Bombarded by ominous talk of widespread radioactive contamination after the Chernobyl nuclear disaster in the U.S.S.R., Americans are anxious about the prospect of future catastrophes closer to home. The concern is particularly acute following the Challenger tragedy and the failure of two unmanned U.S. rockets. Are the marvels of modern technology really under control? “We have been mortgaging our future, ” argues Pulitzer-prizewinning astronomer Carl Sagan, 51, “and giving our children and grandchildren problems of enormously formidable difficulty. ” At home in Ithaca, N. Y., with his wife, author and TV producer Ann Druyan, 36, and their daughter, Alexandra, 3, Sagan discussed the current spate of technological accidents with Assistant Editor David Grogan.