Well, it's more viable than you might think. Putting a 300 degree sphere on the surface would eventually get somewhere. But getting anything useful back is pretty hard, since you'd have a probe that's boiling water below 25km of ice and probably far from the lat/lon it landed at.

You could make it buoyancy-neutral and have it float-melt its way back—heater pointed up.

I'd never considered the pressure under an ice cap... it would scale with depth, no?

So a water channel within the ice (going "up" from the sea below) would have a decreasing pressure gradient as it ascended?

I would imagine the water channel would re-freeze fairly rapidly, so you'd end up with a "bubble" of liquid water around the thing slowly melting itself down.

Point. Is ice sufficiently plastic to exert pressure with depth? I honestly don't know.

E.g. What would the pressure of a bubble of water under 1km of ice vs 15km of ice?

The ice clearly moves, as Europa's surface isn't just same as an airless, solid ball-of-rock's default: craters, but has all sorts of features that reshape the surface, so given enough time, it'll equilibrate. (What 'enough' means is left as an exercise to the reader). There are papers[1] that discuss the ice properties, but it's hard to get a specific answer out of them. There have to be tons of research papers out there about the design criteria for melt-drill probes like this, for Europa, Enceladus, and others.

[1]: https://websites.pmc.ucsc.edu/~fnimmo/website/draft5.pdf

Maybe the main melt probe could leave behind little RTG powered relays as it descends. They'd get frozen in place as the main probe continues melting its way down.

> Maybe not so out of reach after all? Leave a "base station" on the surface at one end of the fibre, and at the other end another "subsurface station" that has a ROV-style tethered- "swimmer"

I think your main issue will be that the hole freezes over behind you, meaning you'll need some serious power to pull the fiber through 20 km worth of ice. You can of course pull the fiber in a sleeve of some kind, if you can keep water out of it, and if you can't there's really not much you can do about it.

I would think the best option would be a somewhat high power radio transmitter/receiver.

Spool goes on the descending module.

> would think the best option would be a somewhat high power radio transmitter/receiver.

If you can figure this one out, the militaries of the world would love to have a chat with you. Water/ice is ridiculously good at attenuating EM. It's a huge issue with submarine communications.

Wire-guided torpedos[1] and missiles[2] are fairly common, and the wire pays out from the projectile-side so it's not progressively dragging more and more. The more recent DM2A4 Seehecht torpedo[3] has a fiber-optic link, probably to reduce EM emissions or the detectability of a km's-long wire/antenna, despite being underwater.

[1]: https://en.wikipedia.org/wiki/Mark_48_torpedo

[2]: https://en.wikipedia.org/wiki/Wire-guided_missile

[3]: https://en.wikipedia.org/wiki/DM2A4

> I think your main issue will be that the hole freezes over behind you.

Getting an ice plug started is a problem, you need meltwater to get fast heat transfer from the melt-head to the ice, but you can't have liquid water at Europa surface pressure.

You'd need to take the cable spool down the hole, so the unspooled part above is already in place.