5. The von Neumann probes are microscopic and densely packed with a chemical structure encoding the information needed to reproduce in a variety of environments

5. The von Neumann probes are microscopic and densely packed with a chemical structure encoding the information needed to reproduce in a variety of environments. Occasionally some of them mutate and get stuck in a planet's gravitational well, where they keep reproducing and mutating into who-knows-what. The urge to explore the cosmos is never completely lost, and eventually they reach a state where it's possible to continue the mission.