Uranus Orbiter and Probe (UOP)

[Don2]

So use Americium instead. It's supposed to be available this year. It does require additional mass, but that shouldn't be a problem.

Plutonium was used by the nuclear weapons business, and they did a lot of work on the health, safety and environmental aspects. NASA can make use of all that data for getting safety approval. I think Americium has similar chemisty, but it is not the same. Getting it approved might be hard.

[Greg Hullender]

So use Americium instead. It's supposed to be available this year. It does require additional mass, but that shouldn't be a problem.

Plutonium was used by the nuclear weapons business, and they did a lot of work on the health, safety and environmental aspects. NASA can make use of all that data for getting safety approval. I think Americium has similar chemisty, but it is not the same. Getting it approved might be hard.

It's already approved for use in the Rosilind Franklin Mars Rover in 2028, so I think that challenge has already been met. Also, it's been approved for use in smoke detectors for decades.

The biggest problem with ^²³⁸Pu (as you probably know) is that it has to be synthesized, which is fabulously expensive. ^²⁴¹Am can be extracted from ordinary nuclear waste with 99% isotopic purity. It's less efficient, but it's literally a thousand times cheaper, and the supply is unlimited. Being able to get away from ^²³⁸Pu is a big benefit of cheaper costs for mass to orbit.

[deadman1204]

So use Americium instead. It's supposed to be available this year. It does require additional mass, but that shouldn't be a problem.

Plutonium was used by the nuclear weapons business, and they did a lot of work on the health, safety and environmental aspects. NASA can make use of all that data for getting safety approval. I think Americium has similar chemisty, but it is not the same. Getting it approved might be hard.

It's already approved for use in the Rosilind Franklin Mars Rover in 2028, so I think that challenge has already been met. Also, it's been approved for use in smoke detectors for decades.

The biggest problem with ^²³⁸Pu (as you probably know) is that it has to be synthesized, which is fabulously expensive. ^²⁴¹Am can be extracted from ordinary nuclear waste with 99% isotopic purity. It's less efficient, but it's literally a thousand times cheaper, and the supply is unlimited. Being able to get away from ^²³⁸Pu is a big benefit of cheaper costs for mass to orbit.

EUROPE has procedures to use americim. The US gov does not. Since its nuclear, I'm sure they won't be super open about sharing all their data with us either to make people feel better.

[ccdengr]

EUROPE has procedures to use americim. The US gov does not. Since its nuclear, I'm sure they won't be super open about sharing all their data with us either to make people feel better.

Huh.  Lots of information about this in "European Radioisotope Thermoelectric Generators (RTGs) and Radioisotope Heater Units (RHUs) for Space Science and Exploration" https://link.springer.com/article/10.1007/s11214-019-0623-9

Open access and everything.

[Greg Hullender]

So use Americium instead. It's supposed to be available this year. It does require additional mass, but that shouldn't be a problem.

Plutonium was used by the nuclear weapons business, and they did a lot of work on the health, safety and environmental aspects. NASA can make use of all that data for getting safety approval. I think Americium has similar chemisty, but it is not the same. Getting it approved might be hard.

It's already approved for use in the Rosilind Franklin Mars Rover in 2028, so I think that challenge has already been met. Also, it's been approved for use in smoke detectors for decades.

The biggest problem with ^²³⁸Pu (as you probably know) is that it has to be synthesized, which is fabulously expensive. ^²⁴¹Am can be extracted from ordinary nuclear waste with 99% isotopic purity. It's less efficient, but it's literally a thousand times cheaper, and the supply is unlimited. Being able to get away from ^²³⁸Pu is a big benefit of cheaper costs for mass to orbit.

EUROPE has procedures to use americim. The US gov does not. Since its nuclear, I'm sure they won't be super open about sharing all their data with us either to make people feel better.

Now you're just not arguing in good faith. I don't think you actually believe this.

