Thank you. Now, how powerful (in terms of output) an electric motor can we run from the most efficient thermoelectric generator, using an identical radioisotope heat source?
Bearing in mind that we shouldnt have any trouble dumping heat from the cold side of the thermopile.
Without human support?
Radioisotope generators are typically on the order of 5% efficient, so you get a 260 watt motor, or 0.35 horsepower. Not much.
Use an advanced technology like a Stirling generator and you might achieve 25%, for 1300 watts, or 1.8 horsepower. Seems way too low to turn a 10 km drill stack, but I know nothing about drilling. No one has flown a Stirling generator so they require a bit more development.
Not to derail the thread, but China is apparently considering it.
Whether or not they’re capable or even serious would be a subject for another thread.
Okay, well that’s nice if your probe consists of a 10 cm sphere of plutonium, but in the real world, a probe to descend through the ice and do anything useful would need to be considerably larger; at least a couple of meters in diameter, especially if you want it to carry a coil of cable several thousand meters long to deployed as it descends to maintain a connection to the surface. And it isn’t just enough to melt the ice ahead of the probe; you would need to keep it liquid in the freezing ambient conditions all the way around the probe unit it has completely passed lest it be frozen as it passes.
Making a qualitative comparison between the energy required for a vibratory or drilling rig would require not only details about the specific system and the hardness of the ice that makes up the surface (which is dependent upon composition and ambient temperature and pressure) but also the energy necessary to keep the evacuated material from re-freezing until the probe has passed. But with vibratory or drill removal, you just need to fracture the ice to sufficient granularity to pass it around or through the probe. To melt the ice, you have to apply enough heat to completely liquify and keep it liquid.
But my experience in developing and working with systems designed to remove ice and snowfall in Arctic conditions, I’ve never seen wide scale removal by melting as being used in application or demonstrated to be a viable option in trade studies, just based on the power requirements alone.
No, there are not automated probes off the shelf that can drill their way through thousands of meters of ice. But guess what? There also aren’t extant automated probes that can melt their way through thousands of meters of ice, either. This would require significant development to have any assurance that it would work either way. And this gets back to the basic point that I (and a couple of others) have already made; that we do not need to drill through kilometers of ice in order to get an idea of what lies in Europa’s oceans. If there are geysers spurting water under pressure through flaws in the icy surface, we can sample those and see if there is a) organic molecules, b) precursors to organic life as we know it e.g. amino acids, and c) any indication of complex structures or waste products indicative of life. Trying to do the hardest possible thing in the most challenge possible way is not how you would plan a successful exploration mission. Even if we can’t sample the ocean directly, just the knowledge learned of conditions on the surface is extremely valuable in terms of being to design a more ambitious mission in the future.
Stranger
I had in mind a fairly small probe–10 cm diameter and perhaps 1 m long. 10 km of fiber optic cable has a volume of well under a liter (with cladding and buffer, but no jacket), and I figure an unreeling system should only take a few liters of volume. That still leaves several liters for science, which should be enough for a handful of basic instruments.
You are of course correct that the probe needs to keep the ice liquid all around it–but here, the low thermal conductivity and high enthalpy of fusion works in the probe’s favor: it should only take a small amount of heat diverted from the radioisotope core to keep a liquid layer around the probe. It only spends around 1000 seconds traveling its length. There just isn’t a whole lot of time for the surrounding ice to refreeze the layer surrounding the probe.
I certainly believe that, but I also doubt that radioisotopes were even under consideration for the applications you’ve worked with (I’m certainly willing to be corrected here!). Also, while radioisotopes are quite inefficient for electrical or mechanical uses (Carnot is a bitch, after all), this particular case only requires the direct application of heat, which they excel at.
Sir, in this forum, we obey the Laws of Thermodynamics!
:dubious:
What instruments can you fit into this body? What mission objectives do you plan to complete with them? Is it to be self-propelled? If so, how will you propel it? Why would you think an “unreeling system” which needs to deploy thousands of meters of cable “should be only a few liters in volume”?
