In 1962 the US military shot what they said were nukes straight up in the sky. They hit the firmament and exploded. In the footage you can see they did not explode in a spherical shape like you'd expect. Rather the explosion went out the sides as if it had an impact on something. They were testing to see how high the firmament was. This was crucial to future rocket launches  so they could account for the firmament, getting as close as possible before arching back down. Simple as that.