From Business Insider (from 2016; hat tip to Tom MacMaster):
The fastest object ever launched was a manhole cover — here’s the story from the guy who shot it into space
The very first underground test was nicknamed “Uncle.” It exploded beneath the Nevada Test Site on November 29, 1951. But the tests we’re interested in were nicknamed “Pascal,” during Operation Plumbbob.
[Astrophysicist Robert] Brownlee designed the Pascal-A test — the first designed to contain nuclear fallout. The bomb was placed at the bottom of a hollow column — three feet wide and 485 feet deep — with a four-inch-thick iron cap on top.
The test was conducted on the night of July 26, 1957, so the explosion coming out of the column looked like a Roman candle. Brownlee said the iron cap in Pascal-A exploded off the top of the tube “like a bat.”
Brownlee wanted to measure how fast the iron cap flew off the column, so he designed a second experiment: Pascal-B.
Brownlee replicated the first experiment, but the column in Pascal-B was deeper at 500 feet deep. They also recorded the experiment with a camera that shot 1 frame per millisecond. On August 27, 1957, the “manhole cover” cap flew off the column with the force of the nuclear explosion. The iron cover was only partially visible in one frame, Brownlee said.
When he used this information to find out how fast the cap was going, Brownlee calculated it was traveling at five times the escape velocity of the Earth — or about 125,000 miles per hour.
This dwarfs the 36,373 mph speed that the New Horizons spacecraft — which people say is the fastest object launched by humankind — eventually reached while traveling toward Pluto.
At the time, Brownlee said, he expected the manhole cover to fall back to Earth, but they never found it.
Since then, Brownlee’s concluded it was going too fast to burn up before reaching outer space. “After I was in the business and did my own missile launches,” he said. “I realized that that piece of iron didn’t have time to burn all the way up [in the atmosphere].”
On October 4, 1957, the Soviet Union launched Sputnik, the world’s first artificial satellite.While the USSR was the first to launch a satellite, Brownlee was probably the first to launch an object into space. Since it was going so fast, Brownlee said he thinks the cap likely didn’t get caught in the Earth’s orbit as a satellite like Sputnik and instead shot off into outer space.
“The pressure at the top of that pipe was enormous,” he said. “The first thing that you get is a flash of light coming from the device at the bottom of the empty pipe, and that flash is tremendously hot. That flash that comes is more than 1 million times brighter than the sun. So for [the cap] to blow off was, if I may say so, inevitable.”