The exploration of space is not limited to the flights of astronauts aboard spacecraft and shuttles launched into space by rockets and boosters. That type of space exploration has a history that extends back only to the mid-twentieth century. At the farthest, humans have traveled only about 252,780 miles (406,720 kilometers) away from Earth—that's the distance to the Moon at its apogee (pronounced AP-eh-gee), or farthest point of its orbit. (The distance between the Moon and Earth varies because the Moon's orbit around Earth is elliptical, or oval-shaped. On average, it is located at a distance of 238,900 miles [384,390 kilometers].) Astronauts aboard space shuttles and space stations have stayed relatively close to Earth, conducting work in space at a distance of about 185 to 250 miles (300 to 400 kilometers) above the planet's surface.

The deep exploration of space has come through astronomical observations, or the study of the sky. For centuries...