# Best propellant for space shuttle?

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?

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?

Xenon has a higher atomic mass and is still quite easy to ionize, so it costs less energy to ionize a given mass of propellant. It also has a higher mass to charge ratio when ionized, which decreases exhaust velocity. This decreases propellant efficiency, but greatly reduces the power needed to achieve a given amount of thrust (remember, momentum = m*v, kinetic energy = 0.5*m*v^2).

And that $1200/kg is only a fraction of the cost of lifting the propellant into orbit. SMART-1 was launched on an Ariane 5, one of the more expensive launchers around, at about$10500/kg. The cost of the propellant was about 2.5% of the cost of just launching the spacecraft...ignoring the cost of all the hardware!

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.

Thorium and uranium don't decay fast enough to be useful in an RTG, and the smallest reactor would be much larger and more complex than the RTG used. New Horizons has a RTG using 11 kg of plutonium-238 oxide (not americium-241) producing 240 W of electrical power at launch, with the entire spacecraft massing 478 kg. The SNAP 10A reactor massed 290 kg and was designed to give about 500 W of electrical power for one year...and it had to keep running to keep its coolant from freezing. The only modern space reactor close to being used is the SAFE-400, which masses 1200 kg and produces 100 kW of electrical power.

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?

It depends on the mission and spacecraft. Hydrogen or helium are theoretically good if you need maximum propellant efficiency more than you need thrust or if you have no shortage of power, but storing them is a problem. Xenon, argon, and bismuth are good options for systems that need more thrust at lower power.