Of all the destinations within our solar system, the one that intrigues me the most is Europa. Every location has its own merits and beauty, but I'm most optimistic about finding life on Europa. It's all under ice, but perhaps there's enough warm water to be hospitable to life. Putting aside the marketing and political barriers, are there unsolved technical problems that would keep us from completing this successfully?
I think the major obstacle is the level of autonomousness required for a full-fledged lander mission.
The communication delay between Earth and Jupiter is between half an hour and an hour (depending on the time of year), one way (so the real delay is twice as long), but intelligent decisions have to be made real-time during the landing, and especially during the drilling phase (and/or when diving, if it is to go down itself or send a probe).
Landing as Europa's surface changes a lot, the spacecraft will have to explore the surface from a distance and somehow select a suitable landing spot. Of course, this can be done from low Europa orbit with the aid of people, since there is no rush yet. But given the communication delay, the landing itself will have to be carried out almost fully autonomously. As this will likely be a multi-billion dollar flagship mission, and the consequences of most failure types in this phase all mean end-of-mission, you cannot afford to have anything go wrong here. This implies a very robust AI landing system, that has withstood even the most challenging of tests. We've practiced a lot on Mars, but still, there is no system yet that the space agencies are willing to bet their billions on.
Drilling after a successful landing, it's time to cut through the ice. Ideally, we've selected our landing spot well enough to have landed near a fracture, or at the very least on a thin piece of the ice crust. But of course, good engineers always design for worst case, so, we've just landed on the thickest, densest, coldest part on Europa's surface ice.
Drilling through through ice is probably a lot harder than just dropping a small nuclear furnace on it, that just melts its way through. This can be a very sensitive issue, as getting to Jupiter is usually easiest after a few Earth-flybys. There was a huge political debate when the Cassini/Huygens probe did this, because there was a slight chance of the probe disintegrating in the Earth's atmosphere, meaning, it could break its RTG and spread several kilograms of plutonium out into the atmosphere. Plutonium is the most toxic substance known to man, so that even those few kg would do a lot of damage, regardless of where it was dropped. So bringing a nuclear furnace could prove difficult to pull off politically, or at the very least, increase flight time significantly because we can do no Earth flybys.
There are other concerns with the nuclear option as well; because of its high temperature, it might destroy any organics it might encounter prior to detecting them. It's also difficult to turn off, so that when it has finally reached the liquid water ice, it will probably have to cut through completely and let itself sink to the bottom. There will be concerns of contamination -- dropping an open nuclear furnace onto what could be pristine life will not make you very popular.
The drilling mechanism, be it an actual drill, a heat source like the nuclear furnace, or something else entirely, will have to be an intelligent thing as well. We don't know too much about the ice, so it may be partially rocky (I'll skip that here), and partially "slush". This means, we have to design for the scenario where liquid water will likely fill up the hole above the drilling mechanism, and re-freeze. The only way you can guarantee to make this work, is to leave the lander on the surface, and drop the intelligent drilling/exploration probe down the ice, with a tether providing communication/power. Note that that entire length of the tether will have to be on the drilling/exploration probe, because you can't slack a tether down re-frozen ice.
Diving once all the drilling problems have been solved, and the drilling/exploration probe has reached the liquid ocean, it's time to do some exploring. Most likely the only thing this thing will do during the first Europa mission is collect some samples in its immediate surroundings and do some biochemical experiments on it. Ideally however, what has reached the ocean is a full-fledged submarine, capable of maneuvering itself to spots of its choosing, not being hindered by any tether (so the tip of its tether will be a WiFi transmitter and battery-recharge point :) and fully-packed with all robotic arms, labs-on-a-chip, and other sensors known to man.
Basically it's a three or four-part mission. Part A will be in low Europa orbit to serve as a communication relay station to Earth. Part B will be the lander, and serve as a relay station of communications to part A. Part C will be the ice-penetrating probe, which will communicate with B, that relays it to A. Possibly, there will be a part D as well -- a relay station in a high Jupiter orbit, to minimize communication eclipses between the Earth and Europa, and/or serve as a temporary data storage device. Data from C will most likely be communicated to A in bursts, because A will only come by one per orbital period (unless you place it in Europa-stationary orbit, which is unstable due to the proximity to Jupiter, and would only further complicate the mission). A will have to store and transfer these large volumes to Earth, and communication rates at such distances are considerably lower than what is possible between A, B and C. So possibly, C will communicate its data over the course of a Europa orbit to D, which stores it and transmits it at the lower data rate to Earth receivers.
This particular mission I've sketched here is the most popular one, but in all honesty, also the least likely to happen anytime soon. The last two phases will have to be done fully autonomously, with space-grade reliability. This is all technology we don't have yet. So there are technological challenges, political challenges, monetary challenges, challenges concerning contamination, etc. etc. etc. It's too big of a leap, given the amount of resources we can realistically hope to spend on it (alas, the Appollo days, where a military budget scale was used, are long gone).
The far more likely mission is one that puts an orbiter around Europa, which is equipped with a very powerful ice-penetrating radar (or similar device) and do high quality remote sensing. The only real challenge here is that radar (of which ground-versions and airborne-versions already exist), which will ideally be able to detect not only the shapes and sizes of sub-surface objects, but also, which substances those objects are composed of.
The answer to your question very much depends upon what your mission objectives are. Let's look at some of the possibilities, in order of difficulty.
- Crashing an object into Europa: I think we can manage this one. We successfully crash-landed the Huygens probe on the surface of Titan in 2005 (incidentally resulting in the only photograph we have from the surface of any celestial body outside the inner solar system.)
- Landing a rover/craft on the surface of Europa: We have fortunately had lots of practice landing rover-style craft on Mars, so the technology to do this successfully is already mostly developed. In fact, NASA is planning on doing exactly this starting in 2020. This could give us valuable information about the viability of life on Europa, though it wouldn't be able to bring us into direct contact with it. These rovers could, for example, be able take very detailed analyses of Europa's chemical composition, paving the way for future underwater probes. Also, over the course of geologic time water from the deep oceans could migrate into the surface ice, which would mean that even surface rovers could detect signatures of ancient life.
- Drilling down into the ice crust to the oceans underneath: In order to successfully either prove or disprove whether there is indeed currently life on Europa, we would need to send a probe of some kind under Europa's surface ice into its liquid oceans. Unfortunately, this presents a number of very difficult (and unprecedented) engineering challenges that we would have to overcome first. If Europa even has liquid oceans (which we don't know for sure yet), they would be buried under at somewhere between 5km to 100km of granite-hard ice. At the upper ends of that scale, we don't even fully understand how ice behaves at those pressures, much less how to drill through it properly. Even assuming we could get Earth's finest drilling equipment to Europa, we would have to take enormously complicated precautions to reseal and reinforce the hole as it is dug, both to prevent the tunnel from collapsing on itself and to keep Europa's internal pressure from suddenly erupting through the hole. An underwater probe for Europa is probably out of the question both because of current understanding of the conditions on Europa, and our drilling technology.