Where is Space? Part 1 of 2

Ask a four year old “Where is space?” and they will likely reply “Up there!” Look in the Outer Space Treaty or virtually any other US law relating to space for a definition of “where is space?” and you’re not going to do much better than the four year old’s answer. Obviously the Earth ends and space begins somewhere but today, as it has been for the entirety of humanity’s manned and unmanned exploration of “up there”, there is no international legal definition of space; no clear indication of where space law applies! This ambiguity is a potential source of confusion and unease for aerospace companies and space farers because different nations (or the same nation) may apply different laws to the physically identical vehicles and flight profiles and may attempt to apply their laws to satellites in orbits that overfly their soil.

Within a year of the first ballooners taking to the skies, governments began applying the rule of law to flying vehicles. Long standing international law recognizes that all countries have the right to control the airspace above their lands, but somewhere above all nations airspace ends and space (or outer space, if you prefer) begins. Different laws apply which assert that this area “shall be free for exploration and use by all.”

Internationally, there is not yet an established boundary marking where space begins. International efforts to define where space begins take three different approaches. The first approach seeks to define where space law applies from a physical prospective. The secod approach is functional, applying space law based on the characteristics of the vehicle involved. The third approach is to legally define space from an arbitrary standpoint that serves one or more sovereign nations’ interests, or definition by fiat.

Physically defined “space”

Physical boundary layers for space are typically defined in terms of physical laws. The most famous “line in the sky” is the Kármán Line. Hungarian physicist Theodore von Kármán determined that at an altitude of approximately 100 km above sea level, the atmosphere becomes so thin that a vehicle would have to travel faster than orbital velocity to generate sufficient aerodynamic lift to be self supporting. But then it would be essentially orbiting the planet already, so why would it worry about generating enough lift to stay aloft? It’s in orbit!

So, let’s define space as the point where you can stay aloft only by achieving orbital velocity and call it a day; simple, right? Wrong. Even Kármán pointed out that his 100 km high line in the sky was approximate. The atmosphere is a dynamic system, thickening and thinning in response to myriad factors like solar output, local chemical composition, temperatures, and air currents. Depending on atmospheric conditions, a lifting body at altitudes significantly higher than 100 km might generate sufficient lift to stay aloft at sub-orbital velocities. And that’s for a craft designed to be a lifting body! Not all spacecraft are lifting bodies, a lot of their physical profiles more closely resemble rocks, rather than 747s.

Another approach would be to define space at an altitude above which satellite orbits don’t rapidly decay. At approximately 200 km, well above the planned apogee of SpaceShipTwo, the atmosphere is so thin that air resistance is almost negligible, but satellite orbits still rapidly decay without boosts from on board thrusters. Even at 300 to 400 km altitudes, where the Shuttle and the International Space Station orbit, regular station keeping thruster firings are required to avoid orbital decay due to atmospheric drag but it would seem ludicrous to define space in a manner that did not include the orbit of the International Space Station!

As far back as 1979, defining space at an altitude in the multi-hundreds of kilometers above sea level was discouraged from many quarters. The USSR proposed that future UN treaties addressing issues of space indicate that space start “at an altitude not exceeding 110 km above sea level.” Others have suggested setting the boundary of space at an astonishing 1500 km above the Earth’s surface. That would include virtually all of low Earth orbit!

Come back tomorrow when we’ll explore two more approaches to legally defining space: the functional approach and the fiat approach.

Happy creating!

Update: Read part 2 here!