Just exactly where is the final frontier?

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 last Thursday.

The atmosphere doesn't abruptly end, but thins out gradually.

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A version of this article appeared in the print edition of The Straits Times on December 17, 2018, with the headline 'Just exactly where is the final frontier?'. Print Edition | Subscribe