Researchers tracked interstellar comet 3I/ATLAS across 37 nights and discovered jet structures inside a rare sun‑facing anti‑tail that extended up to about 620,000 miles (1 million km). The jets wobbled with a ~7 hr 45 min period; that precessional motion implies a nucleus rotation of roughly 15 hr 30 min. Observations captured the coma's evolution from a sunward dust fan to a pronounced antisolar tail as the comet neared the Sun on Oct. 30, 2025.
Interstellar Comet 3I/ATLAS Shows Wobbling Jets in Rare Sun‑Facing 'Anti‑Tail'
(Center) A white light of the comet 3I/ATLAS is surrounded by a blue glow against a black backgroundHubble Space Telescope captured interstellar comet 3I/ATLAS in November. (Corners) Images of the coma of 3I/ATLAS . | Credit: NASA, ESA, STScI, D. Jewitt (UCLA). Image Processing: J. DePasquale (STScI)/ Serra-Ricart, Licandro, and Alarcon
Although it is now receding from Earth and heading back out of the solar system, the interstellar comet 3I/ATLAS continues to surprise astronomers. New observations reveal jet structures inside a rare sun‑facing "anti‑tail" that stretched as far as about 620,000 miles (1 million km) and displayed a clear wobbling motion.
Comet tails and halos form when sunlight heats a comet's nucleus, driving off gas and dust. Most tails stream away from the Sun, pushed by the solar wind and radiation pressure. An anti‑tail is an uncommon geometry in which dust appears to extend sunward, producing an apparent tail that points toward the Sun.
Wobbling Jets and a Spinning Nucleus
Observers measured the jet structures in 3I/ATLAS's anti‑tail wobbling with a period of roughly 7 hours and 45 minutes as the comet approached the Sun. That wobble — interpreted as precessional motion of active vents — allowed the team to infer a nucleus rotation period of about 15 hours and 30 minutes, a somewhat faster spin than some earlier estimates.
"Characterizing jets in 3I thus represents a rare opportunity to investigate the physical behavior of a pristine body formed in another planetary system," the researchers wrote in a paper posted to the arXiv repository.
Observations and Evolution of the Coma
The research team monitored 3I/ATLAS on 37 nights between July 2 and Sept. 5, 2025, using the Two‑meter Twin Telescope (TTT), a robotic instrument at the Teide Observatory on Tenerife in the Canary Islands. Those repeated observations let them follow short‑term changes in the coma and tail structure.
Over the observing window the coma evolved from a sun‑facing fan of dust seen before August into a more pronounced antisolar tail. The researchers attribute this shift to increasing solar radiation pressure acting on coma dust as 3I/ATLAS moved toward a close solar passage on Oct. 30, 2025, when it came within about 130 million miles (210 million km) of the Sun.
The jet feature was visible inside the anti‑tail on seven nights between Aug. 3 and Aug. 29. The jets' precessional wobble provided the evidence used to estimate the nucleus rotation period.
Context and Current Status
3I/ATLAS is only the third confirmed interstellar visitor recorded in our system, following the cigar‑shaped 'Oumuamua (October 2017) and interstellar comet 2I/Borisov (August 2019). The wobbling jets in an anti‑tail represent the first documented instance of such outgassing from an interstellar comet.
The comet made its closest approach to Earth on Dec. 19, 2025, passing at roughly 168 million miles (270 million km). It is now moving back toward the outer solar system and, like 'Oumuamua and 2I/Borisov before it, is expected to eventually depart the Sun's neighborhood for good. Nonetheless, the detailed observations of its anti‑tail jets and rotation provide a valuable record of a relatively pristine object formed around another star.
