Many defense and aerospace engineering job / projects require Security Clearance, but not all do. Which mean you must be US Citizen. There are different tier of clearance. You need a company sponsor to get that and it is lengthy and costly. Top Secret Clarence is good for 5 years, secret 10 years etc. If you got it, it is worth GOLD and usually receives higher salary. During the investigation, they literately call; visit your work site, home, churches etc and look at EVERYTHING. If it is your first job, they probably will call the U. So don't lie, as they can even request for sealed doc.
Security Clearances help in obtaining a job