3I/ATLAS Rotates Every 7.1 hours After Perihelion
How fast does the interstellar object 3I/ATLAS rotate now? Did its rotation period change during perihelion, when it came closest to the Sun on October 29, 2025?
There are two ways to measure the rotation period of 3I/ATLAS. One uses the periodic shift in the orientation of the jet structure launched by 3I/ATLAS as it rotates. The second relies on the periodic modulation of the total brightness of 3I/ATLAS, including the glowing halo of coma and jets around it.
In a new paper that I co-authored with Toni Scarmato (accessible here), we measured the rotation period of 3I/ATLAS after perihelion in both ways.
First, we measured the position angle of the anti-sunward jet on the sky at multiple times by applying the Larson-Sekanina Rotational Gradient filter to Hubble Space Telescope images between November 20, 2025 and December 27, 2025. Second, we analyzed brightness evolution between December 9 and 22, 2025 using data from the 0.25-meter telescope MPC L92 in Calabria, Italy.
We have found that the periodic wobbles of the jet position-angle by +/-20 degrees occur over a period of 7.20 (+/- 0.05) hours. Independently, the periodic variations in brightness by +/-30 percent yield a period of 7.136 (+/- 0.001) hours. The two periods differ slightly, but the small difference is plausibly attributable to systematics and aliasing. The combined data supports a post-perihelion rotation period of about 7.1 hours, triggering a periodic precession of the jet structure around the rotation axis of 3I/ATLAS.
Since the jet is precessing around the rotation axis, we associate its average position angle value of 270 (+/-3) degrees with the rotation axis. Given that the Sun-3I/ATLAS axis is at 290 degrees, we infer that the rotation axis of 3I/ATLAS is aligned with the Sun-3I/ATLAS axis to within 20 degrees. This surprising alignment needs to be explained, as the rotation axis was set in interstellar space, far from the Sun.
Based on the first image of 3I/ATLAS taken by the Hubble Space Telescope on July 21, 2025 (as reported here), less than a percent of the scattered sunlight originates from the surface of its nucleus. The mass loss rate increased significantly near perihelion. This means that the modulation of the post-perihelion flux by tens of percent can only originate from the glow surrounding the nucleus.
As the nucleus rotates, the dominant outflow direction sweeps around the rotation axis as a result of the precession of the jet, which in turn modulates: (i) the column density of dust along the line-of-sight; (ii) the distribution of dust within the photometric aperture; and (iii) the effective scattering phase-function of the dust particles. Changes in jet orientation therefore produce periodic variations in the observed flux of scattered sunlight. The resulting “heartbeat variability” — by which the jets pump dust and gas into the coma like a heart pumping blood through veins into a body — was suggested in an essay that I wrote on November 30, 2025 here. Prior to that, it was argued incorrectly (as mentioned here) that the brightness variations stem from changes in the reflected sunlight from the surface of a rotating nucleus.
In reality, the underlying rotational state of the nucleus can manifest differently in brightness and in jet position angle. If the jet direction is tied to a fixed active area on the rotating nucleus, then the jet orientation would exhibit periodic shifts, and the integrated brightness would vary periodically as the jet alternately points closer to, or farther from, the line-of-sight. Thus, if the jet direction undergoes precession around the rotation axis (e.g., due to a high-latitude source region and changing illumination geometry), the brightness variability period would correspond to the jet-orientation cycle rather than to the nucleus shape as it reflects sunlight. The observed periodicity should be regarded as a jet-driven modulation that traces the rotational state through the jet orientation, rather than a direct measurement of reflected light from the nucleus itself.
Our inferred value for the jet precession period after perihelion of 7.2 (+/-0.05) hours is consistent with the value measured for the periodic jet precession before perihelion of 7.74 (+/-0.35) hours (as reported here). However, it is shorter by a factor of about 2.3 than the rotation period of about 16–17 hours inferred from the brightness variability before perihelion (as reported here and here). The difference in brightness periodicity might have resulted from a change in the number of jets or active spots — likely induced by the perihelion passage of 3I/ATLAS. For example, the perihelion passage could have doubled the frequency by which the coma flux is modulated by jets since the rotation axis is nearly aligned with the Sun-3I/ATLAS axis. Whereas only one of the rotation poles was illuminated by the Sun before perihelion, the second rotation pole was illuminated by the Sun after perihelion, potentially triggering two active jets — which are also viewed from a different angle after perihelion.
Altogether, the new data indicates that the periodic wobble of the jet around the rotation axis did not change during perihelion. The alignment of the rotation axis of 3I/ATLAS to within 10–20 degrees with the direction of the Sun at large distances is anomalous, as it has only a probability of 1.5–6% for occurring at random in interstellar space.
3I/ATLAS will be within 0.69 degrees from perfect opposition relative to the Sun with the Earth situated in the middle on January 22, 2026 (as described in the other new paper that I co-authored here).
On that date, the anti-sunward jet will nearly disappear from view and the sunward anti-tail jet will be pointing at us.
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Speaking about period keeping, let me close this essay with a general note about extraterrestrial time keeping, which recently appeared in the news also in the context of synchronizing clocks on the Moon relative to Earth.
According to Albert Einstein’s General Theory of Relativity, time progresses slower in the presence of stronger gravity — which manifests as curvature of spacetime. The extreme manifestation of gravitational time-dilation can be found near the event horizon of a black hole, where time slows down to a halt from the vantage point of a distant observer. If we were to video-record astronauts falling into a black hole, we would find their final image to be frozen at the instant when they crossed the event horizon. The reason is simple: no information can escape from inside the horizon, so their last image stays forever frozen for distant observers.
For the same reason, time is ticking slower on Earth than on the Moon as the lunar gravitational potential well is shallower than that of Earth. The gravitational time dilation is larger than the net time dilation due to the second-order Doppler effect, which is of order the square of the velocity of the Moon around Earth normalized by the speed of light. The slowing in the progression of time near Earth is a mild version of the black hole extreme.
Time on the Moon progresses faster than on Earth by approximately 56–59 microseconds (millionths of a second) per day. The fractional drift is about 0.66 parts per billion faster than Earth time. Over 46.5 years, the drift amounts to one second. Precise timekeeping is important for maintaining synchronous operation of electronic equipment on the Moon. If synchronicity slips out of control, we would not be able to keep a proper record of events or communication protocols among computers and other electronic equipment on the Moon and on Earth.
Time is ticking even faster on spacecraft than on the surface of the Moon. But irrespective of where they are in the Solar system, all astronauts and earthlings are embedded inside the gravitational potential well of the Milky-Way galaxy. This adds a Galactic fractional slowdown of about a part in a million, much larger than that induced by gravity on Earth. Over a lifespan of 100 years, we (along with all our Milky-Way neighbors) age slower by 53 minutes (!) than average cosmic residents in intergalactic space. Thanks to that, we can observe the Universe during each century for nearly an hour longer than our intergalactic counterparts.
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Earlier today, I received the following uplifting email, titled “Thank you for making me more curious”:
“As a Microbiology lecturer at a Massachusetts Community College, I am usually looking down the tube of a microscope. You have inspired me to turn my gaze toward the stars and look with humble curiosity to the heavens. Thank you for your open-minded scientific questioning. I’ve been following your Medium.com feed, news interviews, and I read Extraterrestrial. Great work.
Best,
Sage Franetovich
Professor of Biology
Greenfield Community College
Greenfield, MA”
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.
