As of November 15, 2025, the interstellar object 3I/ATLAS still displays a prominent anti-tail as well as tails, according to a new image released by Teerasak Thaluang from a 0.26-meter telescope in Thailand.
The physics behind the anti-tail of 3I/ATLAS is unclear. For a natural comet, it could be associated with the release of giant dust particles with an effective radius of ~100 micrometers (as suggested by David Jewitt and collaborators here and here). These particles are a million times more massive than the typical micrometer-scale dust which scatters sunlight most effectively because it is comparable in size to the wavelength of the radiation. Since the ratio of surface area — which scales as particle radius squared, to mass — which scales as radius cubed, is 100 times smaller for these giant particles relative to their smaller counterparts, they will not be accelerated away from the Sun by radiation pressure as effectively as micrometer dust in familiar comets. However, the amount of mass loss in 100-micrometer particles must be 100 times larger than the corresponding value for micrometer particles in order to produce the same brightness of scattered sunlight.
Another possibility is that instead of refractory dust particles that survive solar heating, the scattering of sunlight is produced by fragments of ice which get evaporated before having a chance to turn around and produce a traditional cometary tail away from the Sun (as I suggested with Eric Keto here and here).
Finally, there is the more speculative possibility that the anti-tail is a result of technological thrusters which accelerate 3I/ATLAS away from the Sun through tightly collimated jets that penetrate a million kilometers through the Solar wind because of their high speed. Future spectroscopic data will be able to calibrate the outflow speed and distinguish between natural outgassing which results in a characteristic speed of up to a few hundred meters per second and artificial jets which produce speeds above a few kilometers per second.
Figuring things out through the collection of data resembles the work of a detective. With the flood of data on 3I/ATLAS in the coming weeks — leading to its closest approach to Earth on December 19, 2025, we should be able to figure out its nature.
The public resonates with evidence-based science, as long as the scientists show the humility to learn something new. Here are 4 out of a thousand supportive letters that I have received in recent days:
Letter 1
“Dear Professor Loeb,
I apologize for yet another message, but I have just watched your conversation with Dr Brian Keating and I would like to write that whether it is a natural object or technological, I am so grateful to you for opening up this amazing curious discussion in science! After seeing all the unfair criticism coming your way, I truly hope that it will be technological just to see what these “experts” will say! However, if it is natural, I will be equally grateful to you for opening up this fascinating discussion, and I have no doubt that there are technological objects out there anyway — definitely yet to be discovered! I feel that it would be narrow-minded to believe that they do not exist.
Thank you so much!
With best wishes,
Aleksandra Tryniecka”
Letter 2
“I know that you get many emails these days and I don’t want to take up too much of your time. But I am a software engineer in the video game industry and have grown up very enthusiastically following astro-physics. I truly believe that if Carl Segan were still with us, he would be strongly in your corner and enthusiastic in his support of your position.
Not that I can speak for him. I speak for myself.
Thank you!
Keep it up!
Kevin Normann
Georgetown, TX, USA”
Letter 3:
“Dear Professor Loeb,
I hope this message finds you well. I’m writing to express my sincere support and admiration for your stance regarding the interstellar visitor 3I/Atlas. As an amateur astronomer and passionate astrophotographer, I’ve followed your work closely and deeply resonate with your commitment to scientific curiosity and intellectual courage.
Your openness to unconventional hypotheses is not only refreshing but essential to the health of scientific inquiry. Unfortunately, I’ve experienced firsthand how discussions with mainstream academics often end in marginalization or attempts to assert authority rather than engage in genuine dialogue. This pattern is particularly evident in exchanges I’ve had within certain online communities where such dynamics are quite apparent. I’ve also had similar conversations in private groups with other mainstream professors that further illustrate this tendency.
I wanted to let you know that your assertiveness and integrity in the face of criticism are deeply inspiring. Please continue to stand firm in your approach. Your work encourages many of us who believe that science thrives on pluralism, imagination, and respectful debate.
Wishing you strength and success in all your future endeavors.
Warm regards,
Alexandre
From Portugal”
Letter 4:
“Dear Professor Avi Loeb,
My name is Rafael, and I want to express my deep gratitude to you. You are undoubtedly the driving force behind science, and your work and passion for the world have inspired me to explore it. Every day, I study our 3rd interstellar object, and you are the only one who can provide me with well-reasoned explanations.
While there may be many opinions and criticisms about you, I wonder why I don’t see any numbers or calculations.
This lack of seriousness only discourages people from engaging with science. All these scientists have faced criticism:
Galileo (heliocentrism)
Darwin (evolution)
Wegener (continental drift)
Marshall (H. pylori)
Einstein (relativity).
Thank you for your work!”
ABOUT THE AUTHOR
Avi Loeb is the head of the Galileo Project, founding director of Harvard University’s — Black Hole Initiative, director of the Institute for Theory and Computation at the Harvard-Smithsonian Center for Astrophysics, and the former chair of the astronomy department at Harvard University (2011–2020). He is a former member of the President’s Council of Advisors on Science and Technology and a former chair of the Board on Physics and Astronomy of the National Academies. He is the bestselling author of “Extraterrestrial: The First Sign of Intelligent Life Beyond Earth” and a co-author of the textbook “Life in the Cosmos”, both published in 2021. The paperback edition of his new book, titled “Interstellar”, was published in August 2024.