Access to systems is made available on the basis of a known common secret - your password.
Information is available once access has been gained.
Information spreads from one who has it to others who need it. As NASA and all Federal systems are constantly under attack, information needs to be protected if it is not to spread further than was intended. One of the ways to thwart these attacks is to have users frequently change their passwords. NASA policy states that passwords need to be changed every 90 days. The longer and more complicated the password, the harder it is for a hacker to guess or crack.