Before we send humans into deep space, we'll have to find out just how long the human body can survive in a weightless environment. One problem is that in space there's no physical sensation to let you know when you're upside down and astronauts have to rely on possible clues from their surroundings. A few hours after reaching orbit , one in three of all astronauts will experience space sickness--a feeling rather like carsickness. There is also the problem of protection from the extreme hazards  of space such as constant meteorite bombardment  and radiation. It is going to take some clever technology to keep rockets and astronauts from these dangers. Even with a well-protected spaceship, space travel isn't going to be easy. No matter what you travel in, it is going to be a long flight in space! Science fiction writers often imagine using suspended animation , a kind of forced long-term sleep, as a way for astronauts to escape the boredom of long missions. An even stronger measure might be to freeze the astronauts. We already use cryogenic  techniques to preserve dead bodies and store human embryos . Freezing living adults may not be so far away, but perhaps we don't have to do that. Perhaps we should use our existing technology and send frozen embryos to the far corners of universe. Hundreds of years from now, billions and billions of miles away, the embryos will be thawed  and their hearts will start beating. These astronauts of the future will not grow inside a mother's body but will be produced in a machine. They will be brought up by robot. It may seem strange but one day it might just happen. The passage is mainly about   _  .
the bright future of space flight the possibility to train embryo astronauts the physical and mental conditions of astronauts the problems and the possible solutions to them in long space flights