This, as I understand it, being the closest approach to our orbit, not our atmosphere or so. To put this in perspective, the Earth is some 6.4 Mm in radius, so that's 40-6.4 = 33.6 Mm above the surface.
MOID being minimum orbit intersection distance, for anyone else wondering. Sounded like someID (identifier) to me, was wondering why that value was given in AU
[1] https://www.projectpluto.com/neocp2/mpecs/ZTm0038.htm