All of these images are in the public domain courtesy of NASA.

blue-marble-west.jpg:
http://visibleearth.nasa.gov/view_rec.php?id=2429

cloud.jpg:
www.spacetelescope.org/images/html/heic0601a.html

comet.jpg:
http://hubblesite.org/newscenter/newsdesk/archive/releases/2004/52/

earth-horizon.jpg:
http://spaceflight.nasa.gov/gallery/images/station/crew-7/html/iss007e16249.html

galaxy-ngc3370.jpg:
http://hubblesite.org/newscenter/newsdesk/archive/releases/2003/24/

helix-nebula.jpg:
http://hubblesite.org/newscenter/newsdesk/archive/releases/2003/11/

jupiter.jpg:
http://hubblesite.org/newscenter/newsdesk/archive/releases/1999/29/

sombrereo.jpg:
http://hubblesite.org/newscenter/newsdesk/archive/releases/2003/28/

whirlpool.jpg:
http://hubblesite.org/newscenter/newsdesk/archive/releases/2005/12/image/a

