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Nuclear Potato Cannon

Here is a question for you: What was the fastest1 ever man-made object?

You'd probably guess it's one of the Voyager probes2 - specifically Voyager-1 at currently almost 40,000 mph (> 60,000 km/h).3 And you'd be most certainly and hilariously wrong.

To zero in on the correct answer, you have to learn not to underestimate the curiosity (and nerdiness) of a bunch of nuclear scientists, having really, really big "toys" to play with.

Blowing Shit Up (for Science)

It was the 1950's, a simpler and more innocent time, when detonating dozens of nuclear bombs in the middle of the U.S. desert (and frying more than a thousand life pigs in the process) was considered an acceptable nerd-sport. "Operation Plumbbob" consisted of 29 nuclear test detonations in 1957 with a maximum single yield of 74 kt (equivalent to the explosion of 74,000 tons of TNT).

Plumbbob PriscillaOne of the tests, named Galileo (later renamed to Pascal-A), was exploring the scenario of an accidental detonation in a confined space. The nuclear device was detonated on the bottom of a shaft with an expected yield of less than 1kg TNT equivalent escaping to the surface. The experiment was deemed a failure (and it looks like a spectacular one): the actual surface yield was 55 t (a slight 55,000 times over expectation) as the explosion evaporated a 2 m thick concrete cylinder blocking the shaft and blew the lid off its top.

The response was the obvious one: if you fail, try again - just bigger. In fact, the experimental designer Dr. Robert Brownlee proceeded to create the most powerful potato cannon in history. In the resulting experiment, named Pascal-B, another 2,000 lb (900 kg) piece of steel armor plate was used as a cover to plug the detonation shaft. As a result of the explosion, the expectation was for the steel plate to achieve approximately 6 times earth's escape velocity (about 11 km/s at surface level),4 which turned out to be a quite plausible estimate, as we will see.

In order to satisfy their curiosity, the experimenters this time around trained a high-speed camera on the shaft cover, which managed to partially catch the steel plate on just one blurry frame after the 300 t explosion - before it was gone from view forever. In an apparent attempt to give the experiment some semblance of a serious endeavor, they then proceeded to rate it as "failed" for not containing the blast underground.5

Now, it's not technically possible to derive a speed from just one frame of video, but it's possible to narrow things down quite a bit: assuming the cover started moving exactly at the end of the previous frame and assuming a constant acceleration, Dr. Brownlee established a lower bound for the steel plate's speed of more than 66 km/s, which translates to 150,000 mph (240,000 km/h) - quite close to the original estimate and one hell of a ride.

Of course, if you for example assume that the cover started moving in the middle of the time distance between the 1 ms frames (halving the movement time) you would double the speed estimate.6 In any case, as Brownlee observed, the thing was "going like a bat!"

Where are they (it) now?

So, what happened to the steel plate?

As we have seen, it was definitely fast enough to leave earth's gravity and to reach its own solar orbit. In fact, at launch it was even fast enough to overcome not just earth's, but the sun's escape velocity as well,7 which would suggest it could be on its way out of our solar system as we speak.

Apollo 4 Re-EntryBut this calculation neglects one key point: air resistance and the resulting energy conversion. This means that a part of its kinetic energy would be converted into heat, slowing it down in the process. As a benchmark, the Apollo-4 test reentered the atmosphere (up where air is quite thin) at around 25,000 mph (~40,000 km/h), resulting in its heat shield reaching a temperature of something like 5,000° F (2,760° C).8

With the steel plate going at least 6 times faster and initially being close to earth's surface (where air is a lot denser), it likely experienced quite rapid and substantial (and possibly catastrophic) heating and deceleration. Without air resistance, the straight upward journey to the edge of our atmosphere would have lasted at the most 1.5 seconds9 - still enough time for unspeakable things to happen. If it indeed made it out of the atmosphere in one piece, it probably does not now enjoy the same shape it started with.10

Under these circumstances, the final fate (or generally, the continued existence) of the man-hole-sized steel plate is understandably somewhat uncertain.

Honorable Mentions

Of course, in space - without an atmosphere to cramp your style - high speeds are readily achievable.

Current top scorers are the Juno probe, which entered Jupiter's orbit at a speed of approximately 130,000 mph (210,000 km/h)11, and the Helios probes, of which Helios-2 achieved a maximum speed of 157,078 mph (252,793 km/h) orbiting the sun.12

Looking into the future, we should have a new record coming up in the 2020's: The Parker Solar Probe (launched in August 2018) will make numerous swings around the sun, in the process temporarily reaching a maximum speed of around 430,000 mph (700,000 km/h).13

Photos courtesy of NASA and NNSA.