If the atmosphere had the same density throughout its vertical extent as it does at sea level, where would outer space begin?
A) at an altitude just under 85,000 feet
B) about 20 miles up
C) at roughly 28,000 feet MSL
D) just above FL 450
Answer: Although the actual atmosphere (at least in name) extends many times higher than aircraft can fly (by at least one definition, at about 100 Kilometers or 62 miles), if the atmosphere were the same density throughout that the it has at sea level on a day of standard temperature and pressure (29.92 inches of mercury, or 1013.25 millibars and 15 degrees Celcius) as defined by the ICAO, the theoretical height of a homogeneous atmosphere (the so-called scale height) would only be about 8500 meters, or about 28,000 feet. After that, it would suddenly become the inky void of outer space. (Also, without the gradual slowing provided by the continuing increase in friction within a normal atmospheric density profile, re-entry from earth orbit would be nearly impossible.) The correct answer is ‘C’.