Question

Why are collisions between Pluto and Neptune impossible

Answer 1

Because plutos orbit takes it much higher above the suns orbital plane when Pluto is at the same point as neptunes orbit it actually MUCH higher up than Neptune so the two planets will never be at the same place at the same time

Answer 2

even though the orbits of pluto and neptune intersect,they are locked in a gravitational resonance condition which seems to prevent them from coming closer than a few 100 million miles from one another. that is why it is not possible for both planets to collide