Only for some observers. For observers moving faster, has to be rice.
Since this thread has been revived, can I ask this again?
This article provides a decent summary but in essence although ‘Grand Tour’ trajectories for visiting all four outer major planets was determined, NASA management was concerned (rightly so) that the cost of developing a spacecraft assured to function throughout the 12 years of that mission and allocating the mission funding for that time (potentially across as many as four presidential administrations and seven congressional seasons) would be considered prohibitive, so they instead proposed to twin spacecraft which would perform Jupiter and Saturn flybys in the five year timeframe. However, JPL mission management elected to put Voyager 2 on the trajectory that could still potentially fulfill the Grand Tour (provided that it survived its swing by maneuvers with the two gas giants) and buried plans for a ‘follow on’ mission within the appendices for the mission plan. When both Voyagers were astonishingly successful at their flybys (getting detailed pictures of the unexpected vulcanism on Io, detailed views of Jupiter’s turbulent and colorful atmosphere, the impact basin on Callisto, the striations on Europa indicating deep oceans and ongoing surface tectonic activity, the magnetosphere of Ganymede, then closeup pictures of the composition and distribution of Saturn’s rings, the dense atmosphere of Titan, and the oceans of Enceledus), funding a ‘coincidental’ mission extension to Uranus and Neptune for Voyager 2 became a no-brainer with even those brief flybys giving deep insights into the structure of those planets and their associated moon systems.
There have been subsequent flyby and exploration missions to explore the Jupiter (Ulysses, Galileo, Cassini-Huygens, New Horizons, Juno, and the in-flight JUICE and Europa Clipper) and Saturn (Cassini-Huygens) but no new missions to Uranus or Neptune have or are likely to be planned in the foreseeable future despite the fact that these are examples of common exoplanet types (complex, ‘icy’ gas giants which are very different in composition from Jupiter and Saturn). The difficulty of reaching these planets with an orbiter-type mission without doing multiple flybys makes them expensive propositions that would take decades with conventional propulsion, and aside from planetologist there is little appeal for the costs of such missions. Another limitation is bandwidth; the NASA terrestrial Deep Space Network is already obsolescent and overtasked for interplanetary communications and a high bandwidth mission to Uranus and Neptune begs for a communications relay system in solar orbit. (I worked on a unsolicited study for such a system which would cost somewhere the range of USD 5-8 billion in today’s dollars, which is a pittance for something desperately needed to expand interplanetary missions and especially any kind of crewed mission to Mars but such proposals never even rise to the top in Decadal Surveys because there is no appetite for building this kind of infrastructure to support a capability for missions that are unlikely to come even before the current kerfuffle.)
[Voyager Tales)(https://arc.aiaa.org/doi/abs/10.2514/4.868931) has a lot of detailed insights by people who actually worked on the Voyager program at JPL and elsewhere (and is where I discovered that Gary Flandro, who I knew of through his work on combustion instability in solid propellants, first worked out the Grand Tour trajectory as an intern at JPL) although some of it is quite technical and not a light, casual read. There is also NASA’s Voyager Missions: Exploring the Outer Solar System and Beyond, which I have not read but seems to be well regarded. And Haynes (the automotive repair manual publisher) offers the NASA Voyager 1 & 2 Owners’ Workshop Manual - 1977 onwards (VGR77-1 to VGR77-3, including Pioneer 10 & 11) if you want to learn how to maintain your Voyager spacecraft.
Stranger
Thanks
I have a copy of this and it is fascinating; highly recommended.
Apart from the joy of reading it, it could be called the most useless book in the world.
Speak for yourself. I picked up a secondhand Mariner spare from a Smithsonian yard sale and worked it up into the Voyager configuration. If I can get an old PAM and a cheap rideshare I’m going to send it out to explore interstellar space and report back when it develops a machine society of its own.
Stranger
I teach spacecraft design / mission design at a university…and I’ve heard of this book but not purchased it. Does it go into enough detail to teach readers a bit about spacecraft and mission design? (If so I should get a copy!)
Doesn’t the solid propellant have only a limited warranty (it doesn’t age well…)? I’d be careful! 
Minuteman I motors (M55, M57) that are approaching or over 60 years old and many others like Peacekeeper motors that are approaching 40 years have and continue to be flown; as long as the propellant grain has adequate mechanical properties, hasn’t been subjected to high humidity, and any stabilizers or plasticizers haven’t degraded, all of the ‘soft goods’ like O-ring seals are replaced, and all of the components like the igniter are reliable, they will continue to operate for decades. The PAM is a pretty simple spin-stab motor, so no TVC or guidance system to worry about.
The bigger problem is sourcing some new RTGs for the space probe; just the 238Pu alone runs about half a billion dollars for each unit, and I need enough for three of them if I want the mission to last past Saturn.
Stranger
Yeah…I tell students to pretend that they can count on RTG’s being available even if this may pose an issue IRL.
Is it because there is no longer a manufacturer ? Who does make them?
Plutonium-238 (238Pu) is a synthetic element and not a normal byproduct of power generating light water nuclear fission reactors. The facilities at Hanford and Savannah River that historically produced and separated 238Pu were shut down in the ‘Eighties. We’ve been buying 238Pu from Russia until recently with the High Flux facility came on line, and I believe Ontario Power is producing 238Pu specifically for NASA, but it isn’t as if you can just go on the open market and buy it. Maybe North Korea and Pakistan has some surplus or would be willing do devote some production capacity to making it but I don’t like dealing with despots. I know Japan was looking to offload their excess Pu stockpile but I don’t think they’ve separated the isotopes and I don’t want to get into the centrifuge business because the last guys to do that just got bombed.
Stranger
Interesting, thanks.
Way too much work. My suggestion is to skip the centrifuge part and proceed directly to the getting bombed part.
Or start collecting a metric crapton of smoke detectors and build an RTG with ²⁴¹Am.
But then I’d need about fifteen of them to get the same power output. Plus, I’d end up looking like this guy:
Stranger
Wow, he didn’t actually die of radiation poisoning (though may have if he had lived):
Wiki:
“As an adult, Hahn served in the United States Navy and United States Marine Corps. He was subsequently treated for mental illness, and his death at age 39 was related to drug and alcohol use.”