What do you do if you're young and you've already conquered the world? You look for other worlds to conquer.
Elon Musk, 40, whose SpaceX venture won the right to try the first flight of a private cargo ship to the International Space Station, is just one of several multimillionaires who made their fortunes on Earth and are now looking to the skies.
Musk and other space enthusiasts first made their money in online ventures, travel, media and other fields, and have now launched private space efforts.
There is a cynical saying sometimes used that you really can make a small fortune in space -- all you have to do is spend a large one. But many space entrepreneurs argue that since they are not burdened by government bureaucracy, they can do cheaply what NASA has done expensively.
They say space could be a bit like the old West: Governments sent explorers, such as Columbus or Lewis and Clark, to open the frontier, and then private settlers followed.
Here is a list of some would-be space settlers and their projects.
|Elon Musk, SpaceX|
Elon Musk made his fortune online. He co-founded PayPal, the popular system for sending money by email, in 1999, and sold it to eBay in 2002. The purchase price: $1.5 billion in stock. Musk owned about 12 percent of PayPal at the time.
It was not even his first company. In 1995 he dropped out of Stanford's graduate school to start Zip2, a firm that made software for publishing content on the Internet.
He began SpaceX in 2002, saying he believed humanity's future lay in space. The philosophy of his Falcon rockets and Dragon capsules, he told ABC News, was to keep it simple: "We're building trucks, not sports cars."
Several early launches failed. But in 2010, a SpaceX Falcon 1 rocket became the first liquid-fueled booster to put a satellite in Earth orbit as a private venture.
His principal client is now NASA. SpaceX hopes to carry cargo -- and eventually astronauts -- to the International Space Station, something NASA has been unable to do since the space shuttles were retired in 2011.
|Sir Richard Branson, Virgin Galactic|
Richard Branson started setting records in the record business. He began what became Virgin Megastores in 1972, selling music in England at discounts not seen before.
As his fortunes grew, he expanded into Virgin Atlantic Airways, Virgin Mobile cellphone services, medical care and other ventures.
He was not just a risk-taker in business. He was a serial adventurer, trying to set records in sailing, ballooning, auto racing and flying.
Then came Virgin Galactic. Branson says there is a market for other wealthy people, willing to spend, say, $200,000 for a short ride to the edge of space. He has teamed with the designer Burt Rutan to build SpaceShipTwo, which would carry paying customers more than 60 miles above the ground. Virgin Galactic claims that more than 500 prospective customers have signed up.
|Paul Allen, Stratolaunch Systems|
In 1975 Paul Allen moved to Albuquerque to start a software company, persuading a childhood friend to drop out of Harvard to come join him. The friend was Bill Gates. Allen thought of a name for the firm: Micro-Soft. The rest you know.
Allen, now 59, was the less public of the Microsoft pioneers. He stopped playing an active role at the company after fighting off Hodgkin's lymphoma in 1982. Since then, he's been an investor and philanthropist, putting his money into everything from sports teams (football's Seattle Seahawks and basketball's Portland Trail Blazers) to life in space (a vast array of radio antennae, designed to listen for signs of extraterrestrial intelligence, was built in northern California with his money).
In 2004, with Rutan, he won the Ansari X Prize -- $10 million to the first private company to send a ship more than 100 km (about 62 miles) into space. He did not exactly need the cash. In December 2011, he announced Stratolaunch Systems, a company that would use a giant dual-bodied jet plane to carry a SpaceX rocket into the air. The rocket, launched from cruising altitude instead of the ground, would save thousands of gallons of fuel on its way to space.
|Jeff Bezos, Blue Origin|
Jezz Bezos said he close the name Amazon for website principally because it came early in the alphabet. It has become the world's largest online retailer.
Bezos has also revealed, quietly, that he is a space enthusiast. He started a company called Blue Origin, testing prototypes of reusable spacecraft in the Texas desert. Few other details have been made public.
This spring, Bezos also revealed online that an expedition he backed had found what he says is probably the first stage of the Saturn V rocket that launched Apollo 11 to the moon in 1969. It would be thousands of feet beneath the surface of the Atlantic, off the coast of Florida. Its five F-1 engines -- still the most powerful ever made -- may still be intact. Bezos said he would like to recover them and return them to the American people, to whom they belong.
|Robert Bigelow, Bigelow Aerospace|
Robert Bigelow made his fortune in extended-stay hotels. If you've ever stayed at a Budget Suites, that's his.
He's expanded the idea into space. His Bigelow Aerospace has already launched two Genesis inflatable space modules, which he says could someday be used as orbiting laboratories, space station components, factories -- even space hotels.
When he uses the word "inflatable," don't think of a balloon. The Genesis modules, he says, would be more like tires. They would be compact for launch, but strong and durable in orbit.