This is actually based upon a
manga called "Aa! Megamisama" by Kosuke Fujishima. This is being published in the US by
Dark Horse (and translated by Proteus Studio). If you enjoy the
anime then it is well worth looking for the collected volumes of this because they are much more of the same fun stories.
It is very loosely based upon the Norns or Fates from Norse mythology. As well as Belldandy (a japanisation of the norse name Verthandi) there are her elder sister Urd and her younger Sister Skuld. They have to face a number of problems both from modern and ancient times.