Following its detection on July 1, 2025, the light from the interstellar object 3I/ATLAS showed pulsating variability with a period of 16.16 hours. The periodic variability, reported here, was measured to have an amplitude of tens of percent. The observers related this variability to a nucleus with an axis ratio of about 0.8. However, the association of this variability with the ellipsoidal shape of the nucleus is not warranted.
Based on the Hubble Space Telescope image taken on July 21, 2025 (and reported here), most of the light observed from 3I/ATLAS originates from a glowing halo around it, a coma. This coma is transparent so that the nucleus can be seen through it. The fraction of the total light originating from the reflection of sunlight by the nucleus is unknown, because the nucleus size is not resolved in the Hubble image. However, the brightest pixel in the image presumably overlaps with the nucleus and contains a small fraction of the total light.
If all the light originated from the solid surface of the nucleus, then the effective radius of the nucleus had to be R_eff=10 kilometers in visible light (as reported here) or R_eff=23 kilometers at a wavelength of 1-micrometer (as reported here) for a typical albedo of 4–5%. However, the actual radius of the nucleus must be much smaller since most of the light is coming from the coma.
If the actual radius of the nucleus is R, then the fraction of light that it reflects relative to the total (coma+nucleus) scales as the surface area, (R/R_eff)². For example, the upper limit of R~2.8 kilometers inferred here from the Hubble image, implies that the nucleus reflects less than a percent of the total light at a wavelength of 1-micrometer. If the nucleus is spinning over 16.16 hours, why was the periodic variability at a level of tens of percent in total light?
Over the past month, images of 3I/ATLAS showed multiple jets. If the mass loss in the jets is pulsed periodically, the resulting coma would display periodic variability in its scattering of sunlight.
In the context of a natural comet, this can arise from a sunward jet (anti-tail) that is initiated only when a large pocket of ice on one side of the nucleus is facing the Sun. As a result, the coma will get pumped up every time the ice pocket is facing the Sun. This resembles a heartbeat with a puff of gas and dust serving the role of a stream of “blood” through the coma periodically over the rotation period of 16.16 hours. Assuming an outflow velocity of 440 meters per second as suggested in the Webb telescope report here , the distance that sublimated volatiles can reach during 16.16 hours is 25,600 kilometers.
This heartbeat pattern should have been apparent in a series of well-calibrated snapshots of the coma over several days but none was systematically studied in the published literature. For a technological object, the direction of the pulsing jet could be arbitrary and not necessarily pointing towards the Sun. A movie showing the periodic brightening of the jets around 3I/ATLAS over several days can reveal whether the jets are natural or technological based on the orientation of the heartbeat pattern relative to the Sun.
At any event, it is clear that the reported periodicity over 16.16 hours is not directly associated with the shape of the nucleus but rather with the collimated jets coming from it out to much larger distances.
And speaking about heartbeat, my heart in recent months is with kids who tell me that they aspire to become scientists after hearing me speak about 3I/ATLAS. Here is the most recent example of a message that I had received today before my morning jog at sunrise:
“Dear Professor Loeb,
My name is Brian. I am 19 years old and I live in Uruguay. I am writing to you because your ideas about the universe, life, and the possibility of other civilizations have deeply inspired me.
I come from a humble background and I’m still finding my path, but astronomy has awakened something very strong in me — a desire to understand the world, the universe, and humanity’s place in it. Your work has given me hope that curiosity can lead anyone, no matter where they come from, toward something meaningful.
I know you are a very busy person and that you receive many messages, but if you had even a few words of guidance — a direction to follow, something to study, or simply advice for someone beginning this journey — it would mean a lot to me.
Thank you for your time, your work, and the inspiration you give to young people like me.
Sincerely,
Brian Fernández”
***
Here’s hoping that many from Brian’s generation will choose to become scientists and solve some of the mysteries that my generation failed to solve.
Avi Loeb é o chefe do Projeto Galileu, diretor fundador da Iniciativa Buraco Negro da Universidade de Harvard, diretor do Instituto de Teoria e Computação do Centro Harvard-Smithsonian de Astrofísica e ex-chefe do departamento de astronomia da Universidade de Harvard (2011–2020). Ele é ex-membro do Conselho de Assessores de Ciência e Tecnologia do Presidente e ex-presidente do Conselho de Física e Astronomia das Academias Nacionais. É autor do best-seller “
Extraterrestre: O Primeiro Sinal de Vida Inteligente Além da Terra ” e coautor do livro didático “
Vida no Cosmos ”, ambos publicados em 2021. A edição de bolso de seu novo livro, intitulado “
Interestelar ”, foi publicada em agosto de 2024.
