Only humans have the ability to leave their home planet and explore, settle, and even alter other worlds, and many people want to do all of these things. What is the attraction of these distant worlds that tempts humans to leave the comforts of Earth for uncertain, and probably hazardous, journeys beyond?

The history of human exploration of Earth provides a basis for understanding our motivations for exploring new places. At the same time, humankind's brief experiences with human exploration of the Moon, and the extensive robotic exploration of the solar system, show how space exploration will be different from past voyages of discovery in terms of motivation and operation.

In current and near-term space missions, the search for scientific knowledge has been more prominent, sometimes exclusively so, than it was in historical voyages. Furthermore, unlike the European migrations to the New...