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The interstellar visitor known as 3I/ATLAS is forcing astronomers to rethink what they know about objects drifting between star systems.
Following the earlier arrivals of 1I/’Oumuamua and 2I/Borisov, this newly detected body is already rewriting assumptions about size, activity and detectability.
Fresh analysis now suggests that 3I/ATLAS may measure barely one kilometre across, an unexpectedly small nucleus given the intense activity observed around it. This was pointed out by Harvard astrophysicist Avi Loeb, who cited a recent paper and said the number was the same as his earlier choices in 2025.
If confirmed, the finding carries major implications for how many such objects may be passing unseen through our solar system.
Radio Observations And Power Limits
On 2 July 2025, the Allen Telescope Array made 7.25 hours of radio observations of 3I/ATLAS. The 1-9 GHz frequency band had nearly 74 million narrowband hits. After filtering out radio-frequency interference, only about 2 million hits remained. Among them, only 211 matched the location of the object in the sky, and none of them was deemed to be worth following up.
Researchers determined the isotopic radiated power of 3I/ATLAS by observing the Doppler drift and established that the power must be between 10 and 110 watts. The limit was subsequently determined from data obtained with the Green Bank Telescope, which narrowed the possible transmission power to a simple 0.1 watts.
These measurements suggest that 3I/ATLAS is not emitting any observable artificial impulses, supporting the view that it is a natural object rather than a technological one.
Non-Gravitational Accelerations
The nucleus size of 3I/ATLAS is estimated through the measurement of the non-gravitational accelerations of 3I/ATLAS. The cause of these accelerations is believed to be jets of material ejected from the comet’s surface, which impart momentum to the nucleus. Based on acceleration and mass-loss rate data, scientists have therefore estimated a diameter of about 1 kilometre.
Loeb emphasised that this estimate confirms his earlier estimate; however, he admitted that several uncertainties remain. The NASA database Horizons-JPL has made multiple revisions to the measured acceleration, lowering it since November 2025, for example. The values must be normalised to the telescope’s localisation, and models are needed to estimate variations in acceleration and the distance to the Sun.
Uncertainties And Open Questions
There are many unanswered questions about the mechanisms underlying 3I/ATLAS’s behaviour. This is the last possibility: that the particles of the gas do not give the impulse which they transmit to the nucleus, but to greater icy bodies. Then the mechanics of the jets would not match those of standard comet models.
The angle and speed of the thrown substance are also unknown. As of today, there is no spectroscopic data on jet velocities. The James Webb Space Telescope can significantly contribute to solving this dilemma, as it will monitor and provide relevant information in the future.
Photographs of the anti-tail major jet made by the Hubble Space Telescope showed that the central anti-tail jet appeared to be directed towards the Sun during the pre-perihelion and post-perihelion periods. This means that the jets’ impulses may cancel each other out in the long run, making it difficult to predict the object’s motion.
Implications For Interstellar Science
Suppose that the nucleus of 3I/ATLAS is actually no more than a kilometre across; it would be one of the smallest interstellar objects to be discovered so far. This would also affect theories about the number of such objects in the galaxy. The smaller objects are hard to spot, so many of them will not be detected in the solar system.
The exploration of 3I/ATLAS also underscores the importance of an integrative approach to the examination, which incorporates a range of observational procedures, such as radio astronomy and optical imaging, to obtain a comprehensive picture of the visitors to the interstellar medium. Each of these discoveries adds to our understanding of the formation and development of planetary systems beyond our own.
The case of 3I/ATLAS underscores both the progress and the limits of interstellar astronomy. Each new visitor adds context, but also exposes gaps in understanding.
As Loeb has observed, science advances through revision. With every new dataset from 3I/ATLAS, researchers are edging closer to understanding how these wandering remnants of distant star systems form and how many may be passing quietly through our cosmic neighbourhood.
