The Immovable Sky

In this world, striking an asteroid with a projectile does not measurably alter its trajectory, regardless of the impactor's mass, velocity, or approach angle. The physical mechanism remains disputed, but the empirical result is uniform and repeatable: momentum transfer from kinetic impact dissipates without deflecting the body. Planetary defense doctrine was forced onto a wholly different footing from the first serious tests onward.

Diverged 1994 (first rigorously documented anomalous result, retroactively dated; theoretical confirmation accumulates through 2005) · “Humanity cannot change an asteroid's trajectory through kinetic impact.”