I would drop the 'Adventure' unless it is vital...aren't all sci-fi's in space an adventure anyway of some sort? Add more drop shadow. I think the font will stand out more. Make it pop. I personally like the last cover you posted, the one with the gold lettering. Just lacking a bit of shadow.

Look at your book blurb or synopsis or whatever you have that describes your entire book in a couple of sentences or paragraphs and design a cover that mirrors that. Maybe the cover should reflect the main theme of the book.

Good luck with your book.