KEY FINDINGS
- The data came back quiet.
- The physics, however, speaks loudly.
- And together, they tighten the margins around an object that continues to resist simple explanation.
Fresh radio scans and physics-based modeling narrow what 3I ATLAS can be—and what it is not.
By Samuel Lopez | USA Herald – A new peer-reviewed analysis of radio observations has placed the most stringent limits yet on whether the interstellar object known as 3I/ATLAS is emitting artificial radio signals, while a separate preprint has independently converged on a nucleus size that reshapes how scientists understand its unusual behavior. Together, the findings narrow the space of plausible explanations without resolving the deeper question of why this object continues to behave so unlike an ordinary comet.
The radio study examined 7.25 hours of data collected on July 2, 2025, using the Allen Telescope Array, spanning frequencies from 1 to 9 gigahertz. Within that enormous dataset, researchers initially detected nearly 74 million narrowband signal “hits.” After extensive filtering to remove radio frequency interference and unrelated sky sources, the dataset was reduced to roughly two million candidates. When sky localization was applied—matching signals to the precise position of 3I ATLAS—only 211 hits remained. Each was then visually inspected in the time–frequency domain. None displayed characteristics that would justify follow-up as a credible signal originating from the object.
Accounting for the object’s rapid radial velocity and Doppler drift, the researchers derived an upper bound on any isotropic radio transmission from 3I/ATLAS of approximately 10 to 110 watts across the observed frequencies. That threshold was later tightened even further by independent observations using the 100-meter Robert C. Byrd Green Bank Telescope, which constrained any possible emissions in the 1–12 gigahertz range on December 19, 2025, to roughly a tenth of a watt. In practical terms, the data strongly disfavor the presence of any continuous, beacon-like radio transmission detectable with current instrumentation.
At the same time, a newly released preprint has combined updated measurements of non-gravitational acceleration with estimates of mass-loss rates to infer the physical size of the object itself. Using momentum conservation, the analysis concludes that the nucleus of 3I/ATLAS is on the order of one kilometer in diameter. That figure independently confirms a prediction made in early July, shortly after the object’s discovery, and places 3I/ATLAS firmly in the size range of large cometary bodies—at least on paper.
But the apparent clarity of that estimate masks substantial uncertainty. Since November 2025, NASA’s JPL Horizons system has repeatedly revised downward the measured non-gravitational acceleration of 3I/ATLAS as additional astrometric data were incorporated. Those revisions depend sensitively on how positional uncertainties are modeled and on assumptions about how acceleration varies with solar distance. Small changes in those inputs can propagate into large differences in inferred mass and size.
There is also the unresolved question of what, exactly, is providing the recoil force. The current models assume gas outflow at thermal speeds, but prior work—including a study co-authored with Eric Keto—suggests that the dominant momentum transfer could instead come from the ejection of icy fragments. Fragment recoil would alter both the efficiency and directionality of the force, complicating any back-calculation of size from acceleration alone.
Directionality itself remains one of the most striking features of this object. High-resolution imaging from the Hubble Space Telescope showed a dominant anti-tail jet pointed toward the Sun both before perihelion, in July 2025, and after perihelion, in late November. During that same passage, the Sun’s gravity deflected the trajectory of 3I ATLAS by only about 16 degrees. Under those conditions, sunward jets active before and after perihelion would impart nearly cancelling momentum impulses to the nucleus. Current models do not yet fully incorporate how jet orientation may have evolved over the past six months, nor do they benefit from direct spectroscopic measurements of jet velocities—data that astronomers hope the James Webb Space Telescope may soon provide.
Taken together, the radio silence and the kilometer-scale size estimate do not resolve the mystery of 3I/ATLAS. Instead, they refine it. The object appears large, dynamically active, and energetically constrained in ways that rule out some hypotheses while leaving others intact. As I reviewed the cumulative record—from anomalous non-gravitational acceleration to persistent sunward jets—the pattern that emerges is not one of exotic certainty, but of narrowing possibilities. Each new dataset trims the margins, even as the core questions remain open.
What the evidence suggests, but does not yet prove, is that 3I/ATLAS operates at the edge of known cometary physics. The coming observational windows, and especially any direct measurements of jet composition and velocity, will be critical in determining whether this interstellar visitor ultimately fits within established natural models—or forces those models to expand.
