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New observations reveal that interstellar object 3I/ATLAS — discovered on July 1, 2025 — shows periodic brightness changes every 16.16 hours. Scientists believe the effect is caused not by the nucleus itself, but by pulsating jets of dust and gas forming a transparent, extended coma, reports Medium.
Data from the Hubble Space Telescope (July 21, 2025) shows that most of the light comes from the coma surrounding the nucleus. Because the coma is transparent, the nucleus is visible through it — but its exact size remains uncertain. If all the light were reflected from a solid surface, the nucleus would appear 10 km in radius (visible spectrum) or 23 km (1-micron wavelength). In reality, the nucleus is far smaller, with an upper limit of 2.8 km, meaning it reflects less than 1% of the total light.
Despite this, 3I/ATLAS displays strong brightness fluctuations. Several dust jets recorded in recent images may periodically intensify as the object rotates, creating a “beating” light pattern each cycle.
One possibility is that the effect is tied to a sunward-facing anti-tail that activates only when a volatile-rich region of the nucleus rotates into sunlight. With outflow speeds around 440 m/s, gas could travel more than 25,000 km, driving the pulse-like variations.
Ground-based observations have also captured a bright anti-tail roughly 180,000 km long — a rare and unexplained feature, raising scientific interest as the object leaves the Solar System and likely originates from elsewhere in the Milky Way.
“The light from 3I/ATLAS showed pulsating variability with a period of 16.16 hours and an amplitude of tens of percent,”
says Harvard astrophysicist Avi Loeb.
“But linking this entirely to an elongated nucleus is not justified.”
Loeb notes that no systematic, peer-reviewed study has yet analyzed the imaging needed to confirm the source of the pulses. If the direction of the pulsation is not aligned with the Sun, he adds, it could suggest an artificial origin.
