Until we learned to properly navigate our way across the oceans, early explorers were fearful to lose sight of land in case they’d be lost at sea. They learned to use the water currents, winds, movements of birds and of course, the positions of the Sun, the Moon and the stars to find their way across the seas to distant lands. As we learned to launch spacecraft into orbit and out into the Solar System, mission planners needed to develop entirely new methods of navigation.