Tuna-Fish2 t1_jajnj3b wrote
Reply to comment by 12edDawn in NASA’s DART data validates kinetic impact as planetary defense method | DART altered the orbit of the asteroid moonlet Dimorphos by 33 minutes by mepper
In an inelastic collision, momentum is conserved. So Sum(momentum of all bodies) is the same before and after the collision.
If Dimorphos was a rigid body, the spacecraft would only add it's own momentum to it.
However, because the impact by the spacecraft caused just loads of material to be thrown off, the end momentum of Dimorphos is old_momentum + dart_momentum - sum(momentum of all the ejecta). Since the ejecta is going the other way, the net effect is that dimorphos was accelerated more.
(Remember, kinetic energy = ½mv^2, momentum = mv, so spreading out the energy over more, heavier objects increases efficiency.)
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