That's when it collides with the skybox, like the sailboat at the end of The Truman Show.

David Brin thinks so:

dmd 6 hours ago | parent | prev [-]

Or like Apollo 8 in the incredibly funny book Unsong.

	▲	agg23 5 hours ago \| parent [-]
		Unsong is extremely amusing to me for some reason. Something about how Scott comes up with reasonably sounding similarities and manages to make those relate to an overall story.