This is a 100% hypothetical question, I'm not expecting it to happen... How big and how fast (minimum for both) would an Asteroid have to be for it to alter the orbit or completely just send it off toward Earth? It can either hit Deimos or Mars.
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It'd have to be about the same size as the satellite. It also would need to have a velocity enough that it would increase the total energy of Deimos and release it from gravitational bound. You can see it for yourself, you don't have to (and should't) take my word for it: use what you learnt in high school about kinetic energy and gravitational potential energy and see what is the total energy of Deimos; to get it released from Mars, the total energy has to be at least zero.