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The hole of Space exploration is really hindered by the approach to nuclear power. Many missions that could fly with nuclear batteries don't use them. Rosetta w
by nickik 9y ago
The hole of Space exploration is really hindered by the approach to nuclear power. Many missions that could fly with nuclear batteries don't use them. Rosetta would have been a far better mission with one.
On a larger scale we should have nuclear reactors in space. If we ever want a mars colony, nuclear is way, way better then solar. Elon Musk is gone try with solar but he really doesn't have any other options.
NASA is the one place there can actually be progress on this stuff, no private company can reasonable attempted this stuff because of regulation. We need more nuclear batteries, we need small reactors for space and we need nuclear thermal rockets engines.
- Gravityloss 9y agoIt's not so easy to make space nuclear power light and reliable. Most likely a nuclear powered spacecraft with electric thrusters would be a dog. Nuclear thermal rockets aren't that great either, the temperature and thus the specific impulse is not very high, and they have high dry mass and would be really expensive to develop and hard to make reliable. In space you have 1.3 kW per square meter sun power constantly without the atmosphere or clouds in the way, and solar cells improve constantly.
- chesimov 9y agoI think it's more like ~0.6kW/m^2 out at Mars
- kakali 9y agoI think your 1.3 kW number assumes being near Earth.
- camiller 9y ago"...1.3 kW per square meter sun power constantly..." at Earth's distance from the sun. At Mars it is .715 kW. Inverse square law and all that.
- sdenton4 9y agoNew plan! Colonize Venus! (What's all this about cloud cover?)
- snerbles 9y agoAirship colonies, take advantage of that dense atmosphere and float over the clouds.
- Symmetry 9y agoIf you go high enough in Venus's atmosphere that the temperature and pressure are Earth-like you're getting 1.3 times the sunlight of Earth coming in from above and about 1.0 times reflected off the clouds below.
- snerbles 9y agoAt Earth orbit, yes. Recall that the inverse square law applies for light emitted by the sun. On Mars, solar power per square meter is a bit over 550W.
- redcalx 9y agoThat number sounds high. I make it about 300W for Earth. http://heliosphan.org/solarenergy.html http://heliosphan.org/solarenergy.html
- snerbles 9y agoI should have clarified - this is raw irradiance, not the power you'd see from a photovoltaic cell.
- deleted 9y ago[deleted]
- redcalx 9y agoAhh, I forgot that my calc was based on an average for the Earth's surface - so including high latitudes and day/night. On that basis I figure our numbers broadly agree.
- dTal 9y ago1.4 KW is right: https://en.wikipedia.org/wiki/Solar_irradiance#Earth https://en.wikipedia.org/wiki/Solar_irradiance#Earth It's an unusual methodology to begin by looking up the power output of the sun, which is worked out using the reverse of your calculation. Might you have erroneously divided out the entire of Earth's surface area (which is 4 times the area of a circle the size of the Earth)?
- redcalx 9y agoYeh I forgot that's what I did, it was a while ago now, and I admit the average over entire surface area isn't a very useful number :)
- emeraldd 9y agoSo the next step is to figure out how to build the power plant in space instead of lugging all that material up out of the Earth's gravity well.
- jessriedel 9y agoIt's hard to miniaturize nuclear power. But if you're going to mars, you don't want miniature, you want economies of scale (both for descent/landing and for cruise). And this is what nuclear power provides in spades.
- nickik 9y agoIt only needs to be lighter then the massive amounts of solar panels that would be needed on mars. People have done the calculation and nuclear would be way, way better then solar for a mars colony.
- Gravityloss 9y agoIt's hard to do any kind of nuclear reactor in space. You don't have bodies of water or even an atmosphere so radiative cooling it is, and that requires huge radiators. They alone weigh a lot. You must pipe coolant through them. What about micrometeoroids? You probably have to have valves to shut off leaking radiators. You don't have gravity so you don't have convection or separation of gas and liquid. Hard to test such systems on Earth. You can't make repairs easily. Solar cells start to look really good really fast. Maybe on Mars surface they could make some sense, or on the Moon where the night lasts for two weeks. Though on the Moon one would preferably settle near the poles without such problems. But even on the surface, just regular batteries look really good on the simplicity front, and fuel cells beyond that.
