And what was it's speed as it hit approx. 100,000 ft. altitude?
Enough to keep going. Honestly, its been 20 years since I took physics, so I'm not going to attempt it here. I'm taking their word that an initial speed of 25,000mph at the surface is the speed required to escape the Earth's gravity for this object. Drag coefficient would of course matter, but the object would be through 50% of the atmosphere in .27 seconds.
Shooting objects into space with big cannons isn't rocket science ;-) Anyone who has placed a tin can on top of a firecracker can imagine what several kilotons focused up an air shaft might do.
The object in question was made of steel, 4" thick and about 6' across. I am unsure whether it was round or square. It was welded to the end of the shaft, which would certainly help acceleration. Inertia would prevent it from flopping around too much, it would be very heavy and would resist fluttering. In any event, it would be through 99% of the atmosphere in ~3 seconds.