An Ion-Beam Shepherd for a 140-Metre Asteroid: Beam Momentum Transfer Without Contact

This paper computes an ion-beam shepherd that directs a 1.1 N ion plume at a 140 m asteroid of mass 3,735,563,105 kg from a hovering spacecraft. Beam divergence and imperfect interception reduce the transferred force to 0.792 N, an acceleration of 2.12e-10 m per second squared. Sustained for 4.5 years the shepherd delivers 30.1 mm per second against a requirement of 22.6 mm per second for an 8,000 km displacement with eight years of warning, a margin ratio of 1.33, a computed separation of 10,642 km, and a propellant consumption of 4,977.8 kg. This is a computed study of a deflection system for planetary defence, with no targeting or weapon application. It is simulated, not measured.