Musk used some of the proceeds from the sales of Zip2 and PayPal for SpaceX, the commercial space exploration firm he founded in 2002 that remains perhaps his most ambitious venture yet.
The company has flown cargo missions to the International Space Station for NASA and delivered satellites into space for commercial customers. It has booked more than 50 additional cargo and satellite flights for paying customers through 2017, and hopes to take astronauts to a colony on Mars one day.
SpaceX remains privately held, though it's been the subject of recent IPO chatter.
Elon Musk unveiled designs for the Hyperloop Monday, a superfast tubular transport system promising travel from L.A. to San Francisco in 30 minutes.