Look into Astrobiology and Astrobotany and what NASA is doing. See https://astrobiology.nasa.gov/ and https://phys.org/news/2016-02-nasa-simulate-potatoes-mars-peru.html. If any of this interests you, then you can see what types of majors the people have who are doing that type of research. My guess is that they studied one science, and did not double major. You can minor in a second science a lot easier than double-majoring, so that is always an option. You can also just pursue summer internships and research projects on topics that combine your interests in astronomy and biology.
If you are diligent in your science classes – completing homework, studying for tests, etc. – you should do fine. If you plan to get an MS or PhD, you will want to have as high a GPA as possible (3.0 minimum, but 3.5 or higher ideally). Science courses can be graded harshly, but if you do well in other subjects, then you should be able to keep your GPA up. Since you would be getting a BA instead of a BS, that means you will have more non-science classes and that might help. A BA will allow you to take a foreign language, and there often isn’t room for that in the schedule for a BS major. That could help you use your science degree in an international context later.
I think that something must have led you to start out in International Relations though. Before you change majors, I recommend taking some career aptitude and interest tests. You might actually find that what you really want to do is deal with scientific topics in an international relations context. Some government positions abroad specifically deal with science issues, for example.
Georgetown University’s Walsh School of Foreign Service has a major in Science, Technology and International Affairs, and says: “Now more than ever, science and technology are at the heart of international affairs. The Science, Technology and International Affairs (STIA) major equips students with the knowledge, skills and confidence to engage with the challenges and opportunities this presents. Students follow the regular SFS core curriculum, enroll in core science courses and develop an in depth understanding of one of our areas of concentration: Energy and Environment; Business, Growth and Development; Biotechnology and Health; or Science, Technology and Security.” If you realize that such a program interests you, then you can look at the courses required and try to select a major and minor at your university that would be comparable. More information is here: http://sfs.georgetown.edu/stia/