[VSECOTSPE]

Zeno Power Systems has a $15 million contract with the technology and human space flight arms of NASA to develop a Stirling generator for Am-241 sources, ostensibly to support Artemis operations during lunar night. 

https://spaceref.com/newspace-and-tech/nasa-selects-zeno-to-lead-team-to-develop-radioisotope-power-system-for-lunar-applications/

Frankly, I’m skeptical.  The science arm of NASA spent $272 million on Stirling generator development for Pu-238 sources and that effort was terminated because more work was still needed to prove out the reliability of the system and the money was to do that was needed elsewhere in the planetary program. 

https://spacenews.com/39124lockheed-shrinking-asrg-team-as-closeout-work-begins/

Maybe if the human space flight side of NASA throws low hundreds of millions of dollars at Zeno’s solution, the planetary science program could piggyback on it like they do DOE’s plutonium program.  But I don’t see that happening — the astronauts want a surface reactor, not RTGs.

Even then, I imagine setting up the processing for those Americium-241 sources will cost another order of magnitude.  Again, I don’t see human space flight paying for that when what they really want is a reactor.  And I don’t see science paying for it when they didn’t want to cough up hundreds of millions for a four-fold increase in Pu-238 efficiency in a much better (but still challenging) budget environment.

Lots of hammers in search of nails...

[Blackstar]

Even then, I imagine setting up the processing for those Americium-241 sources will cost another order of magnitude.  Again, I don’t see human space flight paying for that when what they really want is a reactor.  And I don’t see science paying for it when they didn’t want to cough up hundreds of millions for a four-fold increase in Pu-238 efficiency in a much better (but still challenging) budget environment.

Yes, but there's even more to it--the current system for producing RTGs for spacecraft is entirely based upon Pu-238. It's not just the production, it's all the handling, delivery to the launch site, integration, and the launch approval process. That's based upon about 67 years of experience. Change the material, and all that needs to be created for the new material. More money. Not going to happen.

[deadman1204]

So use Americium instead. It's supposed to be available this year. It does require additional mass, but that shouldn't be a problem.

Plutonium was used by the nuclear weapons business, and they did a lot of work on the health, safety and environmental aspects. NASA can make use of all that data for getting safety approval. I think Americium has similar chemisty, but it is not the same. Getting it approved might be hard.

It's already approved for use in the Rosilind Franklin Mars Rover in 2028, so I think that challenge has already been met. Also, it's been approved for use in smoke detectors for decades.

The biggest problem with ^²³⁸Pu (as you probably know) is that it has to be synthesized, which is fabulously expensive. ^²⁴¹Am can be extracted from ordinary nuclear waste with 99% isotopic purity. It's less efficient, but it's literally a thousand times cheaper, and the supply is unlimited. Being able to get away from ^²³⁸Pu is a big benefit of cheaper costs for mass to orbit.

EUROPE has procedures to use americim. The US gov does not. Since its nuclear, I'm sure they won't be super open about sharing all their data with us either to make people feel better.

Now you're just not arguing in good faith. I don't think you actually believe this.

hmmm.... you sound quite certain that it will be simple to tell the us gov were just gonna change everything and switch to americium and all the insane radioactivive material protocols they have will be just fine and nothing will be a problem.....
Or we can be realistic and assume theres more to it that arm chair engineers don't see

[MickQ]

I’m speaking from a position of total ignorance here but as I understand it Pu238 is a radioactive heat source and Am241 is a radioactive heat source.

What great changes in handling and procedures would be needed ??

[Don2]

Huh.  Lots of information about this in "European Radioisotope Thermoelectric Generators (RTGs) and Radioisotope Heater Units (RHUs) for Space Science and Exploration" https://link.springer.com/article/10.1007/s11214-019-0623-9

Open access and everything.

At the time that paper was written, which was the end of 2019, they were at TRL 4. They had built a prototype with output of 10W electric weighing 10kg. They planned to scale that up to a 50W system but had no plans for anything bigger.

NASA's Next Gen RTG will produce 245W at beginning of life and weighs 56kg. The Americium design would weigh 245kg if it had the same power output. The ESA technology is much heavier. It has a small advantage in that the power output decays more slowly. They are claiming about  a 10% drop in 10 years (1% per year) , while the NASA RTG drops 28% in 17 years (2% per year). 

