It may be on its way away from Earth and heading out of the solar system, but interstellar invader comet 3I/ATLAS still has some surprises for researchers.
New research reveals that jet structures in the sun-facing "anti-tail" of this comet, estimated in some observations to stretch up to 620,000 miles (1 million kilometers), were wobbling every 7 hours and 45 minutes as 3I/ATLAS approached the sun. Of course, comets are famous for their tails and haloes, comprised of gas and dust that is blown from their nucleus as radiation from the sun heats them. However, these tails generally face away from the sun and the influx of solar radiation. A rare anti-tail is a cometary tail that points toward the sun, rather than away from it.
3I/ATLAS is only the third object known to have entered our solar system from around another star. The first was the cigar-shaped space-rock 'Oumuamua, discovered passing through the solar system in Oct. 2017, and the second was the first interstellar comet 2I/Borisov, spotted in our solar system in August 2019. Though rare, scientists have seen comets originating in the solar system display a sun-facing anti-tail before, and wobbling jets have been observed in these anti-tails. However, this is the first time that such an "outgassing" has been observed from an interstellar comet.
"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 behind this discovery wrote in a paper published on the paper repository site arXiv.
The team discovered the wobbling jets in the coma of 3I/ATLAS after observing the comet across 37 nights between July 2 and Sept. 5, 2025, with the Two-meter Twin Telescope (TTT), a robotic facility located at the Teide Observatory in Tenerife, Canary Islands.
These observations allowed the researchers to track how the comet's coma evolved from a sun-facing fan of dust before August, to a pronounced antisolar tail. They attribute this transformation to the increasing influence of solar radiation on dust with the coma as 3I/ATLAS headed toward a close approach to the sun on Oct. 30, 2025, when it came to within around 130 million miles (210 million km) of our star.
The jet structure appeared within the anti-tail of 3I/ATLAS on 7 nights between Aug. 3 and Aug. 29, and its wobble or precessional motion implied to the team that the icy heart of this interstellar invader is rotating around once every 15 hours and 30 minutes. This is a shorter rotational period for 3I/ATLAS than has previously been estimated.
3I/ATLAS made its closest approach to Earth on Dec.19, coming to within around 168 million miles (270 million kilometers). Since then, the interstellar interloper has been making its way to the outer solar system. Like 'Oumuamua and 2I/Borisov before it, the comet is expected to eventually leave the solar system for good.However, as this research demonstrates, 3I/ATLAS may soon be gone, but thanks to its impact on science, it is unlikely to be forgotten.
