When the Space Shuttle Challenger exploded in 1986, the entire crew was instantly killed. How many astronauts were on board?

Explanation

The Space Shuttle Challenger was NASA's second Space Shuttle orbiter to be put into service, Columbia having been the first. Its maiden flight was on April 4, 1983, and it completed nine missions before breaking apart 73 seconds after the launch of its tenth mission on 28 January 1986. All seven crew members died. 18 months later it was established that the disaster was due to the failure of an O-ring on its right solid-fuel rocket booster. Challenger was replaced by the space shuttle Endeavour six years later.