Radio signal detected from mysterious interstellar visitor 3I/ATLAS defies comet explanation

On October 24, astronomers using South Africa's MeerKAT radio telescope detected hydroxyl radicals surrounding 3I/ATLAS, an interstellar object now moving through our solar system at extraordinary speed. The discovery marks the first radio detection of the mysterious visitor and has set off a cascade of observations that challenge everything scientists thought they understood about how comets behave.

Hydroxyl radicals—molecules composed of oxygen and hydrogen—leave a distinctive fingerprint in radio wavelengths that instruments like MeerKAT can isolate and measure. From these readings, researchers determined that the object's surface temperature hovers around minus 45 degrees Fahrenheit and that it spans somewhere between three and six miles in diameter. Harvard professor Avi Loeb, who has been tracking 3I/ATLAS since summer, emphasized the significance of this first radio confirmation. The timing mattered: just days before the detection, the object had passed near Earth's orbital plane, positioning it in a way that made detailed observation finally possible.

What came next deepened the puzzle. Optical images captured on November 9 revealed jets of material erupting from 3I/ATLAS in both directions—nearly 600,000 miles toward the sun and almost 1.8 million miles away from it. To put that in perspective, the sun itself spans roughly 865,000 miles across. At a distance of 203 million miles from Earth, these measurements represented the first clear accounting of the sheer scale at which the object was shedding material. For comparison, 1I/'Oumuamua, the famous interstellar visitor discovered in 2017, measured only a few hundred feet across. 3I/ATLAS dwarfs it.

But the size alone is not what troubles astronomers. The speed and force of those jets present a problem that standard comet physics cannot easily explain. If 3I/ATLAS were a conventional icy comet, Loeb noted, the material should be moving much more slowly and would take months to travel the distances now being observed. Instead, the mass, density, and brightness of the outflows suggest something anomalous is at work. The required mass loss, the rapid brightening as the object approached the sun, and the sheer dimensions all point away from natural comet behavior. "The numbers are challenging for a natural comet explanation," Loeb said. The implication hung unspoken: something else might be powering these jets.

The question of what 3I/ATLAS actually is will not be answered until December 19, when the object makes its closest approach to Earth. At that moment, space telescopes including Hubble and the James Webb Space Telescope should be able to measure its velocity, chemical composition, and the total mass of material being ejected. Those measurements could settle whether 3I/ATLAS is simply an icy body behaving in unexpected ways or whether it is something altogether different—perhaps, as Loeb has suggested in other contexts, an object propelled by technological thrusters that could produce similar jets with far less mass loss. NASA's Juno spacecraft will add another layer of investigation on March 16, 2026, when it passes within 33 million miles of Jupiter and uses its dipole antenna to search for low-frequency radio signals from the object.

Observatories worldwide are now monitoring 3I/ATLAS with particular intensity, partly because its trajectory aligns within 9 degrees of the direction from which the famous 1977 "Wow!" signal arrived—a mysterious radio burst that has never been explained. Whether that alignment means anything remains unknown. What is clear is that 3I/ATLAS has given astronomers something rare: an interstellar object close enough and active enough to study in real time. The combination of radio and optical data shows an object shedding enormous quantities of material at incredible speeds and behaving in ways that force a reckoning with what scientists think they know about how the cosmos works.