Let’s make a comparison to something we do know about. The smallest homing torpedo in the US inventory is the Mark 54 Lightweight Hybrid Torpedo. It is 324 mm in diameter and 2.72 m in length, containing a 40 kg explosive charge (which we could replace with scientific payload), and the remaining volume taken up by fuel, a very compact swashplate piston engine, have a fairly simple internal 2D phased sonar array which allows them to identify large voids (e.g. the air inside of a pressure hull), and a simple off-the-shelf inertial navigation system. It operated for a few tens of minutes at low speed. It isn’t wire guided and so doesn’t have to deploy cable behind it. They aren’t designed to operate at the depths, pressures, and temperatures a probe would have to withstand on Europa, nor can it operate for more than a few tens of minutes. What makes you believe that we can send a self-boring probe that is literally less than 4% of the size of a Mark 54 torpedo which can do anything of use?
Stranger
Which raises the question, what sort of weight/size/mass stats would a realistic mission have, using current launchers
Self-propelled? No, not a chance. It just sinks. If and when it detects that it has entered a liquid ocean, it detaches the sphere. Ideally, the remaining part will be only a bit more dense than neutrally buoyant so as to sink slowly once it hits water and give a bit of time before the remaining fiber runs out.
Surviving the pressure while maintaining reasonable internal volume and a low density is a challenge. Modern syntactic foams are pretty good, though–half the density of water and can survive the pressure at 10 km down on Earth. Europa has <15% the gravity and the pressure at depth is correspondingly lower.
It helps that only the instrument core needs to resist the pressure. The fiber unspooler can be somewhat exposed. The fiber itself has a volume of ~2/3 liter (assuming 250 µm diameter). I posit that it’s not an impossible challenge to build an unreeling system not too many times larger than that. A TOW missile does it and it unreels at 300 m/s. This probe unreels at 1 mm/s.
The first instrument–and one which might justify the mission all by itself–is a fiber speed indicator. The unit will sink through the ice at a fairly constant rate. That rate will change when it hits water or a rocky surface. We don’t know yet how thick the ice is and knowing just that would be very interesting. This can be a simple optical sensor that counts the number of loops that have unwound.
An accelerometer of some kind may prove useful. High accuracy probably isn’t very necessary since the probe doesn’t have hard contact with the surroundings; a tiny solid state unit should be accurate enough to give interesting data. Does the tidal flexing cause any seismic activity?
Similarly, a hydrophone to measure tidal activity. Flexing ice should make sound. It can be a flat transducer on the hull.
A thermometer loses much of its utility when connected to a giant heat source, but it’s tiny and it may be possible to use one to infer something about the surroundings. At the least, the equilibrium temperature will change between traveling through ice vs water, so it can act as a partial backup for the reel speed sensor.
A pressure sensor is probably redundant, but heck, science is about being surprised.
Salinity meter. A pH meter would be cool too, but might be too finicky for this application.
The remaining sensors need a view to the outside–probably using quartz windows. It may not be possible to fit all of them, but even one or two would be good.
A simple one would be a particulate meter. This could be as simple as measuring the scatter from a light beam. A fancier unit could use multiple frequencies, have a particle counter, or other measurements.
Somewhat more advanced would be a spectrometer. There are lots of choices here, and some are very small (including ones already in use on other probes).
Perhaps optimistically, a microscope. Fixed focus and magnification for simplicity. Probably only useful if there is some genuine possibility of the oceans teeming with life.
These are of course just some brainstorming ideas. Probably some of them are less interesting or less doable than they sound. But like I said, a probe that’s little more than a smart plumb bob would already give us information we don’t have right now, and convincingly select one of the handful of ice/ocean models.
Just hang a video camera from the cable and look for very obvious life.
One thing that doesn’t seem to have been addressed here is radioactive contamination, which is more than just incidentally significant when one is exploring probably the last extraterrestrial place in the solar system that might harbor life. A probe that penetrates the ice thermally would eventually end up permanently on the bottom and, even if well shielded, the shielding would eventually be compromised, perhaps even in a very short time due to unexpected conditions. Even the PU238 used in RTGs has a half-life of almost a century, even if it’s absolutely pure and not contaminated with other forms with half-lives of thousands of years.
Mechanical drilling – or thermal methods relying on energy supplied from the surface – would seem at least a bit more palatable. Granted, we may have no other option way out there except nuclear, but at least an RTG would stay on the surface.
As for the suggestions of dropping a nuclear bomb on Europa, wouldn’t it be terrific if the Europans (Europeans?) turned out to be intelligent advanced life forms in possession of both nuclear weapons of their own and a stern sense of justice with a propensity for retaliation? 
