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It'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 therm

It'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 ago
I think it's more like ~0.6kW/m^2 out at Mars
kakali 9y ago
I 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 ago
  New plan! Colonize Venus! (What's all this about cloud cover?)
  - snerbles 9y ago
    Airship colonies, take advantage of that dense atmosphere and float over the clouds.
  - Symmetry 9y ago
    If 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 ago
At 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 ago
  That number sounds high. I make it about 300W for Earth. http://heliosphan.org/solarenergy.html http://heliosphan.org/solarenergy.html
  - snerbles 9y ago
    I should have clarified - this is raw irradiance, not the power you'd see from a photovoltaic cell.
    - deleted 9y ago
      [deleted]
    - redcalx 9y ago
      Ahh, 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 ago
    1.4 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 ago
      Yeh 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 ago
So 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 ago
It'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 ago
  It 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 ago
  It'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 ago
      Your 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 ago
Nuclear 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 ago
  Any regulations that would prevent the development of nuclear rocketry on Mars, after a base was established?
  - philipkglass 9y ago
    None 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 ago
  Let'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...

chesimov 9y ago

I think it's more like ~0.6kW/m^2 out at Mars

kakali 9y ago

I 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 ago
New plan! Colonize Venus! (What's all this about cloud cover?)
- snerbles 9y ago
  Airship colonies, take advantage of that dense atmosphere and float over the clouds.
- Symmetry 9y ago
  If 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 ago

At 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 ago
That number sounds high. I make it about 300W for Earth. http://heliosphan.org/solarenergy.html http://heliosphan.org/solarenergy.html
- snerbles 9y ago
  I should have clarified - this is raw irradiance, not the power you'd see from a photovoltaic cell.
  - deleted 9y ago
    [deleted]
  - redcalx 9y ago
    Ahh, 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 ago
  1.4 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 ago
    Yeh 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 ago

So 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 ago

It'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 ago
It 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 ago
It'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 ago
    Your 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 ago

Nuclear 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 ago
Any regulations that would prevent the development of nuclear rocketry on Mars, after a base was established?
- philipkglass 9y ago
  None 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 ago
Let'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...