The Swedish SMART-1 used a Hall effect thruster which used Xenon gas. Xenon costs 1200 USD per Kg. Why didn't they use Helium instead which costs 50 USD per Kg? New Horizons probe launch in 2006 used a Radioisotope Thermoelectric Generator which used Americium-241. Americium cost 1,5 million USD/Kg. But why didn't they use Uranium or Thorium instead? Thorium costs 5000 USD/Kg and Uranium costs 113 USD/Kg. In your opinion, what is the best propellant (gas) for a hall effect thruster which is powered by a nuclear reactor? Is it simply, just a matter of more power for the buck, or increase the volume for a less powerful gas?