Where, exactly, is the edge of space? It depends on who you ask.

With more countries and commercial companies heading into the stratosphere, the debate about how to define outer space is heating up.

Ask someone where outer space is, and they’ll probably point at the sky. It’s up, right? Simple.

Except, no one really knows where “air space” ends and “outer space” begins. That might sound trivial, but defining that boundary could matter for a variety of reasons—including, but not limited to, which high-flying humans get to be designated as astronauts.

Now, with Virgin Galactic seemingly on the cusp of launching paying passengers onto suborbital trajectories, many people are wondering whether those lucky space tourists will earn their astronaut wings. As of right now, they will, according to U.S. practices.

Is that a problem? “No, I think it’s great!” says NASA astronaut Mike Massimino, who helped repair the Hubble Space Telescope.

Here, we take

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