Astronomers have detected the first ever “radio signal” emanating from an interstellar comet, just as the comet 3I/ATLAS passed the halfway point on its one-way trip through the solar system. And while this may seem like ostensible evidence for the comet’s putative alien origins, it’s actually quite the opposite.
3I/ATLAS is the third known interstellar object (ISO) to pass through our nearby universe. It was first spotted in early July, hurtling toward the sun at more than 130,000 mph (210,000 km/h), but observations dating back to May have since revealed it. Most researchers agree that this comet may be the oldest comet ever observed, having emerged from an alien star system in a “frontier” region of the Milky Way up to 7 billion years ago.
But ever since the interstellar interloper was discovered, a small group of scientists led by Harvard astrophysicist and noted alien hunter Avi Loeb has promoted the unsubstantiated theory that the comet is actually a spaceship in disguise. This has led to many misleading stories about the comet, which experts say distract from the actual science surrounding the ISO. (This is similar to what happened with the first-ever ISO ‘Oummua, which Loeb et al also labeled as a possible alien mothership.)
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So when astronomers at South Africa’s MeerKAT radio telescope recently announced they had detected the first radio emissions from 3I/ATLAS, proponents of the lobe theory were probably hoping to find evidence of some kind of secret alien transmission, especially since it coincided with the comet’s closest approach to the sun, or perihelion, on October 29th.
But the signal had no technical origin. Rather, they are the result of the absorption of specific wavelengths associated with the presence of hydroxyl radicals, or OH molecules, in the comet’s coma.
These radicals are formed by the decomposition of water molecules released from comets through a natural process known as outgassing, a telltale sign of comet activity, according to a 2016 study.
This is not the first time astronomers have seen evidence of water gushing from 3I/ATLAS. In early October, NASA researchers discovered water gushing out of the comet “like a fire hose.” But the latest findings show that this water is being broken down by solar radiation, as expected during perihelion.
Loeb confirmed on his personal blog, which currently has more than 100,000 subscribers, that hydroxyl radicals were detected in the latest 3I/ATLAS update. However, he did not say whether this was a sign of classical comet activity.
The new radio signal was first detected on October 24, shortly after 3I/ATLAS briefly disappeared behind the Sun during perihelion. The comet’s approach to the Sun was also notable for unexpected brightening events and temporary color changes. And after reappearing earlier this month, the comet appeared to have lost its tail, but this turned out not to be true.
The comet has previously displayed several other unusual features, including a highly irradiated surface, excess carbon dioxide, and a mysterious tailplane, adding fuel to the fires of alien conspiracy theorists. However, all of these features have been well explained by the astronomical community, which is almost unanimously convinced that 3I/ATLAS is a natural object.
A separate but related alien theory was also shattered this week after astronomers detected a “nearly interstellar object” that made its closest approach to Earth on Tuesday (November 11). Rumors suggested that it might be a probe deployed by 3I/ATLAS near perihelion, but this was quickly invalidated and even Loeb acknowledged that the claim was unlikely. Instead, the newly discovered object, named C/2025 V1 (Borisov), is a typical Solar System comet.
Other reports citing Loeb’s calculations of 3I/ATLAS’ non-gravitational accelerations also suggest that the comet may have exploded due to excessive mass loss. However, recent observations have proven that this is not the case either.
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