There is no sign saying "Welcome to Outer Space" when a ship such as Virgin Galactic's SpaceShipTwo arrives at its edge, as it reportedly did Thursday. The atmosphere doesn't abruptly end, but thins out gradually. The craft's maximum altitude of 82.7 km was lower than the 100-km limit set by the Ansari X prize back in 2004. (This was won by SpaceShipOne, then of Mojave Aerospace Ventures, which maxed out at 112.17 km.)
Is it OK to lower the bar on the edge of space? Who gets to decide where space begins? Virgin Galactic and its competitors in the space tourism trade would like to make it low enough to say their customers are getting there. (Though moving the line won't affect Elon Musk if he pulls off his promised trek around the moon.)
The boundary shouldn't be an arbitrary one. Astrophysicist and space historian Jonathan McDowell argues that the edge of space should be defined by physics. In keeping track of space history, it matters who gets official credit for getting there first. In the mid-20th century, scientists tried to set that limit at how low you can go and still sustain an orbit — an altitude known as the Karman line, named after aerospace engineer Theodore von Karman. At some point atmospheric drag becomes too big a factor to sustain even a highly elliptical orbit — one that swings in close and then out much farther.