Avi Loeb reports new radio constraints and size estimates for interstellar object 3I/ATLAS

The episode followed the release of two new scientific preprints that examine radio observations and non-gravitational motion associated with the object.

The discussion placed these findings in the context of ongoing efforts to understand the physical properties of interstellar visitors passing through the Solar System.

The first set of results comes from radio observations conducted in July 2025 using the Allen Telescope Array, with additional constraints from the Green Bank Telescope.

These observations aimed to detect any radio emissions associated with 3I/ATLAS across a wide frequency range. The second study combines measurements of non-gravitational acceleration with estimated mass loss rates to infer the size of the object’s nucleus.

Throughout the episode, Loeb emphasized the role of observational limits, modeling assumptions, and uncertainty in interpreting these results.

He also referenced public responses and questions raised by readers regarding whether a relatively small object could produce the observed dust, gas, and motion.

The discussion focused on what current data show, what they do not show, and how future observations may refine these conclusions.

Radio observations and limits on emissions

Radio data collected on July 2, 2025, covered 7.25 hours of observations of 3I/ATLAS across frequencies from 1 to 9 GHz.

After filtering out radio frequency interference, researchers identified approximately two million candidate signals, most of which did not align with the object’s position in the sky.

Of the remaining signals, none were found to warrant follow-up after visual inspection in time-frequency space.

Accounting for Doppler drift due to the object’s motion, the analysis placed an upper limit on isotropic radiated power in the range of 10 to 110 watts within the observed band.

A separate dataset from the Green Bank Telescope, spanning 1 to 12 GHz, set a stricter limit of approximately 0.1 watts for observations conducted in December 2025.

During the discussion, Loeb noted, “Science is fun as a learning experience,” while pointing out that non-detections still provide constraints. He also reiterated a guiding principle often cited in his work:

“The foundation of science is the humility to learn, not the arrogance of expertise.”

These results indicate no detected artificial or natural radio transmission within current sensitivity limits.

Non-gravitational motion and size estimates

A separate analysis examined non-gravitational acceleration measured in the trajectory of 3I/ATLAS and combined it with inferred mass loss rates.

Using momentum conservation, the study estimated that the nucleus has a diameter on the order of one kilometer.

This estimate aligns with earlier predictions based on similar assumptions.

Several sources of uncertainty were discussed. Updates to non-gravitational acceleration values in orbital solutions have altered the magnitude of the effect over time.

The mechanism responsible for momentum transfer may involve ejected fragments rather than gas alone, which would change the effective recoil.

In addition, the direction and speed of material ejected from the surface have not yet been directly measured.

Images from space telescopes show a prominent anti-tail jet before and after perihelion, roughly aligned with the Sun.

Because the object’s path was deflected by only about 16 degrees at perihelion, momentum contributions from different phases may partially cancel. As Loeb stated in the episode,

“The current analysis does not take into consideration the evolution in the direction of the jets during the past months.”

Future observations, including planned measurements of jet velocities, are expected to further constrain these interpretations.



Additional uncertainty arises from limited temporal coverage, evolving jet activity, and assumptions about surface composition, which may affect mass loss estimates and momentum calculations over the observation period.

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Stay tuned for more updates.