The most contentious calculations suggest the combined likelihood of these traits in a natural comet is one in a hundred million or even lower.
The Ecliptic Alignment
What it is: The ecliptic plane is the flat "road" where all the planets orbit the Sun. Interstellar objects arriving randomly from deep space should come from every direction, not just along this flat plane.
The Odds: The alignment of 3I/ATLAS's trajectory with the ecliptic plane to within just 5 degrees has been calculated to have a likelihood of about 0.2% (1 in 500) for a random object.
The Non-Gravitational Acceleration
What it is: This is the "extra kick" that pushes the object slightly off the path predicted by gravity alone. For comets, this is natural, caused by gas jets firing off the sun-warmed surface.
The Problem: The non-gravitational acceleration of 3I/ATLAS is large and, crucially, it appears to be directional (locked into the orbital plane, as described in the article). For a natural comet to generate this much acceleration, it would need to have shed a massive amount of material, which has not been fully observed in the debris cloud.
The Close Planetary Pass
What it is: The object's path brings it exceptionally close to Mars, Venus, and Jupiter.
The Odds: The likelihood of a random interstellar object passing close to these three planets, and being unobservable near the Sun (at its closest point, or perihelion), is extremely small, possibly around 0.005% (1 in 20,000).
The "Technological Propulsion System" (Velocity Lock) This is the radio signature recently observed by the MeerKAT Observatory in South Africa detected a razor-thin radio signal locked precisely to the object's speed. This is the observation that is hardest to explain with natural comet physics… pointing toward a highly ordered process that mimics controlled exhaust or a stable propulsion system that could be technological in origin.