[patrickyeon]

Momentum is conserved in the system (total momentum of probe and planet is always the same). Momentum = mass x velocity, so yes, the planet will slow down, but it's a ridiculously small amount. Think of shooting airgun pellets at a tank barelling down on you to slow it down, but even less effective.

[michaelcampbell]

The tank is powered though... if it were COASTING at me, that'd be closer... ;-)