The (Im)possibility of deep space travel
In a previous post, I mentionned this page about the energetic challenge to any kind of interstellar travel. Since the page is currently unavailable (darn, is web.archive.org gone for good?), I consulted my pineal gland about something roughly equivalent. I thus found this nice article on the topic, that explores more detailed challenges that interstellar travel would have to face -- and that seem physically impossible to overcome, as far as we can conceive it. Way beyond our technological horizon, anyway.
I still have faith in mankind, even though space travel may be endeavours that will take millenia for sending a probe one way and back. But this will not be in your and my lifetimes.