Close-up video footage of "Challenger" during its final launch on January 28, 1986 clearly shows that the problems began due to an O-ring failure on the right solid rocket booster (SRB). The hot plume of gas leaking from the failed joint caused the collapse of the external tank, which then resulted in the orbiter's disintegration due to high aerodynamic stress. The accident resulted in the loss of all seven astronauts on board. "Endeavour" (OV-105) was built to replace "Challenger" (using structural spare parts originally intended for the other orbiters) and delivered in May 1991; it was first launched a year later.