In the past two decades, NASA has launched at least three missions that use RTG's:
- Mars Science Laboratory
- New Horizons
Those launches include plutonium, which is a reason for some to oppose the missions in their form, because the launch is involved with considerable risk. For each of them, NASA has published a quite extensive environmental impact statement. This includes an estimate of the number of "latent cancer deaths" in the worst case scenario:
- Cassini worst case, probability "less than 1 in 1 million", nicely formulated as "could be a 0.0005 percent increase above the normally observed 1 billion cancer fatalities". 0.0005% * 1 billion = 5000 deaths. (see Cassini Environmental Impact Statement, chapter 4, page 4-63)
- New Horizons: 1:1 million for 100 "latent cancer fatalities", 1:62000 for 5 fatalities and 10 km² contamination (at \$93–\$520 million/km² cleanup costs, see New Horizons Draft Environmental Impact Statement).
- Mars Science Laboratory, similar probability but with 60 deaths (see Mars Science Laboratory Enviromental Impact Statement).
What alternatives are available? NASA considered solar for Cassini in Chapter 2 of the EIS and for the Mars Science Laboratory in Chapter 2 at the EIS. The risk for Cassini was much higher than for MSL (because it had much more plutonium), but the cost of implementing the alternative was also much higher (It might be possible now, but I question if it was possible when Cassini was launched). There was some discussion, e.g. here. Both Juno and Juice, missions to Jupiter, use solar.
For a mission like New Horizons, but also Voyager 1 and 2, solar power really is not feasible. Is there any alternative here?
One esoteric, science-fiction solution that I can think of would be to produce plutonium in space. It's futuristic, but with a nuclear reactor on an asteroid, operated by robots, one could produce fuel that makes exploration of the outer solar system possible. This is, of course, not currently possible, but would get rid of the launch safety issue.
Another, closer-by alternative would not get rid of plutonium in Earth-based sources completely, but to limit it further. New-style ASRG's use only a quarter of the fuel than traditional RTG's do, and payloads get more efficient, so a future beyond Saturn might do with 10–20% of the plutonium. This was proposed for the now-cancelled Titan Mare Explorer.
Are there any alternatives for exploring the outer solar system that do not carry nuclear launch risks? For the sake of this question, I define outer solar system as Saturn or beyond, where solar has never been used.