Could the end cap of the Pascal B (1) survive its trip through the atmosphere? In 1957, the US had a nuclear test where a shaft was dug 152 meters into the ground.  A 100 mm thick, 900 kg steel plug was installed and welded at the top of the shaft.  Under it was 2 feet of concrete.
The bomb was detonated created a blast of less than 1 kt.  The concrete was vaporized and the steel plug shot into the air at approximately 66 km / second (125,000 - 150,000 mph).
Some believe it became the first man made item to reach orbit, after all, all you need is approximately 7 km/sec to reach orbit, and this greatly exceeded that.
Others believe it was vaporized by the intense pressure of traveling though the atmosphere at this speed.
No one ever reported it coming down anywhere...  
Is there a physics / mathematical way to determine what happened to this steel plug?  
 A: I used the formula for meteoroid mass loss rate from https://www.spaceacademy.net.au/watch/debris/metflite.htm :
$dm/dt = (\Lambda A \rho_a v^3 m^{2/3} ) / ( 2 \zeta \rho_m^{2/3} )$.
See below the legend and the values I chose (SI units, compare with the values at the link):
$dm/dt$ - mass loss rate
$\Lambda$=2 - heat transfer coefficient
$A$=1 - meteoroid shape factor
$\rho_a$=1.25 - atmospheric density
$v$=66000 - meteoroid speed
$m$=900 - meteoroid mass
$\zeta=3\cdot 10^6$ -  heat of ablation of the meteoroid
$\rho_m$=7800 - meteoroid density.
I obtained $dm/dt\approx 2.8\cdot 10^7$. Thus, the meteoroid will lose mass comparable to its initial mass in $\frac{m}{dm/dt}\approx 3\cdot 10^{-5}~\mathrm{s}$. The meteoroid will travel $v\frac{m}{dm/dt}\approx 2~\mathrm{m}$ within this time. Thus, the end cap probably did not survive in the atmosphere.
The results of the calculation seem strange. It is possible that the formula in the link is incorrect, or maybe I made some mistake, so take this just as an initial attempt to make an estimate.
A: The drag equation stands that the an object travelling through the air receives a force of d*v^2*a*q, if the aerodynamic coefficient (q) was 1 and the air density 1.2 then the cap would have received a pressure of 4961 Megapascals, steel can't stand that pressure.
Also the energy dissipated in 0.0001 seconds given by the same equation would be 31 gigajoules and it is required 0.61 gigajoules to melt 900 kg of steel so the only way the cap will survive the first 6 meters is if the 98% of the energy was dissipated to the air, which is hard because of the black body radiation at about 150000 kelvin (1000 J/(kg*k) * 6^3 kg (air around the cap) /31e+9 J). 
Black body heat transfer in 0.0001 s would be 2 GJ .
A: Building off of akhmeteli's excellent answer, I implemented the differential equations from the asteroid webpage in Mathematica.  I then tried to tweak the numbers, within realistic bounds, to get the thing into space.  In no realistic case was I able to get the thing more than a few hundred meters up before it completely burned away.
To maximize the distance travelled, we want $\Lambda$ and $A$ to be as small as possible;  respectively, these correspond to the rate at which heat is transferred to the "asteroid" and  the effective cross-sectional area of the object (taking into account turbulence).  In addition, we want the heat of ablation $\zeta$ (the amount of heat require to vaporize a certain mass of the substance) to be relatively high, since this will reduce the rate at which mass is lost.
The parameter $\Gamma$ also has an effect;  it describes the amount of drag experienced with the atmosphere.  Interestingly, one can actually get the projectile higher by increasing the drag:  a higher drag means the projectile slows down faster, but that means that the projectile can slow down enough enough that it doesn't burn up immediately.
My optimistic estimates are $\Lambda \approx 0.15$ (note that this number is used in the code example on the page) and $A = 1$ (which would be more streamlined than a sphere).  I also used $\zeta = 10\times 10^6$ J/kg, since it was the highest "typical" value in the table.1  Finally, I used $\Gamma = 0.5$, an estimate given on that webpage for the lower atmosphere.
Here's the result of the simulation, with the parameters given above.  The vaporization of the plug is complete at a height of 312 meters.


And here is the simulation for akhmeteli's parameters, with $\Gamma = 0.5$.  The plug does not significantly change its velocity before it burns up;  the final height is a little over 6 meters.  As would be expected, this is within an order of magnitude of akhmeteli's back-of-the-envelope estimate.
 