The Europeans are using Am2O3 which is a ceramic with a very high melting point of 2205ºC (Wikipedia).  The US uses PuO2, which has an even higher melting point of 2744ºC. (Wikipedia) That implies a very strong bond between the plutonium and the oxygen, which likely leads to very low solubility in water and resistance to chemical attack. Americium has 95 electrons, one more than plutonium which has 94. Therefore it is not surprising that their chemistry is very similar. However it is not the same, which would mean that a lot of analysis and licensing stuff would need to be redone.

I don't think Americium is a realistic alternative for UOP.

[Don2]

  But I don’t see that happening — the astronauts want a surface reactor, not RTGs.

I find that hard to understand. RTGs are proven to be very reliable and long lasting. That would seem to be good for human spaceflight. Don't operating reactors emit a lot of neutrons? And even when they shut down, the spent fuel continues to emit a lot of penetrating radiation.

[Blackstar]

I find that hard to understand. RTGs are proven to be very reliable and long lasting. That would seem to be good for human spaceflight. Don't operating reactors emit a lot of neutrons? And even when they shut down, the spent fuel continues to emit a lot of penetrating radiation.

RTGs are very low power.

There have been proposals for RTGs for lunar surface use in support of astronaut operations, but really as the backup emergency power system. Nobody likes them.

This thread has really started to wander. Maybe it should get back to the subject of Uranus missions?

[VSECOTSPE]

I’m speaking from a position of total ignorance here but as I understand it Pu238 is a radioactive heat source and Am241 is a radioactive heat source.

What great changes in handling and procedures would be needed ??

I don’t think we know, yet.  As of 2017, basic things like what chemical form the fuel will take (Am2O3 or AmO2) and how it will be processed into pellets (the one attempt to date in Germany resulted in unacceptably cracked pellets) were unknown.  Other things, like adding more lead lining to the one cooled shipping container certified for Pu238 and recertifying it for Am241, are driven by differences in the type and energy levels of the particles emitted by the two nuclides and were still unknown at that time.  This reports goes into some of the gory details:

https://inldigitallibrary.inl.gov/sites/sti/sti/Sort_877.pdf

I didn’t see this when I scanned that report, but Am241 has about half the thermal energy density of Pu238, which means to get the same watts out of an RTG, you have to about double the mass of the nuclide, which about doubles the volume.  I would guess that drives very basic dimensional issues on the equipment used, among other things.  It’s not like switching out Duracells for Energizers in a toy’s AA-battery compartment.

I don’t know that any of these things are showstoppers.  But they cost time and money to figure out.

I find that hard to understand. RTGs are proven to be very reliable and long lasting. That would seem to be good for human spaceflight.

Astronauts need kilowatts, not watts.  The solar panels on the ISS generate 240 kilowatts max vice ~200 watts from an RTG.  I think you’d need something like a thousand RTGs to power the ISS.  It’s not practical.

[edzieba]

Pu238, whilst extraordinarily nasty chemically, is relatively benign in terms of decay products, mostly producing Alpha particles (with a little Gamma from the occasional cluster product decaying). Am241 likes to chuck out proportionally a lot more Gamma and neutrons along with its decay Alphas. The upshot is that shielding and distancing requirements for an Am241 would be different to a Pu238 RTG of the same power output, making awkward things like the layout of buildings where the RTGs are stored and processed an issue, as well as transport (e.g. suddenly you can't transport it by rail because the track side clearances that were adequate to reduce public exposure to a certain level with for Pu238 are not for Am241). It's not a massive difference, but its enough that you can't just treat it as a like-for-like replacement and follow the same handling protocols.

If you're going to jump through all the very expensive hoops to product and operate a whole new type of RTG (that is worse in every way but a possibly slightly cheaper raw stock supply) anyway, then you may as well go to slightly more expense to produce and operate a full up reactor that is much more capable.
ESA did not face the same calculus: they have no existing Pu238 handling history, so whatever they picked would be worked out from whole cloth anyway, so Am241 traded better simply due to availability.

[Blackstar]

Up-thread I posted some stuff on aerocapture at Uranus. There are people studying aerocapture for other planetary bodies. I just saw this listing for a paper about using aerocapture at Mars to adjust the orbital plane. I don't know how much active study NASA is doing, however.



