Imagine receiving a faint whisper from a visitor passing through our solar system – a message encoded in the very fabric of radio waves. That's precisely what's just happened! Scientists have detected the first radio signal from the interstellar object 3I/ATLAS, and it's not what you might expect. Instead of hearing a clear 'hello,' we're hearing silence... absorption, to be precise. This discovery opens a new window into understanding the composition and behavior of these mysterious wanderers from beyond our solar neighborhood.
The South African Radio Astronomy Observatory's MeerKAT telescope, a powerful instrument boasting a 13.5-meter diameter, has announced the detection of radio absorption lines caused by hydroxyl radicals (OH molecules) associated with 3I/ATLAS. Think of OH molecules like tiny antennas naturally present within or around the object, absorbing very specific frequencies of radio waves. This absorption tells us something crucial about what 3I/ATLAS is made of and how it interacts with its environment.
Now, let's get a bit technical, but I'll break it down. These observations occurred on October 24, 2025, when 3I/ATLAS was relatively close to the Sun in the sky (only 3.76 degrees away, about seven times the Sun's diameter). The MeerKAT telescope picked up the OH absorption signal at two specific radio frequencies: 1.665 and 1.667 gigahertz. What's particularly interesting is the Doppler shift of these lines. This shift indicates the speed at which 3I/ATLAS is moving relative to us. The measurements showed Doppler velocity shifts of approximately -15.6 kilometers per second. The breadth of these absorption lines also provides insight into the temperature and velocity distribution of the OH molecules. Previous attempts to detect these lines with MeerKAT earlier in September 2025 were unsuccessful, highlighting the sensitivity required for this kind of observation.
And this is the part most people miss... The timing is crucial. 3I/ATLAS reached its closest point to the Sun (solar conjunction) from our perspective a few days prior, on October 21, 2025. At that time, it was traveling almost parallel to Earth's orbital plane. Because of this alignment, the Doppler shift we observe is largely due to the object's actual speed, about 98 kilometers per second, multiplied by the sine of the angle between its direction of motion and our line of sight – which worked out to be about 9.2 degrees.
Given that 3I/ATLAS was about 1.38 times farther from the Sun than Earth is, its surface temperature would be lower than Earth's. We can estimate this because the amount of sunlight an object receives decreases with the square of the distance, while the cooling rate depends on the fourth power of the temperature. Based on this, the temperature of 3I/ATLAS is estimated to be around 230 Kelvin (about -43 degrees Celsius). At this temperature, the OH molecules escaping from its surface would have a thermal speed that broadens the absorption lines by about 0.8 kilometers per second – which nicely matches what MeerKAT observed. As a personal aside, I actually taught the theory behind this "thermal broadening" effect in my "Radiative Processes in Astrophysics" class at Harvard just a month before this discovery!
This detection is significant for another reason: it's the first radio detection ever of 3I/ATLAS. Five weeks prior to this observation, I suggested that radio observatories like MeerKAT should search for radio emissions from 3I/ATLAS, noting its path in the sky was remarkably close (within 9 degrees) to the direction of the famous "Wow! Signal" detected in 1977. The Wow! signal was a strong, unexplained radio signal that some speculated could be of extraterrestrial origin (as discussed in my previous article (https://avi-loeb.medium.com/was-the-wow-signal-emitted-from-3i-atlas-d18d4f0d1f1e)). While I was assured that MeerKAT and other observatories would monitor 3I/ATLAS, this OH absorption is the only radio signal reported so far.
But here's where it gets controversial... Does the absence of other radio signals, especially any potential emission signals, mean anything? Does it rule out any possibility, however slim, of artificial origin? Or does it simply tell us about the natural composition of this interstellar visitor? These are the kind of questions that drive scientific inquiry.
Looking ahead, 3I/ATLAS is expected to pass relatively close to Jupiter on March 16, 2026, coming within 53 million kilometers. The Juno spacecraft, currently orbiting Jupiter, will use its antenna to search for low-frequency radio signals (50 Hz to 40 MHz) from 3I/ATLAS at that time. This presents another exciting opportunity to learn more about this interstellar object.
What do you think this detection of OH absorption tells us about the nature of interstellar objects like 3I/ATLAS? Does the lack of a clear emission signal disappoint you, or does it make the mystery even more intriguing? Share your thoughts in the comments below! Could this be a completely natural phenomenon, or is there a tiny chance we're missing something bigger? Let's discuss!
