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Could we fire a heavy, long (e.g., 50 meters), spear-shaped object into Europa's icy-surface from space and then, when momentum stops, gravity gradually moves weights down inside copper tubes the length of the object like electrical-generator components generating electricity to heat the tip for melting the rest of the way down?

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    $\begingroup$ This seems a bit open ended. You can, given enough electrical energy, generate enough heat to melt your way through an arbitrary amount of ice. I think we will need you to provide bounds on the heating part. The spear-throwing approach is going to be limited by speed. once you get above the speed of sound, you're going to get in trouble rather quickly, but I can't figure out how to tell whether the whole system will work without bounds on how much electricity I can draw. $\endgroup$ – Cort Ammon Jan 22 at 23:14
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    $\begingroup$ I will note that Europa is quite a lot smaller than the Earth, so the effects of generating heat via gravitational potentials (i.e. dropping things down tubes) is going to be limited. $\endgroup$ – Cort Ammon Jan 22 at 23:16
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    $\begingroup$ @Dean A 10-30 km thick ice layer at -220 celsius isn't going to be penetrated by the bigest nuclear weapon we've ever made. Try calculating the energy needed to raise a 1 metre diameter collumn of ice that deep to melting temp. You'll be astonished. $\endgroup$ – Measure of despare. Jan 22 at 23:30
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    $\begingroup$ What is the speed of sound on Europa? Also isn't that the one we're supposed to stay away from? $\endgroup$ – Harper Jan 23 at 5:53
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    $\begingroup$ A chemical energy source is would be more compact than a mechanical one. eg: thermite produces lots of heat without much change in volume. $\endgroup$ – Jasen Jan 23 at 9:44
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The major limitation in this plan is going to be the energy of the objects that you let fall. All of the essential equations happen to have a cross sectional area term in them, so we can actually do some useful calculations: how far do you have to drop something to get the energy needed to melt a length of ice?

Let's start with density. The best object to let fall is going to be the densest thing you can find. Osmium, at $22g/cc$ ($22000 kg/m^3$) is the densest thing we can muster. From physics, we know that the potential energy due to gravity of an object is $PE = mgh$. Substituting in $a$ for cross sectional area, $l$ for the length of the Osmium slug, $d$ for the density of osmium, and $L$ for the length of the spear, we get $PE=dalg(L-l)$.

We also know that 1cc of ice takes a lot of heat to melt. Europa's ice is at -160C, and ice takes roughly 2J/cc-K to melt (actually it's 2J/g-K, which is a little different, but they'll be similar enough results for this back of the envelope exercise). That means it takes 320J to raise 1 cc of ice to 0C. We then need to impart 330J/cc or so to melt it (334J/g, technically... and again I'm handwaving the density because this is just back of the envelope). Put them together, and it takes $660ax$ joules of energy to melt a cross sectional area $a$ $x$ cm deep. To switch that over to meters of depth, it takes $66000ax J$ joules of energy if $x$ is written in meters.

These energies will be the same, so $dalgL=66000ax$. This makes it clear that the cross sectional areas cancel, yielding $dlg(L-l)=66000x$. If we're limited by total length, $l+L$, a bit of calculus will show that the best results we get are when the drop length and length of the Osmium slug are equal, $l=(L-l)=\frac{L}{2}$, so we get $\frac{1}{4}dgL^2 = 66000x$. The gravity on europa is 1.31m/s^2. Thus now we get $7205L^2 = 66000x$ or $L=\sqrt{9.16x}$. Since we handwaved a bunch of stuff, I'm going to round it slightly to $L=3\sqrt x$

What does this mean? To get through the 19,000m we need L=413m. Now we've been rounding a bunch, so let's do it once more. Let's call L=500m. This means you have a 250m long slug of Osmium, resting in a half-kilometer long object. That's what it takes to really melt the ice, assuming everything goes absolutely perfectly. Practically speaking, you'll run into issues melting ice fast enough to not waste energy into the surroundings. You're likely to be talking about a kilometer long object!

My personal recommendation is to land kilometer long spacecraft into ice sheets. Land them gently. Don't try to impact them. The structural effects caused by trying to plow through all that ice would be brutal. It is almost guaranteed that you will crumple those tubes you are relying on. Oh, and Osmium is very brittle, so you're likely to shatter your weights.

