Universe Today
Interstellar comet 3I/ATLAS was first detected on July 1st 2025 as it approached the Sun. Multiple telescopes, including the Very Large Telescope, the Hubble Space Telescope, and the James Webb Space Telescope have taken turns observing the comet as it continued on its hyperbolic trajectory out of the Solar System. It reached solar conjunction on October 21st, and on October 29th it came closest to the Sun.
Professional astronomers and amateur astronomers have been observing and imaging the visiting comet ever since its arrival. New images from the Gemini North Telescope on Maunakea in Hawai‘i come from a program where professional astronomers work with the public to show them how scientific observing works. The program is called Shadow the Scientists, and this is the second time the program has brought astronomers and the public together to observe 3I/ATLAS. The first occasion was in September and took place at the Gemini South Telescope in Chile.
In the Gemini South images from September, 3I/ATLAS appeared with a red hue. But in these images, it has a faint green glow. The difference is because as the comet heats up, different chemicals in the coma are evaporating. In the new Gemini North image, diatomic carbon is heating up and emitting the green glow. Diatomic carbon is extremely reactive and is common in comets.
In this Gemini South image of 3I/ATLAS, the comet appears to have a red hue. The difference in colours is because of different chemicals evaporating in the coma. Image Credit: International Gemini Observatory/NOIRLab/NSF/AURA/Shadow the Scientist Image Processing: J. Miller & M. Rodriguez (International Gemini Observatory/NSF NOIRLab), T.A. Rector (University of Alaska Anchorage/NSF NOIRLab), M. Zamani (NSF NOIRLab)
The comet sped up as it approached the Sun, and then slowed down as it passed the Sun and its gravity pulled on it. Despite this, it will easily escape the Solar System. It's travelling much faster than 1I/ʻOumuamua and 2I/Borisov, the only other two known ISOs. But the action won't stop as it moves away from the Sun.
The interstellar comet 3I/ATLAS with its coma as captured by the Gemini North Telescope and its Multi-Object Spectrograph. In this image, the background stars are frozen in place. Below the comet and to the right is an unknown main belt asteroid. Image Credit: International Gemini Observatory/NOIRLab/NSF/AURA/B. Bolin Image Processing: J. Miller & M. Rodriguez (International Gemini Observatory/NSF NOIRLab), T.A. Rector (University of Alaska Anchorage/NSF NOIRLab), M. Zamani (NSF NOIRLab)
The Sun's heat takes time to penetrate into 3I/ATLAS' nucleus, and some comets exhibit a delayed reaction after they pass the Sun. The delayed reaction can release different chemicals, so Gemini and other telescopes will observe the comet to see what it emits. Nickel has been detected in the comet, which is common, but iron hasn't. This is unusual, since nickel and iron are usually found in approximately equal amounts in comets. Maybe iron will make an appearance in future observations as the Sun's heat penetrates the comet.
The Shadow the Scientists program is led by Bryce Bolin, a research scientist from Eureka Scientific. It invites the public to take part in live astronomical observations. The objective is two-fold: to share scientific knowledge and to excite the next generation of scientists.
In Shadow the Scientists, students from Chile and Hawaii, the homes of Gemini South and Gemini North respectively, joined astronomers for a Zoom session during scientific observations of the comets. They got to watch in real time as astronomers operated the telescope and subsidiary equipment in the observatory's control room. They asked questions and followed the progress of the observations.
“Sharing an observing experience in some of the best conditions available gives the public a truly front-row view of our interstellar visitor,” says Bolin. “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.”
A visiting interstellar comet generates a lot of interest, not only in scientists but in members of the public. There's an eagerness to learn more about them, and that may require close inspection, and even a sample return. But by the time one of them reaches the inner Solar System, it's too late to send a spacecraft to visit it. There are concepts for a comet interceptor mission that would park a spacecraft in orbit, where it would wait until an interstellar object (ISO) was spotted, then be sent on an intercept course.
That will only work if one is detected early enough. That will likely be possible thanks to the Vera Rubin Observatory and its Legacy Survey of Space and Time. Scientific estimates show that it will likely find at least 10 ISOs like 3I/ATLAS.
We're poised to learn a lot more about ISOs in the near future. Objects like 3I/ATLAS, and Oumuamua and Borisov, are only whetting our appetites.
