It is also worth noting that NASA doesn't only do space exploration. It also does important work on civil aviation technology, remote sensing, climate studies, etc.
We always frame our arguments to support our claim; here you represent $17 billion as small compared to a $3.7 trillion bucket. If you were arguing the other way, you might have said we spend 3 times as much on NASA as we do homelessness [1].
In fact I just copied/pasted the bit about the NASA's percentage of the overall US budget from the referenced Wikipedia page. It wasn't an attempt at editorialising.
Homelessness is certainly an important problem, but is it more important than (for example) tackling antibiotic resistance ($450 million budget in FY2014 [1])? For any deserving issue/problem it is possible to play this game all day, as illustrated by the Fallacy of relative privation [2].
NASA's budget for 2014 was $17.647 billion, which is approx 0.5% of US federal government spending [1].
Total ESA budget for 2015 was €4.28 billion (US$5.51 billion) [2].
Everyone else (including Russia) spends not very much at all.
[1] https://en.wikipedia.org/wiki/Budget_of_NASA
[2] http://www.esa.int/For_Media/Highlights/ESA_budget_2015