First radio detection of 3I/ATLAS reveals OH absorption signals

The first confirmed radio detection of the interstellar object 3I/ATLAS was achieved through observations made by the MeerKAT radio telescope in South Africa.

According to Harvard professor Avi Loeb, the signal consisted of hydroxyl (OH) absorption lines identified at specific radio frequencies.

This finding confirms the presence of OH molecules interacting with radio emissions from 3I/ATLAS, marking an important step in characterizing its physical and chemical properties.

The detection was reported by D.J. Pisano and collaborators in The Astronomer’s Telegram and later discussed in a post by Loeb.

MeerKAT confirms OH absorption lines in Interstellar object 3I/ATLAS

Detection through MeerKAT observations

The MeerKAT radio telescope, operated by the South African Radio Astronomy Observatory, recorded the OH absorption features during an observation on October 24, 2025.

At the time, 3I/ATLAS was located 3.76 degrees away from the Sun, approximately seven times the Sun’s apparent diameter in the sky.

The instrument detected two distinct absorption lines corresponding to hydroxyl radicals at radio frequencies of 1.665 and 1.667 gigahertz.

The observed Doppler velocity shifts of the two lines were measured at -15.59 (+/-0.16) and -15.65 (+/-0.17) kilometers per second, with full-width-at-half-maximum (FWHM) values of 0.88 (+/-0.37) and 1.26 (+/-0.40) kilometers per second.

These values indicate narrow but clearly defined absorption features consistent with hydroxyl molecular transitions.

According to Loeb’s summary, earlier observation attempts using MeerKAT on September 20 and 28, 2025, did not yield detectable OH absorption signatures, making the October detection the first successful radio observation of this type for the object.

Observation geometry and doppler shift

The geometry of the observation played a role in the measured Doppler velocity. Loeb explained that the solar conjunction of 3I/ATLAS relative to Earth occurred on October 21, 2025, shortly before the successful observation.

At that time, the object’s motion was nearly perpendicular to MeerKAT’s line-of-sight. Based on its trajectory, the full relative velocity between 3I/ATLAS and Earth was approximately 98 kilometers per second.

The observed Doppler shift corresponded to the component of this velocity projected along MeerKAT’s line-of-sight, which was estimated to form an angle of about 9.2 degrees.

This configuration accounted for the measured velocity offsets in the absorption lines.

The detection occurred when 3I/ATLAS was 1.38 times farther from the Sun than Earth, affecting its thermal environment and the behavior of molecules near its surface.

Temperature and molecular broadening

At its observed distance, the surface temperature of 3I/ATLAS was estimated to be around 230 Kelvin.

Loeb noted that solar heating decreases with the square of the distance from the Sun, while radiative cooling depends on temperature to the fourth power.

These factors combine to produce the estimated equilibrium temperature.

The thermal motion of OH molecules at this temperature would generate line broadening on the order of 0.8 kilometers per second (FWHM), which aligns closely with the observed widths of the absorption lines.

This agreement supports the interpretation that the absorption originated from hydroxyl radicals associated with 3I/ATLAS.

Upcoming observation plans

The detection constitutes the first radio confirmation of 3I/ATLAS.

Loeb mentioned that five weeks prior to the event, he had encouraged radio observatories to monitor the object because its trajectory closely matched the sky position of the 1977 “Wow!” signal, though at a different frequency.

Following the detection, Loeb reported that additional monitoring efforts are planned.

He​‍​‌‍​‍‌​‍​‌‍​‍‌ said that 3I/ATLAS will come closest to Jupiter within 53 million kilometers on March 16, 2026.

To look for any such emissions, the Juno vehicle of NASA, during the said flyby, will be employing its dipole antenna at frequencies between 50 hertz and 40 megahertz.

The first-time radio detection of OH absorption in 3I/ATLAS is establishing a confirmed data set for subsequent radio investigations and is instrumental in setting the initial level for continuous interstellar object ​‍​‌‍​‍‌​‍​‌‍​‍‌monitoring.

Stay tuned for more updates.