1. Images of Earth: Gateway to Astronaut Photography of Earth (http://eol.jsc.nasa.gov)
  2. Information about Impact Craters on Earth: Earth Impact Database (http://www.passc.net/EarthImpactDatabase/index.html)
  3. Images of Planetary Worlds: JPL Planetary Photojournal (http://photojournal.jpl.nasa.gov/).
  4. Information about Impact Craters: http://www.lpi.usra.edu/education/explore/shaping_the_planets/impact_cratering.shtml