What counts as a journey into space?
ON JUNE 7TH Jeff Bezos, the owner of Blue Origin, a rocket maker, announced that on July 20th he would be one of the first people to ride into space in the RSS First Step, the companys reusable space capsule. Mr Bezos, who is in the process of stepping down as CEO of Amazon, a retailer, will thus become the first of the billionaires currently building spaceflight businesses to enter the realm himself. But the New Shepard rocket which will send First Step on her way will not give it anything like the speed required to get into orbit. Indeed, the vessel will not leave the Earths atmosphere. The Economist Today Hand-picked stories, in your inbox A daily email with the best of our journalism Sign up  The answer is its altitude. The thinner the atmosphere, the faster a winged aircraft must move if it is to stay aloft. In the 1950s a pioneering aeronautical engineer, Theodore von Krmn, pointed out that there must be an altitude at which the air is so thin that the speed needed for aerodynamic flight of that sort was higher than the speed needed to go into orbitthat is, for the combination of its forward momentum and the Earths gravitational attraction to keep the vehicle going round and round the planet. Calculations suggested that this disjuncturenow known as the von Krmn linesat around 100km above the Earth. Various bodies, including the Fdration Internationale Aronautique, subsequently decreed that to pass that 100km line was to enter space. The New Shepard will be moving fast enough by the time its engines cut out about 40km above the Earth to coast all the way up to that boundary before falling back down. As well as taking them up over the von Krmn line this parabolic trajectory will also provide the passengers with some of the signature experiences of space flighta black sky, a clearly curved horizon, and a few minutes of the weightlessness that comes with free fall. Mr Bezos thinks that selling this experience to rich thrill- and status-seekers will be a nice business. But this 100km definition is not universally accepted. Americas air force treats 50 miles (roughly 80km) as the edge of space, a definition which allowed it to give astronaut wings to five pilots of its X-15 experimental rocket plane in the 1960s. Jonathan McDowell, an astrophysicist at Harvard who takes a keen interest in spaceflight, has calculated that, in practice, the von Krmn line is closer to 80km than 100km, and that this might be the more defensible definition. That would suit Virgin Galactic, a company founded by Richard Branson which has spent over a decade developing rocket-powered aeroplanes that will offer similar suborbital thrills. When the latest test flight of VSS Unity reached 89km on May 22nd Mr Branson was happy to pronounce it a spaceflight. Virgin has one more test flight scheduled before Mr Branson himself will be flown to (or close to) space later this year; after that, it will be open to paying customers. Mr Branson may or may not make it into space before Jared Isaacman, another billionaire, who is expected to take off in September. But Mr Isaacman, who made his money as boss of Shift4, a payments company, will definitely go farther. He has contracted with SpaceX, a company which ferries astronauts to and from the International Space Station (ISS), for a three-day orbital trip in one of its Dragon capsules. Accompanying him on this Inspiration 4 flight will be a health-care worker from St Jude Childrens Research Hospital in Memphis, another passenger selected by a raffle which raised funds for that hospital, and a third passenger selected through a corporate contest. SpaceX has also contracted with a company called Axiom to ferry paying non-government passengers to and from the ISS in conjunction with Americas National Aeronautics and Space Administration (NASA). It is a fair bet that Elon Musk, SpaceXs boss, will follow in their flight paths at some point. A devotee of the settlement of Mars, he has said that he intends to go there himself at some point. And though nothing has been announced, it would not be surprising to learn that he will take some preparatory steps sooner than that. He certainly has the most impressive hardware. Though Mr Bezoss flight on the First Step is scheduled for the 52nd anniversary of the first landing on the Moon, it is to SpaceX that NASA has turned to build the spacecraft which it will use to return to the lunar surface (Blue Origin is contesting the decision). The scope for yet more spectacular and inspiring space-based self-indulgence looks likely only to grow.