It seems that for a blog looking to discuss air and space, a good starting point would be to focus on where one ends and the other begins, so here goes!

We often hear of high flying aircraft or record-breaking skydivers being described as ‘at the edge of space’ but what does this mean and are such descriptions correct? Ask anybody where space is and they could probably point skywards and say ‘Up there somewhere’, but where that ‘somewhere’ is, and who made that decision are slightly more complex.

Although the concept of an end of a sensible atmosphere and the beginning of the vacuum of space had been well understood for many years, it was really the work of Hungarian aerodynamicist Theodore Von Karman in the 1950’s that separated Aeronautics and Astronautics by proposing a dividing line at the point where air density was too low for aerodynamic flight to be possible, but atmospheric drag is low enough to allow orbital speeds to be maintained without application of constant forward thrust.

After much discussion Von Karman settled on the altitude of 100km (approx. 62 miles) as the location for this division. Now known as the Karman Line, this became recognised by the Fédération Aéronautique Internationale (FAI), the international organisation that sanctions aeronautical records.

Von Karman’s work was especially timely, as advances in rocket propulsion, aerodynamics and materials were allowing designers to create craft that could approach or surpass this line. In the United States, the mid 1950’s saw the National Advisory Committee for Aeronautics (NACA) working with the Air Force on the specification for a high speed, high altitude research plane which eventually became the North American X15. Elsewhere in both the United States and the Soviet Union, rocket designers were looking towards orbiting artificial satellites as part of the activities to support the International Geophysical Year (spanning 1957/1958).

So there we go. Space starts at 100km, right?

Well, as I mentioned earlier, things are slightly more complex than that. Although adopted by the FAI, the Karman Line isn’t the only recognised division. Notably, the US Air Force decided to define the boundary between the Mesosphere and Thermosphere as the division between air and space. This comes in as an average of 50 miles (approx. 80km), some 12 miles lower than the Karman Line. Although this made little practical difference to the ballistic rocket programmes aiming for orbit, it had considerable significance to the pilots of the X15.

The first true aerospace craft, the X15 flew to altitudes exceeding 50 miles on many occasions, and for the Air Force pilots such as Bob White, Joe Engle and Mike Adams who flew some of these missions, that meant collecting their Air Force Astronaut Wings (although this award was posthumous in Adams’ case). But what of their NASA test pilot colleagues? Well as they weren’t in the employ of the Air Force they received no such awards. In fact, to add insult to injury NASA didn’t even recognise Joe Walker’s flights above the Karman Line as a space flight at the time either, reserving the title ‘astronaut’ for those involved in the more visible projects such as Mercury, Gemini and Apollo. It wasn’t until 2005 that NASA bowed to pressure to bestow the title on their X15 pilots who had exceeded the 50 mile boundary.

But what of our record breaking skydivers and high altitude aircraft such as the U2 and SR71? Well, none of these come close to either the Karman line or the Air Force’s 50 mile boundary, but they do reach altitudes where the body would react in similar ways if exposed to the ambient environment, with atmospheric pressures and temperatures low enough that death would occur quickly without the use of a pressure suit. Initially partial pressure suits were used for high-altitude flights, but these were replaced by full pressure suits, similar in structure and function to space suits. Under these circumstances it’s perhaps understandable that, however inaccurate it may be, the description ‘at the edge of space’ is regularly used.

Currently, there are no acknowledged government programmes looking at manned flight in the boundary between air and space. Flights to low earth orbit easily exceed these altitudes (for instance the International Space Station orbits at an altitude of approximately 400km) but we are entering an era where such definitions will become important to a new crop of potential astronauts looking to take the trip of a lifetime on suborbital ‘space tourism’ flights.

When SpaceShipOne won the Ansari X Prize in 2004, it did so by flying above 100km on three occasions, thus proving itself capable of repeated spaceflight using the FAI definition. It currently looks like Virgin Galactic’s SpaceShipTwo won’t be capable of repeating that feat, with some commentators casting doubts that it will even be able to manage a 50 mile altitude in its current configuration. Will passengers become astronauts or not? Will companies such as Virgin Galactic, XCOR and Blue Origin create their own definitions for what constitutes space, and if so what basis will be used for this? Would you be satisfied to pay a small fortune and still not quite reach at least the 50 mile mark, and would you sue or expect a second flight if you fell short of what you’d been promised in the brochure?

I guess we’ll just have to wait and see once one of these services actually starts flying.