March 18th 2019

On March 18, 1980, a Soviet rocket exploded on the launchpad and killed 48 people. 

The Vostok-2M rocket was about to launch a new spy satellite called Tselina-D. Military technicians were working to fuel the rocket on the launchpad at the Plesetsk Cosmodrome, a top-secret spaceport a few hundred miles north of Moscow. 

It wasn’t until three years after the explosion happened that the Soviets admitted that this secret spaceport existed. They continued to keep the deadly explosion a secret until 1989. State officials blamed the explosion on human error. But a later investigation determined the cause to be a design flaw with the rocket.

Source: https://www.space.com/39251-on-this-day-in-space.html