Michael Turner
Tue, 01 Mar 2005 05:55:01 -0800
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Larry writes:
"I hope everyone on this list will at least
give their thoughts and ideas on how we can explore Europa. You need not
be a rocket scientist to do so. In fact, that might bring a refreshing
perspective to look at the problem in another way."
I am an absolute amateur, but when has that
ever stopped me from a rambling speculation on Icepick/Europa? So here's
my last "refreshing perspective" before this list goes off the air. Bear
in mind I have no calculations to back up this wingnut idea.
Icepick is a major planetary-surface mission
with unprecedented energy requirements in a very distant environment where
energy seems to be in very short supply. In the face of an apparent lack
of local energy sources sufficient for melting down into Europa's ocean, current
Icepick designs assume sending a nuclear powered cryobot.
Otherwise, ISRU approaches seem attractive
for any large-scale planetary-surface mission (though some suggested missions
require huge complexity, investment, and work before "break-even" is
achieved.) Perhaps ISRU approaches have been prematurely ruled out in the
case of Europa. Let's do inventory of the in-situ resources.
Material resources: Europa has only one
obvious material resource in great abundance on its surface: H2O. If there
is an economical ISRU approach to Icepick, it has to involve finding some way to
use H2O in some process that helps defeat the surface-ocean barrier, which is
itself composed of solid H2O.
What about energy resources? What do
we have to work with?
There's sunlight, albeit at less than
1/20th of the intensity found at Earth orbit. Europa's orbit is roughly in
a plane aligned with Jupiter's orbit, and Europa's rotation is synchronized with
its orbit, so perhaps continuous sunlight, slowly changing in direction, is
available at the poles. Not much to work with, but it's something, and if
solar-power siting is crucial, there are two candidate regions: the North and
South poles. Europa is also very flat, so that helps. As I've
suggested in earlier postings, given the abundance of H2O and the low surface
gravity, concentrator dishes might be made mostly of ice-composite, if some
robotic system for manufacturing dishes could be planted on Europa. This
would reduce the mass of the ISRU "starter yeast" - the mass of any added solar
concentrator requires only the external inputs of mirroring materials, energy
extraction devices at the focus of mirrors, and the lightweight fiber used to
make ice-composite. Except for one problem: what if the energy for making
such a solar dish in this mode exceeds the possible life-time output of such a
solar dish? The bootstrap snaps before it lifts much of
anything.
Surface thermal range: Europa's
surface is probably coldest just before dawn, and the day-night cycle is
long. Perhaps there is some way to drive a heat engine efficiently between
the temperature extremes of pre-dawn and solar concentrator foci. (Liquid
nitrogen "steam engines", anyone?)
Thermal gradient from surface to ocean:
apart from solar heating fluctuations, Europa's ice is probably coldest at the
very top, and warmer the further toward the ocean you go. Perhaps
this is also exploitable, at least indirectly. If it is exploitable, it's
obvious that it is exploitable only gradually, as you descend.
Gravitational potential energy: Europa has
significant gravity, unlike asteroids. The usual way that energy is
generated from gravitational potential is not available, however - there is
no rain, there are no mountains, so you can't have dams. Still, it's
there. Can it be used?
Kinetic energy: Europa is one of several
Jovian moons, and it is possible to sap kinetic energy from large celestial
bodies by momentum exchange. It's not clear how you'd access any such
kinetic energy, however. Perhaps the only way to use it is indirectly:
through the thermal gradient (see above) from surface to ocean, made possible by
gravitational flexion from the other moons. (Europa also travels through
Jupiter's magnetic field, so it may be possible to convert Europa's own kinetic
energy into electricity, but I've read that you'd need a very long conductor to
generate the kind of energy required to melt your way into the
ocean.)
Those are your energy sources for ISRU
purposes. Clearly, in any pure ISRU approach to
exploring Europa, it's important to economize on energy.
Now, what do we know about H20?
In solid form, it's less dense than in
liquid form - a highly unusual property. And thus part of the problem, in
a way: without this unusual property, there wouldn't be the barrier of the
ice. (Then again, there wouldn't be an ocean either, so perhaps we should
be grateful.)
