[extropy-chat] Pluto New Horizons launch -getting ready

[spike]

bounces at lists.extropy.org] On Behalf Of Robert Bradbury
Subject: Re: [extropy-chat] Pluto New Horizons launch -getting ready


Robert Bradbury:
...  For example there isn't a large meteor body at the bottom of Meteor
Crater [1].  Presumably this is because much of it was vaporized upon
impact...  Robert



Robert, ja I didn't make my previous post very clear, so allow me to come at
it with a five part thought experiment which might help illustrate
hydrodynamic impact.

1.  You are sitting on the back of the last car on a kilometer-long bullet
train travelling at 100 meters per second.  You see a vertical rock wall
ahead with a tunnel entrance, but it is a trick!  Some joker has ended the
track at the base of a solid sheer cliff and painted a black semicircular
arc right where the track ends.

At the instant when the front of the train, 1 km away, hits the wall, you
feel nothing at first, but after some short interval of time you begin to
feel increasing deceleration, before your own car crunches.  Perhaps this
deceleration will be sufficient to stop your car before it looks like the
crunched cars up front.  Let us hope so, and be glad this is a mere thought
experiment.

2.  A plane travels at 1 km per second (which is about thrice the speed of
sound) at a constant altitude of 330 meters.  A shock wave forms at the nose
of the aircraft which is kind of like a bunch of sound waves all piled up on
top of each other.  It is an oblique shock wave: it travels in the direction
perpendicular to the wave at the speed of sound, however its track along the
ground travels at thrice that speed.  That's why we know its angle to the
ground is about 19.5 degrees.  Is the shock wave traveling at mach 1 or mach
three?  Depends on how you look at it.

3.  Since this is a thought experiment, I am free to set up some strange
sounding scenarios, such as a shotgun that has a muzzle velocity of 1 km per
second, firing pellets that do not slow down as they travel thru the air.
You are hunting with a high-ranking U.S. government official, standing at a
distance of about 1 km.  Every time he fires, you see a flash and three
seconds later hear a bang.  Suddenly he mistakes you for a duck or a liberal
and fires upon your ass!  This time you actually hear something, not after
three seconds but after only one second.  Why?  Well each pellet is creating
an oblique shock wave, like the aircraft in scenario 2 above.  Now imagine
this shotgun firing 1000 pellets of 1 millimeter each.  The oblique
shockwave of each pellet sort of adds together with all the other shockwaves
to form a composite shockwave traveling at 1000 meters per second.  Now
imagine instead of 1000 pellets of 1 mm, one million pellets of .1 mm.  Or a
billion pellets of .01 mm.  With those scenarios, it is a little easier to
imagine a transverse shock wave traveling at mach 3.  It sounds impossible,
but it isn't.

4.  Imagine you want to make a spacecraft that will endure impact with the
surface of Pluto.  You might wish to create something that would munch on
impact, but the aft end might decelerate sufficiently to survive, like the
last train car example in scenario 1.  At sufficiently low speeds, such a
thing might be possible.  But what actually happens in the hydrodynamic
impact regime is that a shock wave forms, a little like the interface
between the munched train cars and the undamaged cars in scenario 1.  The
shock wave travels aft thru the spacecraft at some velocity which is a
function of the impacting velocity, kinda like the airplane in scenario 2.
The shock wave is transverse, and is driven by the colliding interface
between the planetary surface and the spacecraft, a little like the pellets
driving the transverse shockwave in scenario 3.  

5.  Imagine you are a tungsten molecule sitting in the aft end of the 1
meter long impact lander in the above scenario.  You can see everything,
since the impact lander is transparent to you (atoms are, after all, nearly
all empty space, with a tiny dense wad of nucleus way inside this
probability- wave of electrons).  You approach the planet surface at 10 km
per second, or rather 10 millimeters per microsecond (a microsecond to an
atom is a long time).  Like the train passenger in scenario 1, you watch the
impact begin at time 0.  As the microseconds tick by, you study the
interface between the spacecraft and the quickly vaporizing planetary
surface.  This is a wild place, with solid material being blasted to plasma.
This interface is coming toward you at a rate of 2 mm per microsecond.
Unlike in scenario 1 above, you experience no deceleration at all.  You can
see that the shockwave interface vaporizes everything as it passes, but the
molecules upstream of the shock wave experience nothing at all.  Here's the
punchline: if the spacecraft is in the hydrodynamic regime, there is nothing
that can be done, no explosive devices, no springs, not one thing, that can
save that aftmost molecule from experiencing that bond-shattering shockwave.
Like the meteor that created that crater near Winslow Arizona, physical law
dictates that the impact lander will be vaporized, completely.  The crater
will end up being about five meters deep, with its diameter a function of
the mass and speed of what used to be the impact lander.

Some meteors do survive impact with the ground.  These are going relatively
slow when they hit, a couple hundred meters per second.  But anything above
about 3 km/sec there's no chance of anything surviving, utterly regardless
of all other factors. 

So if we wish to ever land on Pluto, we must figure a way to reduce the
relative velocity of lander and planet.  On these fast missions, ones that
arrive within ten or fifteen years of launch, the relative velocities must
be enormous.

spike