I think you overestimate the risk or the damage that would be caused by an RTG that settles on the rocky core of Europa’s ocean. It’s a big place. I would be far more worried about biological contamination from earth.
At least until the surface is subsumed. In the meantime, now you’re talking about thousands of meters of heavy power cable drawing a lot of current. The cable can’t be spooled from the surface because the ice will re-freeze in the hole. So it would have to be spooled off the descending probe, making it much bigger and therefore driving up the power requirements even more because it would have to displace more ice. Probably not feasible. I would think even a drill would be carrying its own nuclear power source.
Well that’s just silly. But so is the idea of dropping nuclear bombs on Europa.
You’re not serious, are you? You really think we’re going to go to all that trouble just to ‘hang a video camera’ and that’s it?
There’s a lot of science that needs to be done on Europa, and it’s not all about finding life. Any mission we send there is going to be festooned with as many instruments as we can reasonably package and ship.
It was meant to be silly. But we agree: the idea of traveling around the universe going “Hi! We’re from Earth, and we’re going to detonate a nuclear bomb here because, hey, that’s what we do” should not be our standard method of operation in the cosmos. We used to do it all over our own planet, too. Right in the atmosphere. :mad:
Using current rockets? Cassini-Huygens is a good example of the size of mission we could fly. The payload for Cassini-Huygens was 2,523 kilograms. It has an RTG that generates 880 watts of power.
Cassini was launched on an Atlas IV rocket, and to get to Saturn required two gravity assists from Venus, a gravity-assist flyby of Earth, and another at Jupiter. All told, it took the probe 7 years to get to Saturn.
The Galileo mission to Jupiter was about the same size - around 2,500 kilograms. It also needed multiple gravity assist maneuvers to get there, and that required 6 years in transit.
A Europa mission that sniffs the water ejected from the surface, maps the moon in high detail, and maybe even drops a small lander on it would be a mission with a similar profile, I’d think. Both Galileo and Cassini had secondary probes with them - Huygens landed on Titan, and Galileo had an atmospheric probe that descended through Jupiter’s atmosphere. So a small Europa lander should be do-able.
If you want a mission bigger than that, you need something like the now-cancelled Jupiter Icy Moons Orbiter. That mission would have had ion propulsion to allow it to enter and leave orbit of various moons. Instead of a small RTG that could generate a few hundred watts of power at most, JIMO would have had a full nuclear reactor for powering the ion engines. If you wanted to put that reactor on the surface of Europa, you’re looking at a big lander. That means more fuel for the landing, which means even more weight. You’d need a big rocket to launch that, or perhaps an assemble-in-orbit deal.
Fortunately, big rockets are available or will be soon. The Atlas IVB that launched Cassini could put about 5700 kg into geostationary transfer orbit. The Delta IV Heavy can lift more than twice that amount, and SpaceX’s Falcon Heavy will be able to lift almost 4X that amount to GTO.
So we have the technical capacity to build a big mission. Cassini cost about $14 billion, so a similar mission might cost a similar amount (more for development, less for launch costs). A more ambitious mission like the JIMO might cost $20-$30 billion.
If I want to see that in my lifetime, they’d better hurry. Such a mission will probably take a decade to develop, and another 5-8 years to get there.
Well, I don’t think it’s silly because I’m worried about angering the natives or radiation. Rather, it’s a silly idea because dropping an atomic bomb will intensely disrupt the kinds of things we want to examine. Its effects would be unpredictable. I’m not worried about the radiation - Europa is already bathed in radiation from Jupiter - enough to kill you in less than a day if you were stupid enough to be there. And after the explosion subsides, a lot of that water is just going to flow back into the crater and re-freeze.
In any event, it sure as hell isn’t going to penetrate through to the ocean. Our best guess at the depth of the ice covering is somewhere between 10km and 30km. The 10 MT hydrogen bomb the U.S. detonated on the Enewetak atoll dug a crater about 165 feet deep. When you detonate a bomb on the surface, most of the energy is directed upwards and outwards, like a shaped charge. If you wanted to really crack the ice, you’d have to dig deep and plant the bomb way under the ice. But if you can do that, why not just dig down to the ocean anyway?