In solid form, ice is very slippery.
Ice against ice has a very low coefficient of dynamic friction. Static
friction is another thing, though: two flat pieces of ice, pressed hard against
each other, probably pressure-weld very fast.
Apart from being very brittle, ice is quite
strong. An admixture of light, strong fiber can largely offset the
brittleness. If you soak a roll of toilet paper in water, then freeze it,
you get a composite material that seems to rival pebbly concrete - actually less
brittle than concrete, in my experience. Extracting the core long-chain
molecules of wood fiber, and using only those fibers, could yield a very
strong ice-composite that is still 99%+ H20 by weight. Artificial fibers
such as PBO may substantially improve matters, and one can only speculate what
adding carbon-nanotube fiber (even at the short lengths currently achieved in
mass production) might yield. In my experience, ice-sawdust is still
pretty slippery.
H2O has high specific heat. This is
bad - if you have some very cold ice, you need to apply a lot of heat energy to
liquify it. Europa's surface amounts to a whole lot of very cold
ice. However, this might also be a good property, if used
strategically. Ice is a good thermal insulator, odd as that may
seem. Igloos can be comfortable in -30 F degree weather because of
this. If you have a process that involves the huge expenditure of energy
to melt very cold ice, keeping the water above freezing for some other purpose
might not be so difficult.
OK, now what's the problem again? You
want to put a probe down through miles of ice. The ice is very cold, and
quite hard because it's very cold. Even as the temperature goes up with
depth, the pressures become very intense, the further down you go. You
also want the probe to communicate with the surface, and this seems to require
at least a communications cable from surface to ocean.
The approach I will suggest here relies to
some extent on melter-head heat generated by pressure, with pressure generated
by increasing weight. The Icepick-style probes suggested in the past
assume a probe of relatively constant weight (except for comm cable paid out
behind it as it moves down), with an internal energy supply (nuclear
powered). They also assumed that the ice, under great pressure, will close
up behind the probe, encasing a communications cable. The weight of the
melter was not a significant fact in these designs - and it was assumed to be
fixed, or even declining. What I suggest is a melter head that has
above it a cylindrical tube of fibre-reinforced ice, being rotated slowly
but continuously, and being grown from the top down. Inside the tube is
melted water - which is slightly heavier than ice of the same volume, which can
conduct heat relatively rapidly through flows, and through which small pieces of
equipment can be moved, if necessary.
Why a column of water inside an
ice-composite tube? One reason (possibly negligible) is that water is
heavier than ice, contributing to the compression heating at the melter
head. Yet another reason is that a column of water is navigable. Yet
another reason is that flowing water can be a heat transport mechanism.
However, even back-of-the-envelope computation might show that keeping this
water from freezing requires continuous heat input that isn't really worth it
for the above advantages. Also, because water is heavier, it will
exert an outward pressure (through the ice-composite tube) on the surface of the
hole that exceeds the ice pressure, for any given depth. Thermal coupling
through the tube wall to the hole will also cause the ice in the hole that's
been melted to expand. Both the internal and external forces that
result will increase the friction. It would seem better to forgo the
weight advantage of water (slight in any case) and just supply a power/comm
cable through a frozen column for the melter head (which presumably contains a
probe.) But then - where do you put the water melted at the head?
The need for a column of water going to the surface seems to creep back in again
in this scheme, no matter what. Continuous rotation should keep the tube
capable of steady downward movement - the tube won't bind to the walls of the
cylindrical hole being bored. Yes, there will be friction, but continuous
rotation should keep the interfaces very smooth and relatively tight. It
is not necessary that the interface involve melted water - perhaps it's better
if the interface is not much warmer than the surrounding ice. With liquid
water, you get viscosity - and more torque required to overcome it.
