Actually, the grey line (or greenish in my case) is the top of the atmosphere according to Martin. once your position in your orbit intersects this line, you begin atmospheric interface, and depending on speed and angle will either re-enter the atmosphere or skip off of it.
-Christopher Trott
Parker, Colorado, USA
CEO, Green Mountain Airlines (http://www.flygma.com)
SimNASA Aerospace Research Pilot (http://www.simnasa.org)
Director of R&D, Virtualairlines.com (http://www.virtualairlines.com)