phase in the study of this interstellar visitor. With perihelion approaching on 29 October 2025 and follow-up spectroscopy ongoing, astronomers have a limited but valuable window to probe the comet’s composition and behavior. Observations now will help place 3I/ATLAS in context with other interstellar objects and expand our understanding of the diversity of small bodies across the galaxy.
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