I found this article about the 500 day mission, a simulation for 6 astronauts going to Mars and back with two days spent on the planet. In the article appears this:

"A real mission to Mars is decades away because of its huge costs and major technological challenges, particularly the task of creating a compact shield that would protect the crew from deadly space radiation.

Why aren't they using the same "protection" they used going to the Moon?

Full article here:

[link to news.yahoo.com]