FORTUNE — Leave it to a tech billionaire to once again raise the bar on space exploration. With news that Paul Allen plans to build the largest plane ever—the wings will be 385 feet long—geeks all over the world are surely rejoicing. The plane, which will be made by Allen’s new company, Stratolaunch Systems, isn’t for passenger travel, at least initially. It’s meant to launch rockets into space from 30,000 feet, thereby eliminating costly launching pads.
With the announcement, Allen rejoins the “geeks-in-space” fraternity that he walked away from a decade ago after he and Burt Rutan developed SpaceShipOne, the first privately manned rocket to reach space. There are three other prominent members of the frat. Richard Branson (estimated net worth: $4.2 billion) has Virgin Galactic, which is aiming for suborbital space travel. Elon Musk ($680 million), whose SpaceX makes rockets and will make the rockets that hitch a ride on Allen’s plane. (Burt Rutan’s company will make the actual plane.) And Amazon’s (AMZN) Jeff Bezos ($19.1 billion) with Blue Origin, also focused on human space flight.
When asked how much he planned to invest in Stratolaunch—Allen is the sole investor—he demurred. “Obviously, this is not inexpensive,” he told The New York Times. Allen tried to claim there was a profit motive at play: “This is obviously a big initial investment that we wouldn’t be making if we didn’t think there were going to be a lot of customers out there, both for cargo and manned missions in the future.”
Cargo? Come on, Paul. You can admit it—this is not about making money. It’s about doing something cool with those big bushels of cash you have sitting around your house. And for that, we salute you.
As far as the rich go, Allen doesn’t get much respect. Almost ten years ago, Laura Rich, a former colleague of mine, wrote
The Accidental Zillionaire: Demystifying Paul Allen
. The title said it all: the received wisdom was that Bill Gates and Steve Ballmer were the real brains behind Microsoft (MSFT), and Allen, whose fortune peaked at $40 billion in 1999, was, well, accidentally rich. He’s been ridiculed for a number of ill-conceived investments since then, as well as for turning into a scourge of the figurative town that made him—Silicon Valley—in the guise of a patent troll.
In the twelve years since it peaked, Allen’s cash pile has dwindled steadily downward, to a recently estimated $13.2 billion. That said, Bill Gates, who peaked at $85 billion, is down substantially as well. But not by nearly as much: whereas Allen’s fortune is today worth just 33% of its high, Gates has managed to hold onto 69% of his, or $59 billion. Allen, who didn’t respond to a request for comment, was richer than Warren Buffett in 1999—the Oracle of Omaha was just worth $31 billion—but today Buffett is worth three times as much.
But I come here not to criticize Allen, but to praise him. After all, how much money does a billionaire really need?
Some billionaires seem driven to make more and more and more. In other words, they’re playing to win. Other billionaires seem driven by, well, I have no idea. When they write about how Warren Buffett still lives in the same five-bedroom Omaha house he bought in 1957, I am reduced to a state of total perplexity. Doesn’t he hang with Bill Gates? Hasn’t Gates shown him the cool kind of house $150 million can buy?
Back to the tech nerds, though. The four I mentioned above all seem like wide-eyed optimists to me. And what’s not to love about that? Branson obviously has the most style. Musk is probably closest to actually making money off his dreaming, with Tesla (TSLA), his maker of electric cars. Bezos, well, he’s going to be ruler of the entire planet pretty soon, so we might as well let him do what he wants. And Allen? Anyone who builds a rock-and-roll museum on their own dime—and who owns the world’s largest collection of Jimi Hendrix memorabilia—is all right in my book.
Of course, we also have Musk’s co-founder of PayPal, Peter Thiel. Thiel is worth an estimated $1.5 billion, and in addition to scoring big as an early investor in Facebook, he’s thrown money at nanotechnology, space exploration, and robotics. He’s also involved in the Methuselah Foundation, which has a goal of reversing human aging. (If I were a billionaire, I’m sure I’d want to live forever too—more so than I already do.) The problem with Thiel, though, is he’s a self-styled philosopher, and in an exhausting profile in the New Yorker last month, he made it clear that his investments come with a high price tag: we have to listen to him go on-and-on about whatever “important” topic is on his mind in exchange for his opening his wallet.
I’ll take Richard Branson—who epitomizes carpe diem more than any of his peers—over Peter Thiel. But I’ll take Paul Allen, too. I have absolutely no idea what Paul Allen’s opinions are about anything—and I really don’t care either way. And while he’s down $26 billion—meaning he’s lost more money than most billionaires have ever made, save Bill Gates, Warren Buffett, and Larry Ellison—I don’t think that makes him the worst investor of all time. I call this glass half-full: he’s one of the best spenders that ever lived. And he squandered it all in an admirable state of childlike wonder.