- kbenson 9y ago> You don't have bodies of water or even an atmosphere so radiative cooling it is, and that requires huge radiators. This is what I thought. To my (admittedly fairly uninformed) knowledge, nuclear power is really nuclear steam engines, at least the ones I know of. As awesome as a space steam engine sounds, it doesn't seem too feasible for the reasons you presented.
- jessriedel 9y agoYour intuition doesn't even agree with 1970s technology. Radioisotope thermoelectric generators developed then are still more efficient than modern battery power sources. They don't use steam and they have no moving parts. https://en.wikipedia.org/wiki/Radioisotope_thermoelectric_generator https://en.wikipedia.org/wiki/Radioisotope_thermoelectric_ge...
- philipkglass 9y agoNuclear thermal rockets have low specific impulse compared to electrical propulsion, but better ISP than any chemical rockets. They also provide orders of magnitude more thrust than electrical propulsion does. I don't think that they are in the sweet spot for uncrewed probe missions, but the combination of higher-than-chemical ISP and higher-than-electrical thrust seems like a good match for crewed missions to Mars or beyond. Regarding the many comments in this discussion about accidents during initial launch: if nuclear thermal rockets are fueled with uranium 235, and don't reach criticality until after the risky chemical-booster ascent out of atmosphere, they should be quite safe. Uranium 235 has a 700 million year half life, hence very low radioactivity. The fission products from a live reactor of course include very-short-lived, very-high-radioactivity nuclides. But you can have nuclear power in space with extremely low risks to the terrestrial environment if you use it only for missions that leave Earth orbit, and turn on the reactor only after the risky initial ascent on chemical boosters. That said, I agree that it would be expensive to develop nuclear thermal rockets. It's very expensive to test them on Earth, since the exhaust carries away some small amount of fission products and people don't just tolerate such radioactive pollution like back in the 1960s. You'd need some very expensive/elaborate test facility that can capture the hot, radioactive exhaust, or you'd need to do all the actual testing in space (also expensive/elaborate).
- ColanR 9y agoAny regulations that would prevent the development of nuclear rocketry on Mars, after a base was established?
- philipkglass 9y agoNone at present. The 1967 Outer Space Treaty prohibits nuclear weapons in space, but does not restrict peaceful uses of nuclear energy. A Mars base that is developed enough not only to support permanent human habitation but to conduct nuclear rocketry R&D is so far from where we are now that I couldn't speculate about what sort of laws might then apply on Mars, though.
- Gravityloss 9y agoLet's say you have a velocity requirement of 6 km/s. A chemical hydrogen-oxygen rocket might have exhaust velocity of 4.5 km/s so mass ratio is exp(6/4.5) = 3.8. A nuclear rocket with exhaust velocity 9 km/s gets a mass ratio 1.9. Most likely the nuclear rocket uses hydrogen only, meaning the tank is likely bigger than the chemical rocket's! The thrust to weight ratio of a normalish solid nuclear engine is about ten times worse than a chemical one. You do save on the oxygen mass, but that's it. It's probably bigger, heavier, more complex and definitely more expensive and dangerous than an ordinary chemical rocket. For higher velocities, it starts making more sense...
- cletus 9y agoOne problem is that NASA is running out of plutonium [1] as the Pu-238 used to this point was largely a byproduct of the weapons program and not a lot of nuclear warheads have been built lately. [1]: https://www.popsci.com/new-technique-could-double-plutonium-production-for-nasa https://www.popsci.com/new-technique-could-double-plutonium-...
- samstave 9y agoConvert the warheads into batteries?
- sgt101 9y agoThere's a lot of that in Cumbria... about 120 Tonnes.