Orbit Plane Rotation Using Aerocapture
Daniel C. Gochenaur, Michael P. Jones, Johannes J. Norheim and Olivier L. de Weck
Published Online:13 Feb 2025https://doi.org/10.2514/1.A36232
Journal of Spacecraft and Rockets

Abstract
This study investigates the feasibility of performing orbit plane rotations during aerocapture maneuvers. Three-degrees-of-freedom bounding trajectories at Mars are propagated for a range of vehicle lift-to-drag ratios 𝐿/𝐷
and hyperbolic arrival velocities 𝑣∞. The results show that the maximum plane rotation achievable increases with vehicle 𝐿/𝐷 and 𝑣∞. When arriving with 𝑣∞ of 6 km/s, vehicles with 𝐿/𝐷 of 0.25 and 1.0 can achieve plane rotations of up to 11.6 and 45.3 deg, respectively. Heat rate, heat load, and g-loading constraints identified when rotating the orbital plane are not more severe than those observed for two-dimensional aerocapture at a given 𝐿/𝐷 and 𝑣∞. A direct tradeoff between the maximum plane rotation and entry corridor width exists that will affect the ability of lower 𝐿/𝐷 vehicles to achieve large plane rotations. The proposed maneuver can allow the captured orbit inclination and right ascension of the ascending node to be altered in ways that are not possible using typical interplanetary orbit targeting methods. Further, the maneuver offers the possibility of deploying multiple satellites to different orbits around a target destination using a single launch or approach path.

[StraumliBlight]

Design Considerations for Aerocapture Delivery of Uranus Orbiter and Probe: presentation and paper [Jan 8]

Expendable Nanosats Concept for Uranus Exploration [Mar 2]

The Uranus Orbiter and Probe mission has been prioritized in the decadal survey as the flagship mission in the next decade. One key measurement is to measure the magnetic field and plasma in the magnetosphere which would benefit from having simultaneous measurements from multiple locations. We propose the concept of using expendable Nanosats equipped with passive optical wireless communication with modulating retroreflectors and ultra-low power stepped-quantum-well modulators. These Nanosats are dispatched in divergent directions from the Orbiter and data is remotely read out by the laser from the Orbiter up to 10,000km.

This paper will present our analysis of the communication link budget in this mission context, battery and thermal requirements to enable significant Nanosat’s lifetime, and the size, weight, and power for the instrument payloads. We also presented a table-top experimental platform and preliminary optical link test.

[AndrewM]

A paper from APL/GSFC presented at the 56th LPSC looks at the transit time and launch windows for both a SHLV or HLV with SEP since the funding profile will likely result in not launching in 2031/32 as identified in the decadal survey.

The mission plan described in OWL called for a launch in 2031 or 2032, followed by a Jupiter gravity assist on the way to Uranus. However, due to funding realities at NASA, it is unlikely that UOP will begin development in time to take advantage of Jupiter phasing. Therefore, alternate means of getting to Uranus must be considered. In this poster, a robust means of getting to Uranus during any launch year will be described. We will focus on approaches that require no new technology and can be executed with low risk and acceptable cost.

While direct trajectories to Uranus with currently available launch vehicles do not deliver enough mass to make a credible science mission, emerging super-heavy launch vehicles, such as SpaceX’s Starship/Superheavy launch system, can
deliver a useful payload to Uranus in as little as 10years. However, this would not be a single launch. Rather, it would be a campaign of several launches to fill a propellant depot followed by the launch of the spacecraft.

There is a way to consistently deliver sufficient mass to Uranus in any year. A commercial-derived solar electric propulsion (SEP) stage, coupled with off the shelf Hall thrusters, two Earth gravity assists, and a currently available heavy lift launch vehicle can consistently deliver >4000 kg to Uranus orbit in <14 years of flight time. Unlike the super-heavy lift option, the SEP option may be executed with currently available components. Unlike the inner tour option, the mission design is qualitatively the same in any launch year, and therefore robust to programmatic delay in the current funding environment.

Here is a presentation from JPL with their approach on how to handle the expected launch delay as a result of reduced funding. They utilize an expended FH + kick stage to get a 13.4 year transit time with an annual launch window. The spacecraft would need a 42% mass reduction and drop from 3 to 2 RTGs.