The tube can be continuously fabricated by
freezing water/fiber mixture onto the top, as it's being pushed down. You
don't have to cast sections and fit them. You would have to melt some ice
taken from the surface, to top off what has been melted below - liquid water
will have a lower volume than the ice it was melted from. But this is only
a few percent. (Design alternative: fabricate the tube with small vacuum pockets
to make it a better insulator and to offset the volume difference between water
and ice.) Being warmed from the bottom, the warmer water will tend to
rise, of course, and help keep the water above it from refreezing. (Will
it also melt the inner wall of the composite ice tube? To some extent,
although the fibers could provide a significant degree of added insulation, so
it's not nearly as fast a melting rate as if it were just pure ice.
Some equilibrium must be established, and perhaps the water needs to be pumped
continuous, with the fiber filtered out and reused.)
The melter head will require energy to help
melt the ice below it, but as the column lengthens, the amount of added energy
required at the melting head will steadily decrease. You're using the
gravitational potential energy across the distance from the surface down to the
ocean - that column of water encased in ice composite wants to fall toward the
center of Europa, but it can't. Ice is in the way. Melted water is
continuously fed through the melter head up into the column of water, and from
water melted from the surface, so the column keeps getting heavier. The
energy spent in warming the ice to the point of melting isn't thrown away, as it
would be in a cryobot that just lets the water refreeze after it. It keeps
doing melting work, using gravitational potential energy, and providing some
warmth itself to the head by conduction. Perhaps at some break-even point
well before reaching the ocean, it's no longer necessary to actively heat the
melter head - the pressure-heating, combined with the relatively warm water
behind, is enough, and the ice at that depth at a significantly higher
temperature.
OK, that's the basic scheme. Where is
this idea likely to fall down?
The amount of external energy required to
reach some break-even efficiency might still be too high. You end up
needing to ship a nuclear power plant anyway, and if you've got one of those,
why not just put it in a cryobot? And what if there's no break-even point
(see below).
Increasing amounts of torque at the surface
will be required. Even with a very smooth tube-hole interface, friction
will add up, worse than linearly, since the pressures increase with
depth. And maybe my picture of why ice is slippery is all wrong, and
it requires liquid phase (and hence gives rise to viscosity.) Curiously,
last I looked, the theory of the slipperiness of ice was not a completely
solved problem. And I'm talking about ice being slippery under some pretty
extreme conditions. If the benefits of added weight against the melter
head aren't worth the costs of added torque for keeping the weight tube freely
moving, this approach doesn't work. Note that the slower the melt, the
more of the total energy budget goes to torque. The only place rotation
really matters for progress is at the head, which has a constant surface area,
as opposed to the hole, which only increases in surface area as you make
progress.
Yet another problem is the one mentioned
above: the tube is made of ice composite, and keeping a temperature equilibrium
along the entire length that prevents refreezing of the water inside it might be
unmanageably difficult.
I talk blithely about lightweight fiber, and
small fractions resulting in large volumes of high strength material, but even
so, this is a very long tube, and I don't know what it's minimum wall dimensions
would be. Perhaps the amount of material that has to be shipped to Europa
amounts to much more mass than this drafting-board nuclear-powered
cryobot.
And what about obstructions? You can't
rule out a large meteor, or fragment thereof, at some depth. Is the
system reversible enough that you could retrieve a large portion of the
non-recyclables invested, and start boring again nearby?
And what about ice movement? It's all
very well to talk about a long, perfect, cylindrical tube fitted perfectly to a
long, perfect, cylindrical hole, but what if the ice sometimes moves so
fast that the system can't dynamically return to perfect straightness, or simply
stops grinds to a halt and freezes irrevocably in place, never to rotate
again, because there's too much friction all of a sudden?
Well, I'll stop here. I hope this has
been entertaining for some of you, at least. As ISRU concepts go, I rather
like this one for its apparent simplicity - the thing doesn't have to reproduce
half the industrial revolution. It lends itself to relatively easy
modeling, I think - one (dense) page of differential equations might yield a
decent handle on it. But as Einstein was thought to have said (it might
have been Yogi Berra): "Things should be as simple as possible, but not
simpler." Modeling this idea at even a rough level of detail will almost
certainly turn up a showstopper. Perhaps even a few seconds of thought,
for the experts, would suffice to kill it. Even if it survives light
scrutiny ... well, as in everything space-related, the devil is in the
details.
-michael turner
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