- philipkglass 9y agoSpent nuclear fuel from reactors doesn't have usable quantities of the right plutonium isotope. RTGs use the uncommon Pu-238. Plutonium in spent nuclear fuel is mostly Pu-239, 240, 241, and 242.
- nickik 9y agoWhile this is true it is just another reflection of the backwards nuclear policy of NASA and the US government. With different reactors, creation of more Pu-238 would not be a huge problem. The issue is just that there seems to be very little interest in pushing forward any areas and as so often the case many of these technologies are mutually supporting. I would really love a real push forward on Molten Salt reactors that would give you the ability to filter out a number of important substances that have a hole host of different uses. It would also go a long way in solving the nuclear waste problems. Here is some exploration of what is contained in nuclear waste: https://www.youtube.com/watch?v=rv-mFSoZOkE https://www.youtube.com/watch?v=rv-mFSoZOkE
- abtinf 9y agoThe first time I was exposed to the concept of space-based industry was playing PC game Privateer as a kid. You land on these giant asteroids with attached mining bases... to transport ore to planets and refineries. It never made sense to me. Why setup all of this massive capital investment for mere ore? I think one of the very first space industries is going to be automated mining, refining, and production of nuclear fuel. Once freed from the widespread and totally irrational fear of nuclear technology here on Earth, the magnitude of advantage of nuclear power is stunning. Before the regulatory regime killed nuclear investment, it was well on its way to becoming cheaper than coal. Cheap, abundant energy solves virtually every human problem, whether on earth or in space. A stable supply of space based nuclear fuel will lead to exponential growth in space based activity and greatly simplify colonization of our solar system... and beyond.
- jacobush 9y agoIs there any uranium to be had in space?
- abtinf 9y agoI know very little about this. From what I've read, it is very unlikely that uranium ores will be found on asteroids, because concentrating uranium into economically mine-able quantities is believed to require long running thermal process in a planets core. That would mean looking for uranium on larger bodies, like the moon. https://www.space.com/6904-uranium-moon.html https://www.space.com/6904-uranium-moon.html
- twohlix_ 9y agoI'd wager that uranium (as with all the heavier elements) is spread throughout the solar system as it is forged and spread in stars going supernovae/hypernovae
- logfromblammo 9y agoEverything in the solar system is pretty much made out of the same stuff. There is uranium on Earth, the Moon, and Mars, so we can reasonably expect it to be present on other less massive objects in the solar system. But you also have to consider that materials stratify based on their density and chemical affinity. The Goldschmidt classification of an element defines sort of chemical cliques for the elements. Lithophile elements like to oxidize into minerals and float on top of any dense metal core. For instance, the dominant aluminum ore bauxite is frequently found close to the surface of the earth. To find those elements in space, you look for a rock-dominant (oxidized) asteroid. Siderophile elements like to form solid solutions with iron, and may not oxidize to lighter minerals readily. Elements like gold and platinum are considered rare on Earth not because the planet does not have a great quantity of them, but because the vast majority of that mass is way down in the iron core, where we cannot yet reach it. The mass near the surface is only there due to quirks of geology. To find those elements in space, you look for a metals-dominant asteroid. Chalcophile elements like sulfur, and on Earth their rarity (in terms of the amount we can dig up) is between lithophiles and siderophiles, because they tend to stratify above the iron-lovers and below the oxygen-lovers. The only elements that may be difficult to find would be those that form volatile molecules, because they typically get gradually blown away by solar wind, or stripped away all at once by a catastrophic cosmic event. Uranium and thorium are lithophile elements, so they shouldn't be too difficult to find on rocky, oxygen-heavy asteroids. Just look for low albedo and alpha particles from pitchblende. But beware that absent the ore-concentrating processes of large planets, you may have to process a lot of rock to get a usable quantity of fissile elements.
- oculusthrift 9y agowhat would’ve happened if challenger was carrying nuclear material? (i’m actually asking)
- coryrc 9y agoThe Earth would be immeasurably more radioactive than today.
- vanjoe 9y agoUsually immeasurably means infinite, I'm guessing that's not what you meant?
