# How much does launch costs have to come down to make chemical fuel deep space probes feasible?

Considering that NASA have been running short of plutonium for their RTGs, it makes sense to consider alternatives. For the inner solar system up to Jupiter, solar cells have been demonstrated on missions e.g. Juno. However, for missions beyond such as Pluto, solar cells reportedly are inadequate.

I made some quick calculations with Wolfram Alpha reaching on the order of 50 metric tons for a 1kW power budget and a 20 year mission. These figures are not precise but hopefully on the right magnitude.

Question - Will dropping launch costs (from reuse or otherwise) ever make these types of power systems reasonable for deep space missions? Or will some other technology make efficient use of the extra weight possible to launch when costs drop?

• Plutonium isn't the only source of nuclear power - we can also build nuclear reactors. Jul 16 '20 at 7:28
• I did the sums for hydrogen (which is best in terms of energy/mass) and I got about 43 tonnes, most of it oxygen of course, so I think you're in the right ballpark for mass.
– user21103
Jul 16 '20 at 8:49
• Relevant to the comment by @ikrase: the TOPAZ-I reactor was designed to produce $5\,\mathrm{kW}$ for 3-5 years. It weighed about $320\,\mathrm{kg}$: you could lift more than 100 of these things for the effort of lifting the chemical fuel.
– user21103
Jul 16 '20 at 9:01
• @tdb do you have any info on the expense of those? That could be relevant. Jul 16 '20 at 14:59