See the 3I/ATLAS comet strangely shift from red to green in new images
In the spirit of the season, the interstellar comet 3I/ATLAS is changing color as it leaves the sun behind — shifting from a reddish tint to a faint green glow.
That visible change signals the comet is reacting to solar heat and releasing new gases in space, giving scientists fresh clues about what it is made of and how it behaves.
Astronomers used the Gemini North telescope in Hawaii to capture new color images of the comet after it emerged from the other side of the sun on Nov. 26. Earlier images taken from Gemini South in Chile showed the comet looking redder. That ruddy color comes from gases surrounding the comet — called a coma — that heat up and shine as frozen material turns directly into gas, skipping the liquid phase. Some of those gases emit green light, which Gemini’s cameras can detect.
"Interstellar comet 3I/ATLAS has a post-perihelion greenish glow in time for the holidays," said Bryce Bolin, a research scientist from Eureka Scientific who led the observations, in a post on X.
Just days later, a space telescope added another piece to the puzzle. The European Space Agency’s XMM-Newton spacecraft observed 3I/ATLAS in X-rays for nearly 20 hours on Dec. 3. X-rays are a very high-energy form of light that human eyes cannot see.
The new observations offer a rare chance to study material from another star system while the comet is still somewhat close. By tracking its changes across multiple types of light, scientists can better identify what it's made of and consider how interstellar objects form.
Comet 3I/ATLAS came from another part of the galaxy and was later ejected — likely by a gravitational jolt from a planet or passing star — before drifting across interstellar space for hundreds of millions of years. Only two other confirmed interstellar visitors have ever been sighted: 'Oumuamua in 201, which wasn't a comet, and Comet 2I/Borisov in 2019.
The latest visitor entered the solar system traveling roughly 137,000 mph, a speed far too fast for the sun’s gravity to trap it. Scientists say that means it's only swinging by and will never return.
Comets are icy boulders that heat up as they approach the sun, releasing gas and dust in bright, sweeping tails. Most known comets are leftover materials from the solar system’s construction 4.6 billion years ago, though thousands more likely lurk beyond Neptune in the Kuiper Belt and, farther still, in the Oort Cloud.
NASA and ESA have pointed many spacecraft cameras at the comet — including the Hubble and James Webb Space Telescope, the Mars Reconnaissance Orbiter, and the Lucy and Psyche missions — to collect as much information as possible while the space interloper remains in the solar system.
The Gemini North pictures took place during a live public event organized through Shadow the Scientists. The program invites students and the public into telescope control rooms so they can watch astronomers work in real time. As the telescope tracked the comet, it stayed sharp in the images while background stars streaked across the frame in different colors.
Scientists are especially interested in how 3I/ATLAS will continue to change. Comets often respond slowly to the sun's heat, because warmth takes time to reach their interiors. As a result, new gases can begin escaping later, or the comet may suddenly brighten.
"Sharing an observing experience in some of the best conditions available gives the public a truly front-row view of our interstellar visitor," Bolin said in a statement. "Allowing the public to see what we do as astronomers and how we do it also helps demystify the scientific and data collection process, adding transparency to our study of this fascinating object."
Astronomers generally expected the comet to glow in X-rays because gas flowing off it crashes into the solar wind, a constant stream of charged particles blowing outward from the sun. That collision creates X-rays, revealing gases that are otherwise hard to detect.
These X-rays are especially useful because they can point to gases like hydrogen and nitrogen, which are nearly invisible to regular cameras and even to many space telescopes. Learning which gases are present helps scientists figure out the comet's composition.
Because 3I/ATLAS comes from a different stellar neighborhood, scientists expect some of its characteristics to be different from native comets. Early readings show that its carbon dioxide–to–water ratio does not match solar system comets, and researchers have spotted some unusual amounts of metals and dust features.
But none of those traits, NASA officials have emphasized, suggest it's not a comet or that it could be an alien spacecraft, as some have speculated on the internet. Tom Statler, NASA's lead scientist for solar system small bodies, compared these differences to coffees of various regions, such as Kona and Sumatra.
"It's different, and yet it's still coffee," he said. "In this case, we have a cometary body. It resembles the homegrown comets that we have in our solar system, and yet it's excitingly different in particular ways."
