issue of importance
Many colleges are asking applicants about an issue, or social problem they concerns most and what the student hope to do to make it better or solve it. This is the opportunity for you to show your critical thinking skills and sense of social responsibility. It's always better to relate it to your career goal. Colleges want motivated students who has clear goal and a will to make the world a better place, instead of just focusing on themselves.
For the first two ideas, they are too general, it must be a problem that you concerns.
"populous to pursue Space Exploration". To be honest , I have no idea what you mean by this.
If you want to talk about the failing US educational system. " focuses on grade school not university or college" is it a problem within your community? why it's a problem? why you care about it ? and then talk about its impact to your community. Do you want to solve that problem?
"other topic ideas?" It must be your own idea, the problem you concern about. So nobody can help you about this.