rocket scientist

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A scientist who specializes in rocketry.
• noun Informal An extremely intelligent person.

from the GNU version of the Collaborative International Dictionary of English.

• noun a very intelligent person; -- used mostly in the phrases: “you don't have to be a rocket scientist to …” and “it doesn't take a rocket scientist to …”, meaning that the subject that follows is easy to understand.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun countable a colloquial term for an astronautical engineer
• noun countable, idiomatic Someone qualified to understand or handle that which is overly complex, detailed or confusing; a genius.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun an engineer who builds and tests rockets
• noun a clever thinker