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 spacefarers because different nations (or even the same nation) may apply different laws to 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 there, which assert that this area "shall be free for exploration and use by all." From an intellectual property perspective, defining when and where space law applies is important because the United States has specific laws applying patent law to inventions used in space!
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 perspective. The second 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 kilometers 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 von Kármán pointed out that his 100-kilometer-high line in the sky was approximate. The atmosphere is a dynamic system, inflating and deflating in response to myriad factors that vary with time or location, like solar activity, chemical composition, temperature, and air currents. Depending on atmospheric conditions, a lifting body at altitudes significantly higher than 100 kilometers 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 kilometers, 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 onboard thrusters. Even at 300- to 400-kilometer 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!
Functionally defined "space"
Unlike the physical approach to defining where space begins, the functionalist approach focuses on the purpose(s) of the craft in question and what the craft is being used for at a specific time. In many ways, this approach to defining where space law should apply finds its roots in maritime law, the law of the sea. Maritime law can apply to a vessel "in navigation." That term is a legal term of art; it has a specific meaning to maritime lawyers. Like many terms of art, "in navigation" doesn't mean quite what it sounds like it means! A ship is generally considered "in navigation" when it is carrying goods on the rivers and seas. So far, so good! But a ship is considered "in navigation" even when it is not actively voyaging the seas if it has been constructed, commissioned, and not withdrawn from service. Ships are still considered "in navigation" when they're sitting at the pier waiting for a new cargo, moored undergoing repair, or taking on fuel and supplies in preparation for another voyage! Additionally, if a ship has been built but is on a shakedown cruise or undergoing sea trials before it is commissioned, it is not considered "in navigation" and some aspects of maritime law might not apply to it!
A maritime-law-style approach to defining where space law applies has advantages and disadvantages, especially when considering reusable vehicles, like Stiga, Xaero, Dragon, SpaceShipTwo, or New Shepard. A maritime law-style approach would reduce or eliminate confusion about whether a suborbitial vehicle is subject to laws applicable to space. A functional approach to applying space law to vehicles could be based on the design of the vehicle, rather than fluctuating atmospheric properties. Because a suborbital vehicle is designed for space, space laws would apply to it! Potentially, this approach gives the vehicle operator more control over when space law starts being applied to a vehicle. On the other hand, adoption of an "in navigation" analogue for applying space law might lead to space law attaching to a reusable vehicle like the Stiga rocket even when it is planted firmly on Earth and not being actively used. Finally, initial test flights, such as Falcon 9's COTS Demo Flight 1, which successfully orbited and returned to Earth a test Dragon capsule, might be excluded from some laws controlling space activities because Falcon C1 was akin to a sea trial rather than a routine cargo delivery operation.
"Space" defined by fiat
The last approach to defining where space begins is an arbitrary approach. Organizations like the X Prize Foundation and the Fédération Aéronautique Internationale define space as starting at 100 kilometers above mean sea level. This arbitrary line is based on Kármán's calculations. This definition has been criticized by some individuals. For example, José Mariano López-Urdiales, the founder of zero2infinity, argues that a 100-kilometer definition of space is "somewhat arbitrary and the view from 36 km [where his balloons travel] offers essentially the same viewing experience."
In 2002, the case for a 100-kilometer-high gateway to space was bolstered when Australia passed a series of amendments to its Space Activities Act. The act now applies to an "area beyond the distance of 100 kilometers above mean sea level."
The 1976 Bogota Declaration is an interesting approach to creating a legal definition of space. The 1976 Bogota Declaration, also known as the Bogota Convention, states that geostationary orbits (GSO), which are "located at an approximate distance of 35,871 km" above mean sea level, are natural resources dependent on physical phenomena on Earth. These orbits are not part of space and are therefore subject to the control of the countries over which geostationary orbits reside (i.e. equatorial nations). Thus, the Bogota Convention seems to state that space doesn't begin until you're more than 10% of the way to the moon! The equatorial nations of Ecuador, Colombia, Brazil, Congo, Zaire, Uganda, Kenya, and Indonesia signed this declaration. However, none of these nations have a significant, if any, launch capability. Such a launch capability could be used to enforce the Bogota Convention claims to GSO. Spacefaring, non-equatorial nations have taken a dim view of the Bogota Convention and are therefore unlikely to assist in its enforcement.