I think a space nation should actually have a flag that conveys universal intelligence, not some meaningless combination of earth symbols. I did not see many if any flags that did this.
Imagine, I'm a being from another world, perhaps color schemes mean war to them or the peace symbol means stay away. For example, the swastika from natzi Germany means (according to the wiki) "good fortune" but because of natzi's that meaning is no longer true.
So my idea, I'm physicist and I know there are universally recognized intelligence that could convey meaning to another advanced civilization. Not just unintelligible garble, but actual meaning. If we use something that other advanced civilizations would recognize as something important, universal, and even help bridge the gap between languages, then I would use that as my flag.

I propose using the hydrogen emission spectrum.

This is physics, meaning it would be the same to aliens as it would to us. It is a basis of starting a conversation with a species that may have no other way to start communication. If the alien species is advanced enough to leave their planet, than by all means, this series of colors would absolutely mean something to them.