If you are interested in doing other majors/minors in addition to your main degree, by all means, go ahead. But try not to take so many courses until you take the final straw that proverbially breaks the donkey's back. Employers sometimes ask for your mark breakdowns by year (they do in the UK...not sure about the US, but its probably the same. Why shouldn't they?). Its alright to take extra modules/minors/etc. especially if they are challenging, so you can brag about it to your prospective employer. Its another to take so many/the hard ones until your other subjects suffer.
Put yourself in the shoes of your prospective employer. If you wanted to advance your research as much as possible, chances are you'd want to invest in a PhD holder. Having a PhD means more than just having another degree. It shows that you have the mettle and determination to work through and advance uncharted territories (with the help of your supervisor, of course

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If I were you, I'd go for the PhD. Keep your eyes open and speak to professors in your department (and possibly others) which you think may be involved in such work. What aspect of your course do you like? Structures, materials, computing, signals, ...? If you're in your second year, you must have some idea by now. Which one is relevant to what you want to do? Once you find a common subject, make sure you're the best in your class at it. Shouldn't be hard if you're genuinely interested in the subject and put some elbow grease to it.
Suppose you like computing - its related to space exploration because as the exploration distances increase, the communication latency, limited by the speed of light, increases as well. At some point, for robotic probes, we will have to rely on autonomous onboard computers to think for the probe. You could write up a heuristic program to do that, but that's old fashioned any may not cover all the possibilities. Genetic algoritms are the way to go. They are self-evolving programs which adapt to the situation and improve.
Hope this helps.