A PhD and a Bachelors of Science (BS) aren't exactly a double major--I would consider two BS degrees in parallel a double major. You would obtain a PhD and BS sequentially, with a BS first, and then possibly a Masters (MS) or PhD. The PhD is highly specialized, although many of the best researchers do interdisciplinary work, i.e., work combining multiple fields. That allows them to bring new perspectives to other fields, and can lead to major breakthroughs.
It's also not unheard of to obtain a PhD in a different field than your undergraduate degree, especially if they are similar or related, which may be of interest to you. While you wouldn't necessarily switch from one field to another, you could use your knowledge in one field to make unique contributions. For example, if you are doing a PhD in astrophysics, you may focus your research on problems that could be solved with robotics.
You may want to look up some researchers in astrophysics at different universities, and see what kind of research they're doing, and what kind of background their grad students have. That will give you some idea of what sounds interesting to you, although your interests may change over time, and if you are not yet in college they may change quite a bit. However, if you are trying to get a PhD in a different field, it would be good to take some undergraduate courses in astrophysics so that you have some foundation in the field.