Thank you for your input, Captain Strawman.

Wow, really? So you have access to a usable 3D printer for metals,

Yes, everybody on the planet does. Online 3d print services are everywhere nowadays.

I didn't say "cracked into a million pieces"; it simply no longer had the structural integrity to stay in its slot in the door and my attempts to glue together in a manner that would restore its structural integrity without having it ending up looking hideous failed in both respects. It was still one piece though.

Even if a scanner could detect the cracks, it's not hard to picture the interface having a trivial "crack repair" filter. I'd expect it to be part of any basic package along with rust removal, scratch removal, corner wear repair, etc; it's not algorithmically complex by any means; the first is just vertex merging within a given radius, the second could take several forms, the third is a selective gaussian blur filter, the fourth is a selective sharpening filter, etc. Popular 3d editors like Blender already have functions that can do all of these sorts of stuff and more.

In many ways, Titan is an easier world than Mars (no question that Europa is the odd one out, we're nowhere close to being able to get a probe to explore its oceans). If it wasn't for the distance, it'd be far easier and safer. Lower gravity plus a nice thick atmosphere makes it so easy to aerobrake and gives you tons of great ways to get around (balloons, blimps, helicopters, fixed-wing aircraft, tilt-rotor craft, etc). Moving by air is not only orders of magnitude faster, but - so long as the air is stable, which it appears to be on Titan - safer, too. Surface rovers can and regularly do get stuck, and sometimes it's fatal. The low gravity and dense atmosphere helps with lots of other stuff too - for example, huygens was able to touch down just with a simple parachute, no retrorockets needed (touchdown is always a very nail-biting step in any Mars mission, there's so much that can go wrong with whichever approach you use, even with all of our experience). Plus on Titan there's little electrostatic blowing dust, your radiation levels are lower, your temperatures are more stable, etc

The distance of course is the one big pain with Titan, and it majorly affects a number of things - your delta-V reqs, your transit times, your communication bandwidth / power / weight tradeoffs, your launch windows, etc, and also mandates the use of nuclear power, where on Mars it's only one of two options. Ion engines are thankfully helping to "shrink" our solar system, but no question, the distance is a real big disadvantage for Titan vs. Mars.

But it's really the only major disadvantage. It's so easy to land and get around once you're there.

Humans can create new tools and are vastly more flexible in what they can do than any robot.

Okay, you're on Mars. You discover that there's a neat rock that the instruments you were sent with aren't good enough to completely. You could use some more data. Perhaps thermal conductivity? Okay, go ahead and make a thermal conductivity probe on Mars with enough accuracy to be useful. Maybe one possibility is sensitive to, say, hydrofluoric acid? Go ahead, make some hydrofluoric on Mars. Well, what about a streak or hardness test? Aha, a human could come up with a way to do that by using some other tools or other objects for purposes they weren't designed for! But you know what? So could a robot.

You might be able to come up with some edge case that a human could do that a robot couldn't. But "edge cases" don't pay for increasing the budget by orders of magnitude.

There is a huge difference between looking through a webcam at an ocean and actually standing at the shore yourself.

Unless you're talking about "gee whiz" factors, no. A proper payload can replicate your senses in far more accuracy in far less mass. Your eyes are cool and all, but their eyes can zoom, record readings at each spectral frequency, see IR and UV, see polarization, etc. Your sense of touch is cool and all, but theirs can get precise measurements of thermal conductivity, electrical conductivity, etc. Your sense of smell is neat and all, but versus a mass spectrometer? Just no. And on and on. You are not a scientific instrument. The only thing you really have going for you is your brain. And you know what? That works just fine here on Earth, regardless of how it gets its data.

There are economic benefits to developing the technology to send humans

The economic knock-on effects of building a bigger rocket to carry people, tons of supplies, radiation shielding, etc, versus instead using the same money to build dozens of diverse robots with all kinds of diverse scientific equipment? Absolutely no contest there that the latter will pay off better.

There are some things we will only learn if we are there ourselves.

Like, "Wow, that sure cost a DAMNED lot of money!"

What I really want to see is a single ambitious Titan mission that could answer nearly all of the questions we have today (while undoubtedly making tons of new ones... ah, the beauty of science ;) ):