Both APL/GSFC and JPL agree that launch would likely not be before the mid-2030s.

[Blackstar]

Maybe Ariel had an ocean?

https://www.sciencedirect.com/science/article/abs/pii/S0019103525003707?dgcid=coauthor

Constraining ocean and ice shell thickness on Ariel from surface geologic structures and stress mapping
Author links open overlay panelCaleb Strom a, Tom A. Nordheim b, D. Alex Patthoff c, Sherry K. Fieber-Beyer a

https://doi.org/10.1016/j.icarus.2025.116822

    Tides from an orbital eccentricity ≥ 0.04 generates enough stress (≥1 MPa) to brittlely fracture surface ice on Ariel.
    The geographic pattern of surface eccentricity tidal stress aligns with the pattern of fractures on Ariels surface.
    Higher eccentricity tidal stress is a plausible explanation for the geologically recent resurfacing evident on Ariel.
    Ariel may have had a ∼ 170 km thick subsurface ocean due to past tidal heating.

Abstract
The Voyager 2 flyby of Ariel revealed this moon of Uranus to be a geologically complex world. Analysis of Voyager image data has revealed Ariel to host evidence of potentially cryovolcanic features and a system of fractures and basins which could be connected to geologic resurfacing in the past 1–2 Ga. A plausible stress source for this resurfacing is tidal stress from a higher orbital eccentricity in the geologically recent past due to chaotic orbital evolution during a past episode of mean-motion resonance with the other Uranian satellites. In this study, we compare stress modeling and geologic structures on Ariel's surface to constrain the interior structure and past orbital eccentricity which would be necessary to generate sufficient stress through eccentricity tides to result in geologic resurfacing at Ariel's surface. Our results suggest that an eccentricity of e ≥ 0.04 could generate stress sufficient to cause geological resurfacing at Ariel's surface, create a stress distribution consistent with the stress distribution inferred from Ariel's surface geology, and result in a ∼ 170 km thick subsurface ocean that existed within geologically recent (1–2 Ga) time.

[StraumliBlight]

Universe Today: Starship Could Cut The Travel Time To Uranus In Half [Oct 18]

The other capability is to use Starship itself as an aerobraking shield. In the paper, the researchers examined the idea of using Starship, which itself is already designed to deal with the heat of reentry on both Earth and Mars, as a shield against the heat caused by aerobraking in Uranus’ atmosphere. They found that, with a little modification, the basic principle could work. Instead of separating from the probe once its boost was provided, in this case the Starship would accompany UOP to the Uranus system, using its thermal protective system as an air brake to slow the probe down from its interplanetary speed and remain in the system.

From calculations in the paper, the combination of being refueled in space and also using Starship as an aerobrake could cut the time to the Uranus system in half, to six and a half years. It also wouldn’t require any gravitational assists from any other planets on the way. Even with the added cost of taking a Starship along for the ride, this would dramatically decrease the operational cost of the mission by literally halving its travel time.

[Don2]

Universe Today: Starship Could Cut The Travel Time To Uranus In Half [Oct 18]

The other capability is to use Starship itself as an aerobraking shield. In the paper, the researchers examined the idea of using Starship, which itself is already designed to deal with the heat of reentry on both Earth and Mars, as a shield against the heat caused by aerobraking in Uranus’ atmosphere. They found that, with a little modification, the basic principle could work. Instead of separating from the probe once its boost was provided, in this case the Starship would accompany UOP to the Uranus system, using its thermal protective system as an air brake to slow the probe down from its interplanetary speed and remain in the system.

From calculations in the paper, the combination of being refueled in space and also using Starship as an aerobrake could cut the time to the Uranus system in half, to six and a half years. It also wouldn’t require any gravitational assists from any other planets on the way. Even with the added cost of taking a Starship along for the ride, this would dramatically decrease the operational cost of the mission by literally halving its travel time.

I don't think this would work because there is no way to power it at Uranus. I doubt an RTG would provide enough power to run a Starship.
Also, Starship seems to have very leaky tanks and if the fuel leaked out during a long cruise the mission would be doomed. And Starship is not designed to survive the low temperatures it would encounter in the outer solar system.