In 2004, space exploration by private companies was legalized with the passage of HR5382. Since that time, companies such as Virgin Galactic and SpaceX have invested in vehicles and launch capability with hopes of sending private citizens to space -- for profit -- as well as the ability to transport materials and supplies to the space station and possibly mine asteroids for materials in short supply on earth.

SpaceX experienced a catastrophic launch failure in 2016, with a successful launch and landing in early 2017 -- although docking with the international space station took two tries. The SpaceX Dragon cargo craft returned to earth in a parachute-assisted landing in the Pacific Ocean on March 19, 2017, bringing back more than 3,600 pounds of material from the International Space Station, including materials from medical experiments designed to help people with bone ailments and injuries on earth.

Virgin Galactic aims to open space travel to everyone - not just professional astronauts. For a downpayment of $250,000, Virgin Galactic is accepting applications for space travel, though the flight dates have not yet been announced. In March 2017, cosmologist and physicist Stephen Hawking accepted owner Richard Branson's offer of a trip to space.

Several of the individuals who started private space exploration companies made their fortunes elsewhere: Richard Branson of Virgin Galactic is also the principal behind Virgin Airlines and Virgin Records; Blue Origin was founded by Jeff Bezos of fame. Bigelow Aerospace was founded by Robert Bigelow, who first made his fortune through the Budget Suites of America hotel chain.

The first decades of space exploration by governments yielded a variety of inventions and discoveries to benefit life on earth, but there is clearly more to discover. That potential may be the driving force behind wealthy entrepreneurs delving into space as the next frontier.