Many science fiction writers feel that humans will only be able to feasibly explore the Milky Way galaxy (and beyond) when it is possible to travel at, or above, the speed of light. Unmanned probes have already been sent to explore the solar system and beyond. NASA's Voyager I and Voyager II blasted off from Earth in the late 1970s to explore the outer planets and are now far beyond them.

The Light-Year

The dimensions of the universe are so enormous that they overwhelm conventional units of distance (such as the meter, kilometer, or mile). Therefore, a much larger unit distance is needed—the light-year. A light-year is the amount of distance that light travels in vacuum in one Earth year. Astronomers have found light-years to be a convenient distance when measuring the distance between stars and other celestial bodies.

In one second, light travels approximately...