So when The Future finally arrives and we get to send out starships to explore the worlds beyond our own, what logo will we put on our ships? A national flag would only work if we didn't have a united Earth space project, which sci-fi usually has (and oddly enough, it tends to be headed by people of the same nationality as the author). The UN's logo is ugly and has no place in space after its multitude of failures here on earth.
I humbly suggest that we use a design featuring a trident. Since the trident is traditionally a naval weapon, it will emphasize Earth as the planet with the big oceans. And a trident has three points, emphasizing that ours is the third planet from our sun.
I've borrowed this sample logo from a British engineering firm. It could use some work, but I think it's a good start.