  * The craft would consist of (beyond its initial boost) a propulsion/communication module, an exploration module, and an ascent module.
  * The propulsion module would use its RTG power, plus the power from the RTG of the attached exploration module, to run the ion engine, draining its tank to near empty to get into a low Titan orbit through the ionosphere.
  * The explorer and attached ascent module would be released, to slowly have their orbits decay to capture, while the propulsion module would remain in low orbit acting as a communications relay (thus keeping down the weight and power requirements for the explorer).
  * During its time in orbit, the propulsion module would be acting as an electrostatic "scoop" through Titan's ionosphere. Its orbit being about 1500m/s but its exhaust velocity being tens to hundreds of thousands of meters per second, it should be able to scoop up gas to use as propellant without being overcome by drag (most ion engines are propellant flexible). Over the course of a year or so it should be able to fill up its tank.
  * Apart from this feature, the propulsion stage (now an orbiter) is kept simple - for example, no radar or other power hungry, heavy scientific equipment.
  * After a couple weeks of losing velocity and altitude, the explorer and ascent stage's orbit will fully decay. The explorer would be a tilt-rotor aircraft with pontoons. Autorotation of the props would slow down the stage during descent, and then active powered thrust at high throttle would reduce the landing velocity.
  * The ascent stage, a couple hundred kilograms, would significantly outweigh the explorer, at a several dozen kilograms. The explorer would disconnect from it before roaming the planet, an activity which would be done as a series of daily hops: 1) The craft climbs straight up like a helicopter, then 2) tilts its wings forward to fly as an airplane, in order to get maximum range out of minimum power - a few hundred kilometers per flight. 3) It flies to the location decided the day before while collecting aerial imagery, then 4) does a vertical descent. 5) On the surface it rests for about a day while it does surface science, transmits data, and waits for the next day's instructions while its RTG recharges its flight batteries.
  * Small samples (a few grams each) are collected from each location, whether solid or liquid, and individually chambered in a honeycomb-like storage bay.
  * With that sort of range capability and about a year of exploration time, the probe could probably explore pretty much every major interesting feature on the planet (while identifying no shortage of future targets, no doubt).
  * At the end of its mission it returns to the ascent stage and docks with it. If the probe is capable of lifting the ascent stage (most likely the best option), it achieves the maximum altitude and delta-V it is capable of with its flight batteries before firing the stage; the stage would thus require somewhere around 1,7k m/s delta-V. If the ascent stage has to be launched from the surface, then it needs an additional 0,5-1k.
  * In orbit it moves to an intercept orbit with the now-full propulsion stage and ejects the ascent stage. The propulsion stage docks with it and begins spiraling away from Titan to an Earth-return trajectory using the combined power of the two RTGs.
  * Any of several possible recovery methods are used at Earth to recover not just the sample pack from the explorer, but any residual propellant (aka, Titan's atmosphere) from the propulsion stage tanks. If the propulsion stage uses multiple tanks then different tanks could be filled from different areas of Titan's outer atmosphere.

Sample return and detailed imagery from every interesting portion of Titan's surface, seas, and atmosphere. If that doesn't answer questions I don't know what would. And of course the maneuvers needed to enter and leave Saturnian and ultimately Titanian orbit would give plenty of flyby opportunities for other Saturnian bodies, possibly even some sampling from the far outer reaches of Saturn or Enceladus's plumes using the same electrostatic scoop.

Good question. NASA seems to be on a search obsessively focused on the concept "liquid water touching bedrock equals life, anywhere without liquid water touching bedrock equals no life". There's so many things wrong with this concept I don't even know where to start. We don't even know if the first forms of life on our own planet developed that way, let alone whether it's common or rare and whether other possibilities are common or rare.

It bothers me because it causes them to obsess over certain bodies (Mars, Europa) while ignoring others. Personally, if I was hunting for life, of all the places in the solar system outside of Earth, I'd pick Titan (which usually gets ignored because it's so cold).

  * It's bigger (although not heavier) than Mercury, and has a predominantly nitrogen atmosphere denser than Earth, with a full meteorological cycle.
  * We know that there's complex organic chemistry going on en masse there. Today.
  * We've detected dozens of types of complex organic chemicals already even with our limited study and we know we're only scratching the surface. Unidentified chemicals around 10000 daltons have been detected in the atmosphere. There's probably even more complex chemicals on the surface. There's so much complex organics there that it blankets the surface in places.
  * There's not one type of liquid on Titan but multiple - an underground sea (which reaches the surface through cryovolcanoes, we're pretty certain) and surface seas of hydrocarbons of what appear to be significantly varying compositions.
  * Titan's methane is regenerating itself. We don't know why. On Mars they treat the presence of unexpected methane as an incredible sign of possible life, on Titan it's treated just as a "Huh, weird" thing
  * Before the details of what was going on on Titan it was theorized in peer-reviewed research that if life existed on Titan, it would most likely consume ethane and acetylene as fuel, burn it with hydrogen instead of oxygen, and produce methane instead of CO2. Subsequent measurements revealed that Titan's surface is unexpectedly ethane-poor, highly acetlyene poor versus how much is being produced in the atmosphere, and one tenative study reveals that hydrogen is disappearing at the surface too.
  * A recent study shows that if it reached sufficient concentration, any acrylonitrile dissolved in Titan's hydrocarbon lakes would naturally form membranes with properties almost identical to the properties of phospholipid membranes on Earth. It just so happens that we've already detected acrylonitrile in Titan's atmosphere.

And on and on. Does any of this mean that there "is" life on Titan? No, not at all. But it's orders of magnitude more evidence than we have for life being at any of the other "popular" places like Mars with its peroxide-rich regolith that destroys organics on contact or Europa's undersea ocean that we know virtually zilch about. And there's an awful lot of mysteries about Titan that warrant solving, life or not. For example, even if there was some non-organic catalyst on Titan breaking down acetylene on the surface, it'd sure be amazing and potentially quite useful to know what sort of natural inorganic catalyst could do that at 100K. And even if Titan turns out to be the worst case - a "frozen early Earth" - well, geez, the knowledge we'd gain toward understanding where we came from in studying the organic chemistry there would be amazing.

