Interstellar comet 'switches on,' gaining speed from a blast of sunlight

On a late August night in Chile, the Gemini South telescope swiveled toward a traveler from deep space – a true outsider passing through on a one-time visit. The target was Comet 3I/ATLAS, only the third confirmed interstellar comet ever seen.

Then, the images rolled in – showing a brighter, puffier coma and a newly lengthened tail streaming away from the Sun.

In astronomical terms, 3I/ATLAS had “switched on,” warming enough for sunlight to lift gas and dust off its surface and turn a dim speck into a classic comet.

The images were captured by the Gemini Multi-Object Spectrograph (GMOS) on Gemini South, part of the International Gemini Observatory operated by NSF NOIRLab.

The photos reveal a broad, glowing coma and a tail about 1/120th of a degree wide in the sky. It looks tiny to the eye – yet it’s a clear sign of a comet gaining speed and heat as it cuts inward through the solar system.

Watching the comet in real time

This wasn’t a closed-door observing run. As the telescope gathered photons, students and curious sky watchers logged into the Gemini South control room over Zoom.

The session, organized by NOIRLab with Shadow the Scientists, turned a two-hour block of cutting-edge observing into a live, guided field trip.

Participants from Hawaii, La Serena, and far beyond watched the comet come into focus and filled the chat with questions. They witnessed the messy, exhilarating reality of real-time science: choosing filters, adjusting exposures, and celebrating the first clean spectrum.

Project lead Karen Meech, an astronomer at the University of Hawaiʻi Institute for Astronomy, established clear goals. The team wanted precise colors – clues to dust sizes and composition in the coma – and, crucially, a spectrum, the rainbow barcode that reveals a comet’s chemistry.

“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,” said Meech.

Chasing color and chemistry

Comets are icy, dusty leftovers from planet formation. When they heat up, ices sublimate, lofting dust and gas that build a hazy atmosphere (the coma) and a tail pushed away by sunlight and the solar wind.

In the new data, 3I/ATLAS’s coma looks broader and brighter than before. Its tail, a neat sliver pointing directly away from the Sun, reveals that the comet is actively shedding material.

Those changes aren’t just pretty; they’re a physical record of what happens as a frozen body from the void gets its first serious blast of sunshine in who knows how long.

Images reveal comet shedding

Alongside the images, the team captured the comet’s spectrum. By breaking light into its component wavelengths, spectra reveal the fingerprints of molecules and the properties of dust grains – information that images alone cannot provide.

Bin Yang, an assistant professor at Universidad Diego Portales in Santiago, walked the audience through what to look for. Broad color trends can flag dust size, sharp features can betray gases like CN or C₂, and together they show how everything evolves as the comet brightens.

Early indications are that the dust and ices of 3I/ATLAS look a lot like those in homegrown comets. This hints that planet-forming disks around other stars may cook up comet ingredients in ways familiar to us.

Why interstellar comets matter

Interstellar objects are the definition of rare. Before 3I/ATLAS, there was 1I/ʻOumuamua in 2017 and 2I/Borisov in 2019.

Unlike solar system comets that return on predictable loops, 3I/ATLAS is on a hyperbolic path: it will flare briefly and then slide back into interstellar space. That makes the timing urgent and the science precious.

The material we see fizzing off its surface formed around another star long ago. Catching and decoding it is one of the few ways to sample the chemistry of faraway planet nurseries without leaving home.

“As 3I/ATLAS speeds back into the depths of interstellar space, this image is both a scientific milestone and a source of wonder,” Meech said.

“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.”

Hands-on cosmic learning

If you’ve never watched a spectrum appear line by line on a monitor at two in the morning, this session delivered the next best thing.

Viewers saw how astronomers center a faint, moving target in a narrow slit and how they check focus and tracking. They also saw how raw frames – streaked with cosmic rays and dotted with calibration lines – become clean, interpretable data.

The back-and-forth with the team demystified a process that, from the outside, can feel like magic. It’s not magic; it’s patience, practice, and a lot of careful decisions made on the fly.

“These observations provide both a breathtaking view and critical scientific data,” said Bryce Bolin of Eureka Scientific, who joined the run.

“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.”

Preparing for comet’s return

3I/ATLAS was first flagged on July 1, 2025 by the ATLAS (Asteroid Terrestrial-impact Last Alert System) survey. Since then, it’s been a race to grab photometry and spectroscopy before the comet disappears behind the Sun. The Gemini South data add carefully calibrated colors and a first cut at chemistry.

Come November, when 3I/ATLAS emerges from solar glare, Bolin will host a follow-up session from Gemini North on Maunakea. It will give the public another live window into how scientists chase a fast-moving target.

There’s a reason this effort resonates. It blends the thrill of discovery with a hands-on lesson in how discovery actually happens.

Programs like this knock down the wall between the control room and the classroom and, in the process, show a new generation that they belong in both. The comet will be gone soon enough. The know-how – and the sense of wonder – stick around.

Image Credit: International Gemini Observatory / NOIRLab / NSF / AURA

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