The age of space exploration opened in the sixth decade of the 20th century. In its first two decades, robot probes and then human beings first ventured beyond the limits of the Earth's atmosphere and landed on another celestial object, the Moon. Probes have since gone on to explore the far realms of the solar system and beyond.
Although the space age is still young, it is based on a long history of theoretical and practical developments. Long before the enabling technology for entering space was developed, a theoretical basis had been laid by science (see astronautics). The key to space…
