Shields up! How spaceships can save themselves without science fiction

This post is part of Science of Sci-Fi, Mashable's ongoing series dissecting the science (or lack of science) in our favorite sci-fi movies, TV shows, and books.
Thanks to fictional depictions, we tend to think of spaceships as well-fortified machines. But in reality, even in the emptiness of outer space, their hulls would be under threat of bombardment from near-invisible enemies. 
In the Star Wars and Star Trek franchises, ships are usually fitted with deflector shields — zones of e