4 ms·

> It gets ignored because it is a symptom of not being able to cheaply launch probes. Is it, though? Let's do a quick and very superficial analysis of the cost

by qayxc 5y ago

> It gets ignored because it is a symptom of not being able to cheaply launch probes. Is it, though? Let's do a quick and very superficial analysis of the costs here. Say we want to send a probe the outer solar system, like the moons of Jupiter or the Saturn system. There are two unique challenges that such probes are faced with: • on Jupiter, the magnetic field of the planet requires hardened electronics and shielding • in the Saturn system and beyond, nuclear power is the only option for powering your instruments Neither of these points are in any way shape or form correlated with launch costs. In fact, in order to get RTGs you need to build, maintain and operate an entire specialised nuclear facility. Alternatively you could use small nuclear reactors, but that may or may not interfere with certain instruments. The cost of hardening electronics and shielding is also not going away even if launch costs were non-existent. It also doesn't matter much whether you produce 10 or 1000 of such specialised processors - the cost will still be orders of magnitude higher than comparable commodity options. It's not getting any better if we turn inwards instead and consider Venus or Mercury. Both again pose incredible and unique engineering challenges as do most scientifically interesting targets in the solar system. How exactly would a cheaper launch cost solve the problem of a Venus sample-return mission? Is it the expensive launch that prevents us from sending an orbiter out to Pluto? How does more and cheaper payload help with getting a probe into the oceans of Europa, Ganymede, or Enceladus? I could go on, but I think you get what I mean. edit: there's also the human factor that I conveniently left out - every mission requires a staff of people (both scientists and engineers) for the entire mission duration; that cost is also unrelated to launch costs

cookingmyserver 5y ago
> The cost of hardening electronics and shielding is also not going away even if launch costs were non-existent. Of course, but what about hardened electronics exempts it from the principles I discussed with scientific instruments in general? > the cost will still be orders of magnitude higher than comparable commodity options. Irrelevant, no one is comparing the cost to commodity hardware, we are comparing it to what the cost would be if we don't increase the number of scientific missions and continue with the current demand. > in order to get RTGs you need to build, maintain and operate an entire specialized nuclear facility. Yes, needing more material is going to cost more money, but does the cost per unit stay the same or go up the more you buy? We are going to need to increase production Pu used in RTGs anyways, if we are going to build facilities to might as well utilize them to the max. > How exactly would a cheaper launch cost solve the problem of a Venus sample-return mission? There will be unique engineering challenges that require unique solutions, but engineers will have a large shelf of parts to choose from if they have a specific issue they don't have to invent the wheel for. > that cost is also unrelated to launch costs That one is true. Missions that involve complex maneuvers, landings, driving rovers, etc, will require large amounts of staff and engineers. Other missions that don't involve those things could utilize a shared staff, so something like a general purpose probe network taking measurements across the solar system. Fleet management tools, policy, and procedures need to be developed so we can manage assets like we do with other LEO constellations.
ncmncm 5y ago
Just incidentally, a RTG powered by strontium-90 works as well as or better than a plutonium-powered one. Strontium-90 builds up as a waste product in ordinary uranium fuel rods, and can be (pretty) cheaply extracted from what we call nuclear waste. I suspect the use of plutonium for RTGs in spacecraft is mainly because people already had a pipeline for extracting and concentrating plutonium for the weapons industry. I don't know of any other use for strontium-90, outside RTGs. An industry delivering strontium-90 on demand could be very valuable for space operations, despite or even because of its uselessness as weapon material.
- qayxc 5y ago
  > I suspect the use of plutonium for RTGs in spacecraft is mainly because people already had a pipeline for extracting and concentrating plutonium for the weapons industry. You suspect wrong. In order to be a decent RTG power source, an isotope needs to have: 1. Good power density, since the current energy conversion efficiencies are quite low 2. Good half-life, since it is aimed to operate the spacecraft as long as possible 3. Require little to no shielding, in order not to interfere with sensitive instruments aboard and also to protect humans (i.e. scientists and engineers working on the spacecraft) 4. High power/mass ratio, to minimize the propulsion requirements to launch it into space (which would be less of a factor in this context) Pu-238 is an ideal fuel due its very good performance in all of these characteristics. Nothing to do with nuclear weapons. The next best fuel would be Am-241, which is more readily available but is a more aggressive alpha-emitter and therefore performs poorer in (3). Strontium-90 has a lower power density and a significantly lower half-life. This makes it OK for terrestrial applications, where operation times of a decade are perfectly acceptable, but not ideal for interplanetary missions. Keep in mind that a lot of power capacity is lost during transit if we're talking about missions to Saturn and beyond. Cheap cost to LEO doesn't pull the outer planets any closer so probes will still take the better part of a decade to get to their destination. Overprovisioning is also not really an option, because excess heat needs to be radiated away, which is very difficult and costly to do in space.
  - ncmncm 5y ago
    Strontium-90 has better power-density per unit mass than Pu, albeit with larger volume. Half-life is comparable. Safety is better: less gamma, no neutrons, no shielding needed. Overprovisioning is needed in every case, and excess power has to be dealt with the same way. Low cost to orbit means more fuel / more ∆V is practical to provide, thus much shorter transit times, translating to less up-front decay.

by qayxc 5y ago

cookingmyserver 5y ago

> The cost of hardening electronics and shielding is also not going away even if launch costs were non-existent. Of course, but what about hardened electronics exempts it from the principles I discussed with scientific instruments in general? > the cost will still be orders of magnitude higher than comparable commodity options. Irrelevant, no one is comparing the cost to commodity hardware, we are comparing it to what the cost would be if we don't increase the number of scientific missions and continue with the current demand. > in order to get RTGs you need to build, maintain and operate an entire specialized nuclear facility. Yes, needing more material is going to cost more money, but does the cost per unit stay the same or go up the more you buy? We are going to need to increase production Pu used in RTGs anyways, if we are going to build facilities to might as well utilize them to the max. > How exactly would a cheaper launch cost solve the problem of a Venus sample-return mission? There will be unique engineering challenges that require unique solutions, but engineers will have a large shelf of parts to choose from if they have a specific issue they don't have to invent the wheel for. > that cost is also unrelated to launch costs That one is true. Missions that involve complex maneuvers, landings, driving rovers, etc, will require large amounts of staff and engineers. Other missions that don't involve those things could utilize a shared staff, so something like a general purpose probe network taking measurements across the solar system. Fleet management tools, policy, and procedures need to be developed so we can manage assets like we do with other LEO constellations.

ncmncm 5y ago

Just incidentally, a RTG powered by strontium-90 works as well as or better than a plutonium-powered one. Strontium-90 builds up as a waste product in ordinary uranium fuel rods, and can be (pretty) cheaply extracted from what we call nuclear waste. I suspect the use of plutonium for RTGs in spacecraft is mainly because people already had a pipeline for extracting and concentrating plutonium for the weapons industry. I don't know of any other use for strontium-90, outside RTGs. An industry delivering strontium-90 on demand could be very valuable for space operations, despite or even because of its uselessness as weapon material.

qayxc 5y ago
> I suspect the use of plutonium for RTGs in spacecraft is mainly because people already had a pipeline for extracting and concentrating plutonium for the weapons industry. You suspect wrong. In order to be a decent RTG power source, an isotope needs to have: 1. Good power density, since the current energy conversion efficiencies are quite low 2. Good half-life, since it is aimed to operate the spacecraft as long as possible 3. Require little to no shielding, in order not to interfere with sensitive instruments aboard and also to protect humans (i.e. scientists and engineers working on the spacecraft) 4. High power/mass ratio, to minimize the propulsion requirements to launch it into space (which would be less of a factor in this context) Pu-238 is an ideal fuel due its very good performance in all of these characteristics. Nothing to do with nuclear weapons. The next best fuel would be Am-241, which is more readily available but is a more aggressive alpha-emitter and therefore performs poorer in (3). Strontium-90 has a lower power density and a significantly lower half-life. This makes it OK for terrestrial applications, where operation times of a decade are perfectly acceptable, but not ideal for interplanetary missions. Keep in mind that a lot of power capacity is lost during transit if we're talking about missions to Saturn and beyond. Cheap cost to LEO doesn't pull the outer planets any closer so probes will still take the better part of a decade to get to their destination. Overprovisioning is also not really an option, because excess heat needs to be radiated away, which is very difficult and costly to do in space.
- ncmncm 5y ago
  Strontium-90 has better power-density per unit mass than Pu, albeit with larger volume. Half-life is comparable. Safety is better: less gamma, no neutrons, no shielding needed. Overprovisioning is needed in every case, and excess power has to be dealt with the same way. Low cost to orbit means more fuel / more ∆V is practical to provide, thus much shorter transit times, translating to less up-front decay.