Humans are more liability than asset when it comes to space travel. While humans can think outside the box more readily than machines, developments in cognitive science and computer design have resulted in ever more competent computers that are gaining more and more the ability to solve problems on their own. Furthermore, humans are physically fragile; in the event of even slight damage to a spacecraft, all humans on board could be killed. They are also susceptible to health risks in the form of radiation damage and muscle and bone loss due to the zero gravity conditions of space. [1] Unmanned ships are far more capable of surviving the dangers of space, and of completing missions successfully.  [1] Ad Hoc Committee on the Solar System Radiation Environment. "Space Radiation Hazards and the Vision of Space Exploration." National Research Council.