Stop worrying :) I am a mechanical Engineering by degree and I work in aerospace! And lots of my coworkers are not aerospace engineers. They are civil, electrical, and mechanical. At my company, we build commercial airplanes. So you need to have all types of engineers involved. Even if you were making spaceships, there are mechanical systems (heating, cooling, hydraulics, engines) and electrical systems (networks, lights), and structure (civil) that needs to be designed and built. If you specialize in aerospace you'll have more information on aerodynamics, but it is nothing you cannot get with your undergrad in mechanical. If you are interested in aerospace, within your mechanical studies, you can take vibrations, helicopters, composites, or any other aero-type of class. There's about 90% overlap from ME and AE.