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    $\begingroup$ Won't it be much higher than that? Not only do you need to melt the ice, you need to keep it melted long enough for the probe to pass. Making the object longer simply means more surface area to keep heated. Your formula isn't taking into consideration the heat loss. Not only do you need to melt the ice, you need to keep it melted in a -160C environment. $\endgroup$ – Thorne Jan 23 at 5:40
  • $\begingroup$ you don't need to keep it melted so long as it does not stick to the surface of the spear. $\endgroup$ – Jasen Jan 23 at 9:42
  • $\begingroup$ Excellent calculations and great way to tell this story... just make sure whoever is funding your project doesn't realize that melting ice does not mean it disappears from existence! $\endgroup$ – Tim Jan 23 at 17:10
  • $\begingroup$ Thank you Cort Ammon for your level of technical knowledge backed by mathematics even though admittedly it's over my head. I'm very impressed by the imagination and creativity gone into the all the answers on this site. Given the degree of materials, the size and uncertainty of using a spear-like approach along with the density of the ice, I can see that it would not be practical to fire such an object in this way. I kind of got the idea from bunker-busting bombs and thought the generating system would be a minimally polluting one considering alternative Nuclear energy options. Thanks $\endgroup$ – Dean Jan 24 at 9:38
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No

Heating the tip doesn't heat the rest of the spear and the ice will just close and hold the spear along the rest of the shaft.

I'm also doubtful a moving counterweight could generate enough energy to melt a path for the spear.

Why not just do it like NASA has already planned? Land a lander on the surface that runs on a nuclear battery and use the waste heat to melt the ice. The lander sinks into the ice and keeps melting while the water above refreezes.

Seems much easier than smashing a giant spear and hoping it will keep going based on a crude gravity based generator.

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    $\begingroup$ I'm not so sure the gentle water-dwelling creatures under the ice will be impressed when the scaldingly hot, radioactive killer robot from Earth comes crashing through the SkyIce. $\endgroup$ – Oscar Bravo Jan 23 at 15:03
  • $\begingroup$ After reconsideration, I agree that the 'spear' will likely get jammed etc. I like the idea of a lander, but I'm not keen on a nuclear battery sinking permanently into the pristine environment beneath the ice. Perhaps leave the battery on the surface and run a ultralight weight conducting cable of some kind down behind the sinking lander. Perhaps the sinking lander could be a shorter spear/missile shaped vessel. $\endgroup$ – Dean Jan 24 at 9:46
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I don't think we need any special technique for getting through the ice. We have drilled about that deep through rock for oil right here on Earth.

I think there are two much bigger problems to deal with. First, think about how big the equipment for drilling through ~10km of ice will be. Now remember you have to get all that junk all the way out to Europa. And once it's out there, you have to land it; and Europa doesn't have an atmosphere so you can't use parachutes. Look at how much trouble SpaceX has had landing mostly-empty boosters. Now imagine trying to land an oil rig ~630 million kilometers away.

Second, what happens when your drill finally breaks through the bottom of the ice? The ocean down there must be highly pressurized; ~10km of ice weighs a lot after all, even in reduced gravity. So what's stopping that water from shooting up the bore hole at the speed of sound and destroying all your equipment?

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    $\begingroup$ The Moon is likewise airless,and quite a number of vehicles - Surveyors, Apollo landers, Russian landers, the Chinese Chang'es - have landed there. Nor would you use an actual drill. Instead use hot water, as the recent project to drill into an Antarctic subglacial lake did. Except that you'd put a nuclear reactor in the probe, and just unreel a thin fiberoptic cable behind as it melts down. $\endgroup$ – jamesqf Jan 23 at 4:03
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    $\begingroup$ You'd have to put together a team of the best dang drillers on Earth. You'd need a cowboy one, a wise-cracking skinny one, an over-eating one, an African-American one (who's into motorbikes), a clever, handsome one, and head up the team with a tough, heroic leader who's cooler than the ice they have to drill through. $\endgroup$ – Oscar Bravo Jan 23 at 15:08
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Possibly. This is Worldbuilding.SE, not Space.SE. There could be a way to make your spear practical, we just need a realistic reason why it would (since the others have covered why it won't work outright).

Let's start off with the ice sheets. This is how NASA is envisioning the sheets and underlying ocean working. The red areas indicate thermal currents under the surface (we know they happen from the ice jets shot into space, but not much else)

Europa cutaway

And that ice sheet is estimated to be 19-25km thick by using the unique topology of its impact craters. But this tidbit is interesting

There is strong evidence that Europa’s icy shell is somewhat unstable and has been (or is) convecting. This means that blobs of deep crustal material rise upward toward the surface where they are sometimes exposed as domes several kilometers wide (think Lava Lamp, except that the blobs are soft solid material like Silly Putty). Any ocean material imbedded within the lower crust could then be exposed to the surface. This process could take thousands of years, and the exposure to Jupiter’s lethal radiation would be unfriendly to say the least! But at least we could investigate and sample what remains behind.

So the ice isn't static but shifting over time. Let's say we combined some elements here. The following has some plausibility to it (even if it is unlikely)

  1. NASA detects an asteroid on a collision course with Europa with enough time to launch a probe
  2. The asteroid strikes a thermal vent

If your spear followed close behind the asteroid, there could be just enough weakness in the ice at the impact crater to get your spear through.

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