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Astronomers and students working together through a unique educational initiative have obtained a striking new image of the growing tail of interstellar Comet 3I/ATLAS. The observations reveal a prominent tail and glowing coma from this rare celestial visitor, while also providing new scientific measurements of its colors and composition.

On 27 August 2025, researchers used the Gemini Multi-Object Spectrograph (GMOS) on Gemini South at Cerro Pachón in Chile to obtain deep, multi-color images of interstellar comet Comet 3I/ATLAS. Gemini South is one half of the International Gemini Observatory, operated by NSF NOIRLab.

These observations were taken as part of a public outreach initiative organized by NSF NOIRLab in collaboration with Shadow the Scientists, an initiative created to connect the public with scientists to engage in authentic scientific experiments, such as astronomy observing experiences on world-class telescopes. The scientific program was led by Karen Meech, astronomer at the University of Hawai'i Institute for Astronomy (UH IfA).

In the images captured during the session, the comet displays a broad coma—a cloud of gas and dust that forms around the comet's icy nucleus as it gets closer to the sun—and a tail spanning about 1/120th of a degree in the sky (where one degree is about the width of a pinky finger on an outstretched arm) and pointing away from the sun. These features are significantly more extended than they appeared in earlier images of the comet, showing that 3I/ATLAS has become more active as it travels through the inner solar system.

Members of the public, including students from Hawai'i and La Serena, Chile, were invited to remotely join the Gemini South control room in a special two-hour Zoom session where they could interact directly with astronomers, ask questions about interstellar cometary science, and follow the progress of the observations in real time. The event was followed across the world with people joining from Europe, New Zealand, and South America.

More than just capturing stunning images, the main scientific motivation of the observing session was to collect the comet's spectrum, which refers to the wavelengths of light that it emits. A spectrum can tell scientists information about the comet's composition and chemistry, which allows them to understand how the comet changes as it passes through the solar system.

The interstellar object was first detected on 1 July 2025 by ATLAS (Asteroid Terrestrial-impact Last Alert System). The new observations suggest that the dust and ice of 3I/ATLAS are broadly similar to those of comets native to our solar system, hinting at shared processes in the formation of planetary systems around other stars.

During the observing session, Bin Yang, assistant professor at the Instituto de Estudios Astrofísicos (IEA) at Universidad Diego Portales, guided participants through the interpretation of the spectral data, while Meech led a discussion about the importance of interstellar objects for understanding the formation and evolution of planetary systems.

"The primary objectives of the observations were to look at the colors of the comet, which provide clues to the composition and sizes of the dust particles in the coma, and to take spectra for a direct measure of the chemistry," says Meech. "We were excited to see the growth of the tail, suggesting a change in the particles from the previous Gemini images, and we got our first glimpse of the chemistry from the spectrum."

Interstellar comets are extraordinarily rare: 3I/ATLAS is only the third confirmed example after and Comet 2I/Borisov in 2019. Unlike comets bound to the sun, 3I/ATLAS is traveling on a hyperbolic orbit that will eventually carry it back into interstellar space. Its brief passage through the solar system gives astronomers a once-in-a-lifetime opportunity to study material that formed around a distant star.

This effort builds on NOIRLab's tradition of combining cutting-edge science with public engagement, ensuring that remarkable cosmic events are shared as widely as possible. By involving learners directly in observing sessions and data collection, programs like this one not only advance knowledge but also inspire the next generation of explorers.

"As 3I/ATLAS speeds back into the depths of interstellar space, this image is both a scientific milestone and a source of wonder," says Meech. "It reminds us that our solar system is just one part of a vast and dynamic galaxy—and that even the most fleeting visitors can leave a lasting impact."

Also present during the observing session and lending his scientific expertise was Bryce Bolin, research scientist from Eureka Scientific.

"These observations provide both a breathtaking view and critical scientific data," he says. "Every interstellar comet is a messenger from another star system, and by studying their light and color, we can begin to understand the diversity of worlds beyond our own."

In November 2025, when Comet 3I/ATLAS reemerges from behind the sun, Bolin will host a follow-up Shadow the Scientists observing session, this time bringing the public into the control room of the Gemini North telescope in Hawai'i.