The 'needing humans for digging and cracking rocks' remark is particularly stupid in light of the fact that the current probes already dig and crack rocks to some extent, and NASA's about to launch a probe that takes digging and rock cracking to a whole new level (although for a different purpose). The concept that humans are some sort of ideal digging, rock cracking system is crazy.

The ability to conduct science using human physical and sensory capabilities is highly limited. Science is conducted using scientific equipment. Humans operate it either way, the only major difference between a robot and a physical presence is higher latency for robots, and orders of magnitude greater cost for humans. Given the length of time between missions anyway due to budget constraints, the latency issue is borderline irrelevant.

Of course, objects that are visible in the visible spectrum can be translucent or transparent at other frequencies. That said, translucency (and reflection for that matter) are confusing for LIDAR. But there's potential there for some day in the (non-immediate) future.

See, that's the problem with you people. First you insist that 3d printers can only produce low quality plastic junk, and then when faced with the reality that they actually can produce superb products, you treat them as if they're just some sort of incremental change on earlier systems, as if 3d printers are just some form of advanced CNC system.

They are not.

3d printing is a completely different technology to milling, moulding, die forming, etc - your "traditional" manufacturing techniques. You could say that new 3d printers are expanding on the capabilties of 3d printers, but well, that's about the most meaningless truism one could make about technology.

3d printing is not some universal make-all technology. But it has provided for a way to fill a previously poorly met niche: rapid low-volume production of small objects with incredibly complex, nearly arbitrary geometry. For people who need this capability, the rise of 3d printing has been invaluable. Your continual attempts to downplay it as "printing plastic junk" as you'd been doing, or "extrapolation of current capabilities" as you're doing now, are ridiculous.

The current generation of high end 3d printers are very rewarding, and I say this as someone who's used them. The print quality is superb. And no, you don't generally go to existing metal shops which have a CNC miller, you need a shop that has a 3d printer, the two tools are used for different roles. If you have some big part with simpler geometry that you need, you use CNC milling. If you have some small part with complex geometry, you use 3d printing. 3d printing services are already plentiful on the net. Today. They're not at every corner "FedEx Office" today, but they're certainly heading in that direction.

And please, let's stop with the "they're not useful" stuff. I've used them. I know others who have used them. I know multiple companies that use them. Just because you don't has no relevance.

Why is it that this day in age we're still needing to point out that Not All 3D Printing Is Makerbots? If you wanted a professional looking poster printed out would you try to do it on a cheapo home inkjet? You're declaring the term "3d printing" as only applying to "crappy plastic extruders". The "industrial ones that can print in metal", as you put it, are *also* 3d printers. And home users *can* get prints from them, there's lots of online print services. My personal favorite is iMaterialize, as I like their materials and finish selection.

What part of "3D printers can currently only produce mediocre quality plastic representations" do you not understand?

What part of "you're completely and utterly wrong" do you not understand? Makerbots != All 3d printers. There are excellent sintering and spraying metal printers out there, and lots of 3d printing services that 3d-print the initial mold for lost wax casting. People churn out huge numbers of 3d-printed metal parts. Brass, gold, silver, bronze, steel, titanium, you name it. I've purchased such parts myself. I even know of a rocketry startup that's 3d printing inconel aerospikes with a laser sintering system, having it churn out one version after the next in evolutionary sequence so they can maximize the real-world performance.

Welding services do metal just fine (actually, more than "fine", I'd say "superbly"). I designed and printed out a detailed brass medallion as a gift before and it came out just beautiful. Most services these days just use lost wax casting, but there's also metal sintering, and the newest player is thermal spraying, such as laser spraying.

There's also some tremendous software needs in this regard. Sure, the basics are just being able to take scans and print them (although as we all know there's nothing simple about that). But just on the UI side, there's going to be good demand for filters / tools that can repair common types of damage / wear on the part. And many users are going to want to be able to customize their part, so you need everyday-user-friendly 3d modelling that's ideally possible to do straight on the smartphone. Then what about the people who don't have a part in front of them to scan? Some may want it through photos of something they don't have, which means better photogrammetry software that takes fewer images and can properly understand shine/reflection and transparency. And some may even want to be able to sketch it on a piece of paper and scan that in, only having to do tweaks to their sketch, and with the software trying to make a reasonable interpretation of their awful drawing skills.

And of course, regardless of how the models are made, we're going to want a massive collaborative model database built-in to any such apps, designed to encourage people to properly tag and share their models.

Any sort of future where this sort of thing becomes ubiquitous is going to rely on rapid production and delivery - not just of unprintable parts (as above), but also, say, prototype parts. Because let's face it most people aren't going to be sure if what they're printing is really going to fit / meet their needs. It may take a couple iterations, and they may want to make earlier prototype iterations be hollow / out of a cheaper material / with poorer finish / etc until they're satisfied that it meets their needs. If they have to wait a couple weeks between iterations, obviously that's going to suck.