Harry Clement Stubbs, who under the pen name of Hal Clement wrote science fiction for 60 years and was named a Grand Master, died in his sleep at his home in Milton, Mass., Oct. 29, the Science Fiction and Fantasy Writers of America reported. He was 81. Clement, considered an exemplar of science-based "hard science fiction," was best known for 1954's Mission of Gravity, about Mesklin, a vast and fast-spinning planet with varying gravity, the SFWA reported. Clement's other work included two more Mesklin novels, Close to Critical and Star Light, as well as Iceworld, Cycle of Fire, The Nitrogen...