If you tweak the unknown parameters of my "optimistic" case above, you can attain a height of 1 km if:

*

*$\Lambda \approx 0.064$ (more than twice as small)

*$A \approx 0.031$ (more than three times smaller)

*$\zeta \approx 23.5 \times 10^6$ J/kg (over twice as large.)

*$\Gamma \approx 1.6$ (much more drag—this slows it down sufficiently before too much of it burns away)

All in all, it seems unlikely that the plug got anywhere near space.

Mathematica Code:
Feel free to tweak this code as you see fit.  The code stops integration when either the mass of the steel falls below 1 gram, or the speed falls below 1 m/s.  The code does implement a height-dependent atmospheric density via a simple exponential model, though it turns out not to be all that relevant for realistic parameters.  The acceleration due to gravity is assumed to be constant.
Needs["DifferentialEquations`InterpolatingFunctionAnatomy`"];
Λ = 0.15;(*heat transfer *)
A = 1;(*shape factor *)
Γ = 0.5; (*drag coefficient*)
ρa0 = 1.25 ;(*atmo. density *)
v0 = 66000; (*initial velocity*)
m0 = 900 ;(*initial mass*)
ζ = 7*10^6;(*heat of ablation*)
ρm = 7800;(*steel density*)
h = 7000; (*atmospheric "height"*)
a = Γ A ρa0 / ρm^(2/3);
b = Λ A ρa0/(2 ζ ρm^(2/3));
soln = NDSolve[{x''[t] == - a Exp[-x[t]/h] x'[t]^2/m[t]^(1/3) - 9.8 m[t], 
   m'[t] == -b Exp[-x[t]/h] x'[t]^3 m[t]^(2/3), x[0] == 0, 
   x'[0] == v0, m[0] == m0, 
   WhenEvent[{m[t] < 0.001, x'[t] < 1}, "StopIntegration"]}, 
    {x, m}, {t, 0, 1000}]
{ti, tf} = First[InterpolatingFunctionDomain[x /. First[soln]]]
Plot[x'[t] /. First[soln], {t, ti, tf}, PlotRange -> {0, 66000}, 
 AxesLabel -> {"Time (s)", "Velocity (m/s)"}]
Plot[m[t] /. First[soln], {t, ti, tf}, PlotRange -> {0, 900}, 
 AxesLabel -> {"Time (s)", "Mass (kg)"}]
x[tf] /. First[soln]


1 It is not clear to me whether these are the appropriate units for $\zeta$; the page is unclear.  They're dimensionally correct, though.
A: In my view, there is a very strong chance that the armored borehole cover ejected in the Pascal B (1) nuclear test did indeed survive Earth's atmosphere. And likely maintained enough velocity to escape the Sun as well. IMHO, though, much of the data related to this event is likely still classified nuclear weapons test research.
While physics do indeed enter into the borehole cover mystery equasion, they are not central to the issue. Metalurgy is IMO, what will make this borehole cover literally interstellar.
Some intriguing facts and variables remain:

*

*The durability and heat resistance of the borehole cover.

*The incredible velocity achieved due to the specific construction of the borehole

*The lack of borehole debris or remains.

*The lack of an observation of a fireball, explosion, or vapor trail disintegration.
Potentially classified info preventing an absolute resolution of the issue:

*The specifics of the steel armor alloy used.

*The specific formula of the concrete plug.

*Unrelated overlapping classifed weapons or materials research.

IMO, universally overlooked variables, though, I believe might shed better light on the subject. No mention was made of the specific steel alloy used in the borehole cover. Which was reported to be an armor alloy. With some alloys, of course, much more durable and resistant to heat.
And remember, this was a cover designed to actually endure a small nuclear blast. Albeit at a reasonable distance from the epicenter. And otherwise protected by tons of an unknown (and possibly classified) concrete formulation. Among other aspects of this tests was to test containment structures. And the reason, as well, for the high speed, calibrated observational film camera.
Also noteworthy is the somewhat arbitrary values ordinarily given for aerodynamic stresses. An unstabilized armored disk is likely to seek the path of least resistance in the atmosphere. In that regard, the disk would have turned sideways, as a Frisbee flies. Or the disk would have folded in on itself from aerodynamic stress, and the effects of ablation softening the alloy. As this illustration of an Explosive Formed Projectile demonstrates:

