If said scientific instruments need to go on a rover, mass still matters. More mass can limit rover mobility and at some point the rover requires more power than can be provided with RTGs or solar cells.
There's nothing gained from sending 50t of metal to the outer solar system. Scientific instruments are usually purpose-built and while lifting mass restriction can bring down cost, you'd still have to account for redundancy and precision because no one's going to fly out to Jupiter to properly unfold that darn antenna.
It gets ignored because it is a symptom of not being able to cheaply launch probes. You spend all of this time researching and developing a scientific instrument only to make two of them. One to launch the other to go on a diagnostic model (maybe a third for a launch backup). You spread out the cost of development over more units and bring down the per unit cost of manufacturing (although probably not that much given we are still talking quantities in the tens or at most hundreds of units). But without a cheap launch system there is a fixed cost per unit of hundreds of millions to get it into space, so there is very little point in producing more than what you need for just that mission.
Of course certain missions are going to require special specs, but I do think that generic, somewhat modular spaceprobes are around the corner now that launch costs are going down. If you are going to retire a space craft that you've made a profit off of shuttling cargo to LEO might as well discard of it by sending it on one last mission into deep space with a couple of scientific missions onboard.
We will end up spending more money overall, but cost per unit of science (however you want to measure that) will go down.
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
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.
An industry delivering strontium-90 on demand could be very valuable for space operations, despite or even because of its uselessness as weapon material.
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.
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.
The power density of Sr-90 fuel is 15% lower by mass than Pu-238 - 0.46kW/kg thermal compared to 0.54kW/kg thermal. You must have been looking at the wrong figures. Since the Sr-90 fuel rod doesn't get as hot as a Pu-238 fuel rod of the same size, energy conversion efficiency drops significantly and you need up to 100% more Sr-90 fuel (and consequently more shielding as well) to get the same power.
> Half-life is comparable.
A factor of 3x is not really comparable (87.7 vs 28.7 years).
> Safety is better: less gamma, no neutrons, no shielding needed.
Where did you get that from? Strontium-90 and its daughter product Yttrium-90 emit high-energy beta particles and thus give off high energy bremsstrahlung which requires heavy shielding. The BUP-500 battery (the largest RTG ever built, designed to provide 500W electric after 5 years; 1.8m³ in size and 3.6 metric tons in weight) used a tungsten alloy for shielding; its predecessors used depleted uranium...
Pu-238 on the other hand is pretty much exclusively an alpha emitter and has no short-lived daughter products (U-234 has a half-life of >200ka).
> Overprovisioning is needed in every case, and excess power has to be dealt with the same way.
Again, where did you get that from? A space mission is designed to a certain power spec and if you need your primary instruments for a 15 year mission, you plan your power such that after 15 years all primary instruments can still be powered. With Pu-238 that'd be about 75%-80% of the initial RTG capacity (some losses due to development time and efficiency losses from thermocouples are included).
Due to its properties, Sr-90 would require about 3x as much fuel just to make up for its shorter half-life in addition to the up to 100% increase in required fuel mass from its lower temperature.
Since it's a beta emitter, the additional radiation requires heavy shielding as well. So no, there's a pretty substantial difference there.
> Low cost to orbit means more fuel / more ∆V is practical to provide, thus much shorter transit times, translating to less up-front decay.
That's not how interplanetary missions work. Chemical rockets are incapable of providing enough delta-V to substantially shorten transit times to the outer solar system. If you want to do anything other than fly-by missions, more chemical fuel isn't going to help with that - you'd need nuclear or electric propulsion [1].
[1] https://www.esa.int/gsp/ACT/doc/PRO/ACT-RPR-PRO-ISTS2004-Plu...
So, I am obliged to conclude Wikipedia's figure is off by 2x, undermining my argument.
That said, if there were no Pu, Sr-90 would have to do. It has the advantage of being very cheap, as radiothermal isotopes go, and not requiring G7-military-grade security. Its bremsstralung radiation is only in X-ray range, so easily shielded, yielding heat.
When it gets cheap to loft 150 tons to orbit, in a couple of years, the economic and timing verities of outer solar system transport will be up-ended. Cassini was lofted at 6 tons, and took 7 years to get to Saturn. Much of its cost was driven by limits on mass. Given 150 tons to work with, it is hard to imagine any detail of the mission remaining similar, except its rousing success. It certainly would not have needed 7 years to get to Saturn. It might not even have needed RTG power.
A four-year transit to Pluto using FRC direct fusion propulsion is more appealing than RTG-powered ion drive, with the additional advantage of much less likelihood of wholesale civilizational collapse in the interim. But FRC fusion needs development work; its research budget is currently diverted to dead-end Tokamak projects.
If you drop the launch costs by 100x, this means being able to launch 100x JWSTs for the same price, meaning it makes sense to optimize for production costs instead of reliability - as if one of your telescopes fail, you can always launch/use another. So now it makes sense to focus on using mass-manufactured parts instead of specialty ones, which drops the production costs significantly. It might make sense to design a telescope platform instead, opening more opportunities to exploit economies of scale.
As the GP pointed out the launch cost of the JWST is a small fraction of the program's overall cost. Launching a hundred JWSTs would mean building a hundred JWSTs and doing so is not cheap. Even with economies of scale producing various components for the hundred JWSTs those all need to be tested and verified. That's an expensive process that doesn't really benefit from economies of scale.
For an instrument the size of JWST you can't just spam launches with the assumption some will fail. It's a giant instrument. A catastrophic failure would see it land mostly intact in a populated area or fill a huge orbit with dangerous debris. Even if it was just non-operational it would not occupy an orbital slot that's unusable by other instruments.
Launch costs are almost always a small portion of overall mission costs. Just because launch costs go down doesn't mean space missions can magically scale up. The goal is to launch useful scientific/commercial instruments or space vehicles. We don't just launch hunks of lead into orbit for funsies. Lower launch costs are nice but they're not going to magically make all space missions cheap or easy.
My point is that all those processes are expensive because the final result has to work the first time, because launches are expensive and infrequent. Drop launch costs enough, you don't need so much reliability, so everything across the board suddenly becomes much cheaper. Which then opens many further opportunities to reduce costs.