Complex sentence: It includes only flights that escaped Earth orbit and reached the vicinity of another planet, asteroid or comet; flights that were planned but not executed, were destroyed at or shortly after launch, or which missed their target entirely are not included.
Split into simpler sentences: It includes only flights that escaped Earth orbit and reached the vicinity of another planet, asteroid, or comet. Flights that were planned but not executed, were destroyed at or shortly after launch, or missed their target entirely are not included.