Harry Clement Stubbs (May 30, 1922 - October 29, 2003), better known by the pen name Hal Clement, was an American science fiction writer, a leader of the subgenre hard science fiction.

He was born in Somerville, Massachusetts and went to Harvard, graduating with a B.S. in astronomy in 1943. While there he published his first story, "Proof", in the June 1942 issue of Astounding Science Fiction.

His further educational background includes an, M. Ed. (Boston University 1946), and M.S. in chemistry (Simmons College 1963). During the World War II he was a pilot and copilot of the B-24 Liberator and flew 35 combat missions over Europe with 8th Air Force. He served in the Army Air Corps Reserve, and retired with the rank of colonel. He taught chemistry for many years at Milton Academy in Milton, Massachusetts.

Clement received the 1998 recognition as a Grand Master by the Science Fiction and Fantasy Writers of America (SFWA). In 1996 he retroactively received a 1946 Hugo Award for his short story "Uncommon Sense".

His best-known novel, Mission of Gravity, is the account of an overland expedition across the superjovian planet Mesklin to recover a crashed scientific probe. The natives of Mesklin are centipede-like intelligent beings about 50 centimeters in length. Various episodes hinge on the fact that Mesklin's fast rotational speed causes it to be considerably deformed from the spherical, and its effective surface gravity to vary from approximately 3 G at the equator to approximately 700 G at the poles.

Clement's article "Whirligig World" describes his approach to writing a science fiction story:

"Writing a science fiction story is fun, not work. ... the fun... lies in treating the whole thing as a game. ... the rules must be quite simple. They are; for the reader of a science-fiction story, they consist of finding as many as possible of the author's statements or implications which conflict with the facts as science currently understands them. For the author, the rule is to make as few such slips as he possibly can. ... Certain exceptions are made [e.g., to allow travel faster than the speed of light, but] fair play demands that all such matters be mentioned as early as possible in the story..."

He also painted astronomically oriented artworks under the name George Richard.

Clement was a frequent guest at science fiction conventions, especially in the eastern United States, where he usually presented talks and slide shows about writing and astronomy.

Bibliography

• Needle (1950)
• Iceworld (1953)
• Mission of Gravity (1954)
• The Ranger Boys in Space (1956) (for children)
• Cycle of Fire (1957)
• Close to Critical (1964)
• Star Light (1971) (sequel to Mission of Gravity)
• Ocean on Top (1973)
• Through the Eye of a Needle (1978) (sequel to Needle)
• The Nitrogen Fix (1980)
• Still River (1987)
• Fossil (1993)
• Half Life (1999)
• Noise (2003)