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From these inputs, one can calculate how the density of gas in a jet declines with distance. As the gas expands, it thins rapidly—falling off with the square of the distance from the source. Meanwhile, the solar wind, moving at roughly 500 kilometers per second, applies a relentless opposing pressure. When the pressure of the solar wind equals the pressure of the outflowing gas, the gas can no longer continue sunward.

That balance point occurs at approximately 5,000 kilometers from the nucleus.

This distance is not theoretical. It matches the observed size of the coma itself.

In plain language, if 3I/ATLAS is a normal comet, gas cannot remain in a sunward jet beyond the visible halo. Any structure extending far past that boundary must be composed almost entirely of solid dust grains, not gas.

Dust behaves differently. Larger grains—on the order of ten microns—are less affected by solar radiation pressure and can continue traveling sunward for much greater distances before being slowed or deflected. This explains why anti-tails exist at all. However, dust-only streams tend to broaden, curve, and disperse. They do not remain narrow, rigid, and sharply collimated over extreme distances.

The jet visible in this image challenges that expectation.

Its length suggests a transport mechanism that maintains directional coherence. Its narrowness suggests confinement. And its persistence across multiple observations argues against a transient or chaotic release.

Loeb outlines a decisive test. If gas is detected streaming along the sunward jet beyond the 5,000-kilometer limit, then the assumption of low-velocity sublimation breaks down. Gas cannot survive that far sunward unless it is ejected at much higher speeds—kilometers per second rather than hundreds of meters per second.

Such speeds are not achievable through passive heating of ice.

They are, however, characteristic of propulsion systems.

A chemical exhaust traveling at five kilometers per second would allow gas to remain sunward out to roughly 25,000 kilometers. An ion-driven exhaust, with velocities approaching 90 kilometers per second, could sustain gas streams to distances exceeding 100,000 kilometers.

This does not mean such a system exists. It means that if gas is present at those distances, the natural-comet explanation fails on physical grounds alone.

What this image contributes is geometric evidence. The long, oddly placed jet does not behave like a diffuse dust fan. It does not widen appreciably with distance. It does not arc away under radiation pressure the way dust typically does. Instead, it maintains a disciplined alignment toward the Sun, consistent with a directed outflow rather than passive shedding.