What event led to this speech and why was it unique?

Answer:
The explosion of the Space Shuttle Challenger that happened on January 28th, 1986
Explanation:
The accident occurred a little more than a minute after liftoff , it killed the orbiters 7 person crew, including the first teacher to be launched into space. The primary goal of the shuttle mission 51 - L what's the launch the second tracking and data relay satellite