Since 1994 (see my article on the X-15 in Quest I have been arguing that space should be considered to begin at 80 km, rather than the value of 100 km adopted by the majority of writers in the late 20th/early 21st centuries.

In 2018 I published a formal academic paper in which I lay out the arguments in detail: McDowell (2018), Acta Astronautica 151, 668.