Not only is it difficult to build equipment that won't let the extremely hostile environment quickly kill the people we send there, but the environment itself has long-term risks and costs that we don't know how to protect against.
I still long for interplanetary flights, but those may just not be practical any time soon.
Deep Space Radiation Caused Heart Problems For Apollo Astronauts
The rate of heart disease is greater among the astronauts of the Apollo space program.
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I've recommended this before, but maybe not here: "Packing For Mars", by Mary Roach, is a particularly good book about the various engineering problems involved in deep space travel. There's some discussion of radiation, although she spends more time talking about how you might build and test a zero-g toilet.