Axiomatic Unicorn said:
A claim I have heard many times. Funny thing is, nobody is ever able to support it.
If the government is giving more to the high producers, why don't they just stop?
"Giving"? Who talked about giving? There's more to a government than just providing services (including police, welfare and military). Any government spends a very large share of its resources to control its economy so that it benefits its industry. Also, the most costly infrastructures (large motorways, for example) are usually built for economical reasons... if the motorway passing through my city vanished, to
me it would mean having to drive fifteen more minutes to reach my university, but it would be a major disaster for the whole Italian economy.
And, friend, when a country's economy prospers, everyone benefits,
but the high producers benefit proportionally more, since it's that economy that allows them to be high-gainers too.
A SF tax to fund NASA, OTOH, is unfair because everyone benefits from NASA research in the same way.
Ranger REG said:
You try to convince those who prefer social service that space exploration can benefit them. 
Honestly, if it weren't for the military and the intelligence community, space exploration would just be a memory of our victorious race to land on the moon before the Soviets.
I never said you should cut the military to fund NASA. Honestly, I
thought so, but I didn't say it. Ooops, I said it now.
Seriously, the military and intelligence is important, but often the funds given to military are so overwhelmingly more than those given to research, that moving a 1% from military to research would mean a major boost for science.
Since the original ideas was to raise taxes, however - for adding a tax on SF is simply raising taxes for a minority of population - I would suggest to cut nothing and raise taxes equally instead. A few dollars per year more for each person in every first-world country would give a hypothetical global space research organization some billions per year. With careful spending and some other sources of income, that could be enough.