well regardless of what may happen to the human race and to what artifacts we may leave behind, there are two that will serve  mankind for eon's they will never (in our life time return to earth.......clue) but will go forever forward (missing asteroids hopefully) into the unknown of interstellar space, the two probes voyager 1 & 2 carry photo's messages and all of our (as when launched) maths and physics with a diagram of how to find earth. these are our legacies because if mankind exterminates itself through nuclear bombing there will be nothing lest of our civilisation, apart from those two tiny craft which bears man kinds signature, and tells whoever finds them, humans are or were here.

that's what we will leave behind.