List three limitations of human space travel

 another limitation is the span of the human life. assuming that you want to go outside of the solar system, it is much too far to travel for a human. 
in addition there is the speed limitation. of course, Einstein predicts that in order to travel at the speed of light, the substance must have no mass. therefore, it is impossible for a human - let alone a spaceship - to travel at that speed, crushing our dreams of interstellar travel (in a human lifespan).

One of the biggest problem is radiation. Once you get out if the earth's magnetic field you have your astronauts exposed to solar radiation.
Even sending a team to mars is not possible because the radiation would almost certainly kill them. Until some way is figured out to shield the astronauts from the sun's radiation and other cosmic radiation we are pretty much stuck.