> You could, for example, use some of the money from a "billionaire tax" to fund NASA.

Alternatively, you could allow billionaires to donate some of their billions, untaxed, into non-profits which they control and which have humanity-benefiting goals like space colonisation or clean energy or medical programs.

There could be problems, however, with these non-profits hoarding vast amounts of resources long after the billionaire dies, at which point a wealth tax on the non-profit's unused assets might make sense.