Yeomans, Donald K. (2005). "Comet". World Book Online Reference Center. World Book. Archived from the original on April 29, 2005. Retrieved December 27, 2008.
Yeomans, Donald K. (2005). "Comet". World Book Online Reference Center. World Book. Archived from the original on April 29, 2005. Retrieved December 27, 2008.