free space vs outer space

free space

noun
  • Used other than figuratively or idiomatically: see free, space. 

  • vacuum; a space free of matter. 

outer space

noun
  • Region outside explored space. 

  • A bluish shade of black. 

  • Any region of space beyond limits determined with reference to boundaries of a celestial system or body, especially the region of space immediately beyond Earth's atmosphere. 

How often have the words free space and outer space occurred in a corpus of books? (source: Google Ngram Viewer )