After 24 successful shuttle flights, "Challenger" was torn apart 73 seconds after the launch of STS-51-L on January 28, 1986; the first shuttle launch from Pad 39B and the first U.S. crewed launch failure, killing the seven crew members. An O-ring seal in the right booster rocket failed at liftoff, leading to subsequent structural failures. Flights resumed on September 29, 1988, with STS-26 after modifications to many aspects of the shuttle program.