A、It may endanger the life on the earth and in space. B、The investment is vast more than we can afford. C、It is prohibited to us

W: Dr. Desai, the difficulties of space travel are enormous, aren’t they?
M: Vast. When you think that the nearest star is 4. 2 light years away, a spaceship using current technology would take seventy-two thousand years to get there, much more than the entire known history of this planet.
W: What are the possibilities of building a spaceship that travels much, much faster?
M: Well, I’d say that with research and development it could be technically feasible, (23)but it would probably need to be powered by a nuclear explosion to get it travelling at a speed close to that of light. The explosion would of course have to be set off at a good safe distance from the Earth, (24)but you have to remember that current treaties prohibit nuclear explosions in space, so at present it’s a non-starter.
W: In your book you mention non-nuclear technology and in particular a generational spaceship. Can you explain for listeners exactly what this is?
M: Yes, indeed. (25)Given the vast distances and the time involved, even a very fast spaceship using non-nuclear technology would take several thousand years to reach another star. This means that the people volunteering to go on this journey, won’t live long enough to reach their destination, nor will their grandchildren or their great grandchildren, but many generations later on. They can do nothing in a confined space during all this time. So what would be the point of their lives? And there’s always the risk that they could degenerate into wildness and fighting amongst themselves like some small isolated societies here on Earth.
23. Q: What would happen if the spaceship is powered by a nuclear explosion?
24. Q: Why can’t we use nuclear-powered spaceship at present according to the man?
25. Q: What do we know about traveling in a spaceship using non-nuclear technology according to the man?