Space, as a famous Starfleet captain once said, is the final frontier. But where is the actual frontier? There are no customs posts as you leave Earth, and 50 years after the Outer Space Treaty, there is still no internationally agreed definition of where space begins. With growing interest in tourist trips to the edge of space, it seems like a good time to try to nail this down.
In the 1950s the renowned aerodynamicist Theodore von K谩rm谩n threw his hat in the ring. He argued that by comparing forces from the atmosphere and from orbital dynamics, the boundary could be defined as the altitude where an aircraft relying on conventional lift would have to move so fast to stay aloft on the remnants of atmosphere that it would go into orbit instead.
The resulting 鈥淜谩rm谩n line鈥� marks this altitude and is usually said to be 100 kilometres up, a value sanctioned by the Federation Aeronautique Internationale, which governs air sports. However, that round number turns out to be a relatively recent figure, with most earlier estimates in the 80 to 95 kilometre range.
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Few have argued for much higher values for the air/space boundary than 100 km, but some 鈥� especially those involved in high altitude balloon projects 鈥� would like it to be as low as 30 or 40 km, so their flights can be said to reach the edge of space for example.
Pilot vs astronaut
Others suggest that setting a fixed boundary is undesirable because there is no abrupt end to Earth鈥檚 atmosphere. This is unrealistic: with commercial suborbital flights planned by Blue Origin (which recently announced tickets costing from $200,000 will go on sale next year) and Virgin Galactic expected to soar to the boundary region in the near future, how to define 聽a spaceflight is highly topical. Which is why I have attempted to do so in a .
I initially confronted the question of the K谩rm谩n line when I set out to study the history of the suborbital spaceflights of the . I was faced with the need to define what I would count as a spaceflight. Which of the X-15 pilots were astronauts?
I soon realised that the widely accepted 100 km altitude didn鈥檛 make much sense. There were cases of orbiting satellites with a closest approach to Earth less than this and they weren鈥檛 crashing to Earth as a result. Sixty years after Sputnik, a rich database of data on satellite orbits is available, and I was able to show that satellites with elliptical orbits are often observed to survive with perigees 鈥� their lowest heights 鈥� close to 80 km. This is arguably a similar altitude to the highest structural boundary in Earth鈥檚 atmosphere: the mesopause.
Fine line
Dipping below that is an almost certain guarantee of immediate re-entry. On the other hand, vehicles that need air to stay aloft 鈥� such as high-altitude balloons 鈥� never do so much above the atmosphere鈥檚 stratopause at about 50 km. All this hints at a better solution for the Earth-space boundary, backed up by a formal calculation as per Karman鈥檚 idea.
You might assume the crucial thing is that the density of the atmosphere changes rapidly with height. But what could complicate defining the edge of space is that the density of the atmosphere at a given height is variable, thanks mostly to solar activity. When I began this investigation, I thought this would make the effective Karman line poorly defined, but I was surprised to find that wasn鈥檛 the case.
Solar effects make atmospheric density jump up and down a lot at 200 km, but lower down it鈥檚 much more stable and only introduces a few km of variability into the calculated boundary height. When looked at for a range of satellite types the calculation suggests the boundary should lie somewhere in the 70 to 90 km range.
This agrees with the empirical observation that a satellite can鈥檛 stay in orbit below about 80 km. This leads me to propose that we should drop the artificially round number of 100 km as our guideline, and adopt 80 km as the point where outer space begins. Space just got a bit closer.
Jonathan McDowell is an astrophysicist at Harvard-Smithsonian Center for Astrophysics in Massachussets
