SpaceShipTwo (SS2) in the hangarWith all the hubbub surrounding the Monday unveiling of Richard Branson's new SpaceShipTwo, billed by Virgin Galactic as "the world's first manned commercial spaceship," some of us began to wonder: What makes a spaceship a spaceship? After all, the flights that Virgin Galactic is hawking for $200,000 do not put passengers into orbit. And as for one of the purported thrills of space tourism, weightlessness, that experience is attainable at or below altitudes reached in a typical commercial airline flight—in NASA's "vomit comets," for instance.

So when is a traveler a space tourist and not just an airline passenger with a slightly better view and a much lighter wallet? Slate tackled the question back in 2004, when SpaceShipOne cracked the "official" space barrier in the course of winning the $10-million Ansari X PRIZE. That barrier, sometimes known as the Kármán line after Hungarian–American engineer Theodore von Kármán, lies at 100 kilometers above Earth.

According to the World Air Sports Federation, a governing body that keeps world records for flight, the Kármán line is the boundary separating astronautical flight from aeronautical flight. Although the federation's secretary general told Slate that the choice of 100 kilometers was "fairly arbitrary," it is rooted in science. Von Kármán and his colleagues calculated that somewhere around that altitude, the atmosphere thins out so much that aeronautical vehicles would need to sustain essentially orbital velocities to generate lift. Rather than trying to ascribe a precise location above Earth where this transition took place, the federation adopted the round-number approximation.

So SpaceShipTwo, which is expected to rocket to about 110 kilometers, should indeed breach a barrier in both the terminology and the physics of flight. But that does not mean that suborbital flights such as those sold by Virgin Galactic have always basked in the same heroic glow as orbital shots. When the U.S. sent its first astronaut into space (that is, past the Kármán line), lofting Alan Shepard 187 kilometers into the sky in 1961, Soviet premier Nikita Khrushchev reportedly scoffed that Shepard's 15-minute flight was a mere "flea hop." After all, the U.S.S.R.'s Yuri Gagarin had already orbited the planet.

Tom Wolfe, author of The Right Stuff, echoed that sentiment in a July op-ed for the New York Times, published as the 40th anniversary of Apollo 11 drew near. Recounting the U.S.'s initial lag behind the Soviets in human spaceflight capability, Wolfe denigrated (seemingly only half-seriously) Shepard's flight and that of his successor Gus Grissom as "two miserable little mortar lobs."

So perhaps Virgin Galactic passengers won't find their names enshrined in the Astronaut Hall of Fame. But at least, we imagine, they'll get to skip the indignity of the TSA checkpoint.

SpaceShipTwo (center) attached to its mother ship Eve: Virgin Galactic/Ned RocknRoll