You’ve certainly put a good amount of thought into this, and all of your recommendations are at least plausible (though you are going to need more than a 250 µm diameter cable over that distance). But that is a very minimal instrument package with limited capability for such an expensive effort to deliver the probe to Europa. Realistically, for this scale of effort you’d want to have at a minimum, you’d want to have a gas chromatograph and broad range mass spectrometer; along with the ionization source (laser or electrical), vacuum pump, and analyzer, not to mention the power supply for all of this, is going to be way larger than your probe.
I really don’t intend to offend, but do you realize how completely absurd that is? Other than the fact that visibility is likely to be measured in distances of centimeters and will therefore pretty much provide a screenful of digitized static, what would you gain from having a camera? Either you would see nothing and therefore get no zero value from the probe or you would see…something, and have next to nothing. We would have an almost negligible expectation of finding complex life (fish, cephalopod, the Great Cthulhu) in the ocean of Europa, so we would really be looking for organic molecules and precursors rather than giant extraterrestrial lampreys or Nessie the Loch Ness Monster.
Stranger
I agree that those instruments are implausible for a probe of this size. Furthermore, I kept my instrument selection to those which do not require breaks in the pressure vessel–any instrument which requires taking in an outside sample is going to vastly increase the complexity of the system. At most, I had in mind a fused silica “tunnel” (with a magnetically coupled propeller for circulation) to provide some degree of isolation.
This is getting to matters of politics and the like, but I wouldn’t agree that it’s not worth the effort without those instruments. I just don’t see any other way to probe Europa’s depths in a shortish timescale. Given that we know almost nothing at the moment, even very primitive data tells us a great deal. There’s no reason this has to be the last Europan probe.
Of course, the surface lander can house more advanced instruments, and more. I’ll let someone else put together a wish list for that.
One serious flaw with my proposal is with sourcing that much Pu-238. We spent a long time without any production, instead buying it from the Russians. That deal is basically over, and only fairly recently did the US restart production. But it’s slow going and seems like it will take the better part of a decade to produce 10 kg of the stuff.
But this is exactly what argues against trying to do something as quickly as possible, rather than to design a mission to obtain as much information as practicable. Just getting a probe to Jupiter and landing a robotic probe on the surface of Europa is probably a US$10B-US$15B effort; the cost difference between the simple probe that you describe (and all of the issues with relying on radioisotopic heating to “melt” its way through the crust aside) and a more complex probe are, while not lost in the noise, only a small portion of the overall cost and only add modestly to the complexity of mission operations. Getting primitive data which is conconclusive or provides nothing new will be a flat negative in terms of justifying other missions, and even a total success in finding new conditions or organic precursors hardly justifies a more elaborate follow-on. When NASA-JPL successfully executed the Voyager 1 and 2 missions, which despite their crudity delivered vast new insights into the outer planets and a wealth of information that kept planetologists occupied for years analyzing data, the assumption was that this would justify many follow-on missions to study the complex moon systems and curious phenomena of Jupiter and Saturn, and (in the hopes of a few irrepressible optimists) the fascinating enigmas of Uranus and Neptune. Instead, it tool over twenty years to launch an orbital mission to the Jovian system (Galileo), and longer to go to Saturn (Cassini-Huygens). The ambitious Jupiter Icy Moons Orbiter was cancelled. The Juno polar orbit mission won’t start excuting primary objectives until over two years from now). New Horizons did a quick pass on Jupiter before proceeding to the Kuiper belt. There are no other active NASA missions to explore the outer planets in the works. No missions to further explore Uranus and Neptune have survived a preliminary study phase. NASA’s current focus is excuting the current crewed capability objectives despite not really having any clear roadmap, necessary technology, or funding to do so.
The lesson to be learned here to planetologists is that if you want to execute a planetary mission, go big or go home, because you probably won’t get a chance for a follow-on mission in your career. Instead of launching two or three planetary, solar, and outer system missions a year–entirely feasible within the current budget allocation to NASA–we get about two per decade to the outer planets, and four or five major missions to Mars and Venus, which is a pretty damn shameful situation for an organization chartered with exploring the solar system.
Your point about the scarcity of RTG-grade plutonium is well taken, and is frankly an argument against using RTGs for planetary exploration. They provide so little power that the capability and communications bandwidth is highly limited compared to what could be provided by a small scale nuclear fission reactor even with the cooling requirements and low efficiency. In fact, a subcritical reactor using minimally processed thorium fuel would be almost ideal for this type of application in terms of simplicity/weight/power output balance.
Stranger