The 2010 TIME 100

Elon Musk makes no sense — and that's the reason I know him. When I was trying to bring the character of genius billionaire Tony Stark to the big screen in Iron Man, I had no idea how to make him seem real. Robert Downey Jr. said, "We need to sit down with Elon Musk." He was right.

Musk, 38, is a rocket scientist. He designed the Falcon 9 booster that may serve as NASA's next vehicle to transport cargo and humans into space.

He is also a green pioneer. He helped create Solar City, the largest provider of solar-power systems in the U.S. And he designed the Tesla, one of the first electric cars of the modern era. Bob Lutz, the vice chairman of General Motors, credits him with catalyzing GM to move toward electric cars.

Even Elon's humble beginnings are not so humble. The little Internet start-up that put him on the map is called PayPal. You may have heard of it.

Downey was right. Elon is a paragon of enthusiasm, good humor and curiosity — a Renaissance man in an era that needs them.

Favreau is the director of the Iron Man movies

TIME 100 Social-Networking Index: 780