STS-51-D was the sixteenth flight of NASA's Space Shuttle program, and the fourth flight of Space Shuttle Discovery. The launch of STS-51-D from Kennedy Space Center (KSC), Florida, on April 12, 1985 was delayed by 55 minutes, after a boat strayed into the restricted Solid Rocket Booster (SRB) recovery zone. STS-51-D was the third shuttle mission to be extended.

On April 19, after a week-long flight, Discovery conducted the fifth shuttle landing at KSC. The shuttle suffered extensive brake damage and a ruptured tire during landing. This forced all subsequent shuttle landings to be done at Edwards Air Force Base, California, until the development and implementation of nose wheel steering made landings at KSC more feasible.

Electrophoresis System (CFES) III, which was flying for sixth time; two Shuttle Student Involvement Program (SSIP) experiments; the American Flight Echo-cardiograph (AFE); two Getaway Specials; a set of Phase Partitioning Experiments (PPE); an astronomical photography verification test; various medical experiments; and "Toys in Space," an informal study of the behavior of simple toys in a microgravity environment, with the results being made available to school students upon the shuttle's return.

During the shuttle's landing at KSC on April 19, 1985, extensive brake damage was suffered, and a landing gear tire ruptured. This prompted future shuttle flights to land at Edwards Air Force Base, California, until effective nose wheel steering could be implemented to reduce risks during landing.