- ac29 9y agoImmeasurably means "incapable of being measured", which could refer to too small to measure or too large to measure, because English is fun like that. It does generally refer to the latter, though.
- robotresearcher 9y agoThe Earth would in fact be the same radioactive as today, assuming we had used stockpiled Pu. It's the distribution that matters. A little bit of ocean floor over Florida would be much more radioactive than today if it was scattered with Pu-238.
- maxerickson 9y agoIt would depend a great deal on what it was and how it was packaged. This seems to have some good discussion: https://space.stackexchange.com/questions/16608/how-does-the-launch-risk-for-a-plutonium-rtg-and-a-uranium-fission-reactor-compa https://space.stackexchange.com/questions/16608/how-does-the...
- 14113 9y agoOne problem with nuclear reactors on space craft is what happens if/when the craft re-enters the atomosphere? How do you stop the reactor from becoming a high-altitude dirty bomb?
- devrandomguy 9y agoYeah, we should probably not do this in LEO, where orbits decay and half of the potential acceleration vectors lead back down to Earth. Fortunately, it takes a lot of energy and precision to de-orbit anything heavy that is in geosynchronous orbit or beyond. If the fissile material could be acquired by asteroid mining, then it would never need to pass through the Earth's atmosphere. I know that still sounds way out there, but I think that uranium or thorium would actually be a great material for kicking off asteroid mining. A small amount will pay off in a big way, it is a very dangerous thing for humans to mine on Earth, and, like the parent mentioned, we really don't want to fly it around where it can fall on us.
- olegkikin 9y agoIsn't cooling a big problem for reactors in space? The ISS has massive radiators just for all the power solar panels put out.
- dredmorbius 9y agoYes. I forget what specifically the energy gradient per square meter for radiators is, but it was surprisingly low. A third of a kilowatt comes to mind though not with any strong certainty. Calculations here suggest 20 kW/m^2: http://www.nss.org/settlement/nasa/spaceresvol2/thermalmanagement.html http://www.nss.org/settlement/nasa/spaceresvol2/thermalmanag...
- mtgx 9y agoNo rocket has had 100% reliability. And you need to be pretty close to that if you want to make sure your nuclear-powered rocket won't explode either at take-off or when landing. Even if rockets were as reliable as airplanes, it still wouldn't be good enough. We've had several major plane crashes just in the past decade. Now imagine if each of those made a nuclear explosion. Do you think that would be acceptable? We don't have fission-powered rockets because the math works against them, not because of humans' "irrational fear of nuclear power."
- nickik 9y agoNuclear thermal rocket would be best for upper stages or mars-earth transfer vehicles. Nuclear power to use on ion thrusts would only be used in Space.
- Tuna-Fish 9y agoThere is very little point in using a fission rocket low in the atmosphere (wouldn't play to it's strengths, would not be much better than chemical rockets), but there is no problem in lifting fission rocket engines to orbit, because a fission reactor made out of U-235 is not significantly radioactive before it's turned on. So you can ship it into orbit in some configuration where it cannot turn on by accident (half of fuel removed and in storage or on a different rocket), and then only turn it on when you are in a safe orbit pointing away from earth.
- fishcolorbrick 9y agoMIT astronautics faculty did a study which compares 13 different power generation methods for use on Mars [0]. They determined that: "For latitudes close to this optimum latitude (31" north), photo- voltaic power systems exhibit mass- and volume- based performance that is comparable to that of nuclear fission power systems. For global access for human Mars surface missions, nuclear power is required either in the form of fission reactors or radioisotope generators." ...further noting: "...solar-based Mars surface power systems should be seriously considered as an alternative to nuclear fission surface power." The way I read the study, if you're happy with staying around the "near-equatorial north", you should use PV solar... if you want to site your colony somewhere else, you should use nuclear power. [0]: http://systemarchitect.mit.edu/docs/cooper10.pdf http://systemarchitect.mit.edu/docs/cooper10.pdf