Hubble finds additional evidence of water vapor plumes on Europa

The existence of plumes would make studying the moon's interior ocean easier.

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SiberX[/url]":192lq6d1]Whenever discussion about probes on Europa comes up, I wonder about the feasibility of landing a two-part probe consisting of:

-A surface module, that does the initial landing and contains high-gain antennas to communicate with earth
-A submersible/penetrator module, heated and powered by RTGs to keep it at 70-80 celsius
-A long (5-10km) spool of fibre optic cable, held inside the sub and spooled out as the probe slowly melts its way through the icy crust

The whole assembly lands, and then the lander just plunks the warm sub module on the ice and lets it work its way down to the subsurface ocean. A few years later, there's a few km (hopefully they find a thin part of the crust to make it more manageable) of communication cable connecting the surface with the ocean, and your sub could then send data back to the surface for transmission. If you were clever enough you could even detatch the communication tether and roam deeper (returning to upload gathered data) but simply dangling under the tether cable might provide lots of useful data already.

Is such a plan possible? Is a long enough (very thin) fibre optic infeasibly large? Do the ice sheets move too much to prevent cable breakage? Do RTGs not put out enough heat to melt through the ice?

RTGs currently put out enough heat to melt the ice. The nice thing about RTG is that is you need more heat you just bring more nuclear material. You can find the right amount to melt a path for your probe in a reasonable amount of time. In essence you would pump water past a heat exchanger and direct it downward where it melts more ice. The probe would descend in a bubble of self created liquid water. The formerly liquid water above the probe would eventually freeze solid. So you would have a slowly descending bubble of liquid water in an otherwise solid block of ice.

Communication is the big challenge. Any trailing fiber optic cable would be in solid ice very quickly. The ice does move sometimes meters a day so designing a cable and deployment system that would allow your "landline" to survive would be a challenge.

Even if you solved communication we are probably at least two decades away from any such mission. NASA funding and the lack of detailed information on Europa being the major obstacles.

That and we are dropping a dirty bomb on a place we think might have life.

The real problem with RTGs is actually a lot more mundane, and something nobody has brought up yet. Namely, we don't have the materials to build any.

RTGs rely on plutonium to generate heat and power. Plutonium does not occur naturally in the universe except at trace levels. So all plutonium we have is manufactured in nuclear reactors. Back in 1988 the United States shut down plutonium production, and didn't reactivate it until recently, though production is limited to a handful of grams per year (scaling up is hard).

Limitations on nuclear technology designed to curb nuclear weapons programs also made it very difficult to produce plutonium for peaceful purposes. So, after 1988, basically all the plutonium we got came from Russia, and is of low quality (why would they sell the good stuff?). We have 35kg left, of which half has decayed so much that it's no longer useful for RTGs. The ~17kg left is essentially all already claimed.

NASA claims they aren't limited by plutonium, but this is at best misleading. It's a bit like saying you have enough materials to finish your work because you'll stop working once you run out. Similarly, NASA would be planning far more ambitious, and frequent, missions if they had more plutonium. For example, there was the proposed Jupiter Europa Orbiter mission, which was scrapped in favor of a more economical mission. Probably because the Jupiter Europa Orbiter mission would have required more plutonium than there currently exists on the planet in any form.

TL;DR: Earth is out of plutonium, and we don't know when we'll have more. Any plans predicated upon easy access to large amounts of plutonium are non-starters for at least a few decades.