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.

ERRATUM (2024): In the paper I cite Jastrow's 1957 proposal as being 100 km. In fact, he proposed 100 MILES (160.9 km). This change does not affect any of the arguments in the paper, but it would be interesting to know when 100 km was first proposed.