Original sentence: The mission also marks the first flight of an Educator Astronaut, the successor program to NASA's Teacher in Space Project which ended when the Space Shuttle Challenger broke apart 73 seconds after lift-off during STS-51-L
Edited sentence: The mission also marks the first flight of an Educator Astronaut, the successor program to NASA's Teacher in Space Project (which ended in tragedy when the Space Shuttle Challenger broke apart 73 seconds after lift-off during STS-51-L)