Uploaded October 2025 | Updated September 2026, 2 weeks ago
What happens when a visitor from another star system drops by? Join planetary astronomers Franck Marchis and Ariel Graykowski for a special SETI Live all about Comet 3I/ATLAS — only the third known interstellar object ever detected!
Astronomers around the world, including citizen scientists in the Unistellar Network, are racing to learn as much as possible about this rare cosmic traveler. 3I/ATLAS is swinging through our neighborhood, reaching perihelion on October 30, 2025, just inside the orbit of Mars — a front-row seat for spacecraft like Lucy and Psyche.
While it’s currently hidden behind the Sun, it won’t stay that way for long. By December 2025, 3I/ATLAS will reappear, ready for a fresh round of observations from Earth and its Lagrange-point observatories.
We’ll dive into what scientists have discovered so far, how they’re studying this interstellar visitor, and what it might reveal about the chemistry and dynamics of other star systems. Don’t miss it — interstellar comets don’t come around every day!
Various papers and references:
The report of an observed coma, soon after discovery: Jewitt et al., Jul 3 2025, The Astronomer’s Telegram: astronomerstelegram.org/?read=17263
The estimate of nucleus size from Hubble data (radius less than 2.8 km). This paper also includes an explanation of the anti-tail observed during this time: David Jewitt et al 2025 ApJL 990 L2: iopscience.iop.org/article/10.3847/2041-8213/adf8d8
The observation of a CO2 gas-dominated coma and very little H2O gas in the coma. There was also a strong detection of H2O ice. This paper emphasizes that the coma accounts for the vast majority of the flux, not the nucleus: C. M. Lisse et al 2025 Res. Notes AAS 9 242: iopscience.iop.org/article/10.3847/2515-5172/ae0293/ampdf
The IAWN astrometry campaign, which emphasizes that 3I poses no threat to Earth, is simply a great opportunity for the community to learn comet astrometry to refine comet orbits as much as possible: iawn.net/obscamp/3I_ATLAS
A report about the anti-tail. The report is recent, but the data is from August 2. This is the same anti-tail as reported by Jewitt et al, 2025 above. Serra-Ricart et al, Oct 15 2025, The Astronomer’s Telegram: astronomerstelegram.org/?read=17445
High Nickel-to Iron ratio, which is becoming more “normal” (when comparing to our solar system comets) as it gets closer to the Sun:
Rahatgaonkar, R. et al, 2025, Accepted to ApJL: arxiv.org/pdf/2508.18382
Hutsemékers, D. et al 2025 accepted to A&A: arxiv.org/pdf/2509.26053
Hoogendam, W. B. et al 2025: arxiv.org/pdf/2510.11779
Before 3I’s orbit took it behind the Sun from our perspective on Earth, we noticed a rapid, unexpected brightening. This brightening is confirmed to have continued through perihelion thanks to space-based solar chronographs and heliospheric imagers: Zhang, Q. & Battams, K 2025, submitted to ApJL: arxiv.org/pdf/2510.25035
📺 SETI Live is a weekly livestream from the SETI Institute, featuring researchers and discoveries that deepen our understanding of the universe—and our place within it.
#Comet3IATLAS #InterstellarVisitor #SETILive #SETIInstitute #PlanetaryScience #Astronomy #SpaceExploration #InterstellarObject #CitizenScience #Unistellar #LucyMission #PsycheMission #CometScience #SolarSystem #SpaceResearch #NotAliens
If you like science, support the SETI Institute! We're a non-profit research institution focused on understanding the nature and origins of life in the universe. 💸 Donate here: seti.org/donate
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What happens when a visitor from another star system drops by? Join planetary astronomers Franck Marchis and Ariel Graykowski for a special SETI Live all about Comet 3I/ATLAS — only the third known interstellar object ever detected!
Astronomers around the world, including citizen scientists in the Unistellar Network, are racing to learn as much as possible about this rare cosmic traveler. 3I/ATLAS is swinging through our neighborhood, reaching perihelion on October 30, 2025, just inside the orbit of Mars — a front-row seat for spacecraft like Lucy and Psyche.
While it’s currently hidden behind the Sun, it won’t stay that way for long. By December 2025, 3I/ATLAS will reappear, ready for a fresh round of observations from Earth and its Lagrange-point observatories.
We’ll dive into what scientists have discovered so far, how they’re studying this interstellar visitor, and what it might reveal about the chemistry and dynamics of other star systems. Don’t miss it — interstellar comets don’t come around every day!
Various papers and references:
The report of an observed coma, soon after discovery: Jewitt et al., Jul 3 2025, The Astronomer’s Telegram: astronomerstelegram.org/?read=17263
The estimate of nucleus size from Hubble data (radius less than 2.8 km). This paper also includes an explanation of the anti-tail observed during this time: David Jewitt et al 2025 ApJL 990 L2: iopscience.iop.org/article/10.3847/2041-8213/adf8d8
The observation of a CO2 gas-dominated coma and very little H2O gas in the coma. There was also a strong detection of H2O ice. This paper emphasizes that the coma accounts for the vast majority of the flux, not the nucleus: C. M. Lisse et al 2025 Res. Notes AAS 9 242: iopscience.iop.org/article/10.3847/2515-5172/ae0293/ampdf
The IAWN astrometry campaign, which emphasizes that 3I poses no threat to Earth, is simply a great opportunity for the community to learn comet astrometry to refine comet orbits as much as possible: iawn.net/obscamp/3I_ATLAS
A report about the anti-tail. The report is recent, but the data is from August 2. This is the same anti-tail as reported by Jewitt et al, 2025 above. Serra-Ricart et al, Oct 15 2025, The Astronomer’s Telegram: astronomerstelegram.org/?read=17445
High Nickel-to Iron ratio, which is becoming more “normal” (when comparing to our solar system comets) as it gets closer to the Sun:
Rahatgaonkar, R. et al, 2025, Accepted to ApJL: arxiv.org/pdf/2508.18382
Hutsemékers, D. et al 2025 accepted to A&A: arxiv.org/pdf/2509.26053
Hoogendam, W. B. et al 2025: arxiv.org/pdf/2510.11779
Before 3I’s orbit took it behind the Sun from our perspective on Earth, we noticed a rapid, unexpected brightening. This brightening is confirmed to have continued through perihelion thanks to space-based solar chronographs and heliospheric imagers: Zhang, Q. & Battams, K 2025, submitted to ApJL: arxiv.org/pdf/2510.25035
📺 SETI Live is a weekly livestream from the SETI Institute, featuring researchers and discoveries that deepen our understanding of the universe—and our place within it.
#Comet3IATLAS #InterstellarVisitor #SETILive #SETIInstitute #PlanetaryScience #Astronomy #SpaceExploration #InterstellarObject #CitizenScience #Unistellar #LucyMission #PsycheMission #CometScience #SolarSystem #SpaceResearch #NotAliens
If you like science, support the SETI Institute! We're a non-profit research institution focused on understanding the nature and origins of life in the universe. 💸 Donate here: seti.org/donate
Learn more about the SETI Institute and stay up-to-date on awesome science:
🛎 Subscribe to our YouTube channel at youtube.com/c/SETIInstitute
🕹 Watch our streams over on Twitch at twitch.tv/setiinstitute
🎧 Listen to our podcast feeds.libsyn.com/462636/rss
📧 Subscribe to our weekly newsletter seti.org/signup
👕 Buy merchandise from Chop Shop chopshopstore.com/collections/seti-institute/SETI
Don't forget to like and subscribe! Ring the bell for notifications of when we go live.
