This week the rocket motor on the Scaled Composite built SpaceShipTwo was fired in flight for the first time. The Burt Rutan designed rocketplane accelerated beyond Mach 1and reached an altitude of about 55,000 feet during the short rocket burn. It was an important step in testing what will probably be the first civilian craft to carry passengers into space. The initial suborbital flight is expected to happen next year. Price of a passenger ticket is said to be $200,000.
This second space race, one being conducted this time by private industry instead of cold war enemy governments, raises the question of just what is space? Where does space begin? How does one know when they reach space?
Actually, this is a question that has been around for more than 50 years. And the group most interested in what is space and where it begins can be traced more to the FAI (Federation Aeronautique Internationale), the international body that maintains aviation records, than to astronomers and other scientists.
The FAI dates back to the earliest days of flight. In 1905 the leaders of a group of national aero clubs got together to form an international aero club to create uniform standards for establishing and recording aeronautical feats and the FAI was born. The National Aeronautic Association is the U.S. national aero club and is a member of the FAI.
For the first 50 years or so the task of aeronautical record keeping was pretty straight forward. Who flew fastest, farthest, climbed highest, carried the most payload, flew quickest between city pairs and so on were the record holders. And during those first 50 years records were falling like dominos, with many only lasting a few days before they were topped.
But in the 1950s the exploits of the rocket-powered research airplanes such as the X-15 began to bend our traditional understanding of flying and aeronautics. Because the rocketplanes carried their own oxygen along with fuel they could “fly” to unbelievable altitudes at astonishing speeds. Were these craft really aircraft, or something else because their engines did not need air to breath.
In 1957 when the Soviets launched Sputnik into orbit it became clear that every speed, altitude and distance record in the FAI book was a goner. Suddenly a craft could circle the globe in less than two hours at an altitude measured in miles at a speed that made Mach anything seem puny.
What to do? Clearly there needed to be a demarcation between a space craft and an aircraft or aircraft records would be meaningless.
A possible definition of a space craft could have been one that carries its own oxygenator to power its engine. But some rocket planes flew at levels where air breathing engines could operate so that was limiting.
A nice place to split aircraft and spacecraft would be at the point where the atmosphere ends. But where is that? The earth’s atmosphere kind of peters out with tiny amounts of particles extending far above the surface of the earth with no definite line where there is no more atmosphere.
Without any real absolute definition of where flying ends and space travel begins the FAI decided on an altitude 100 kilometers. It’s a nice round number. And it’s pretty high above the earth. Below that level—about 328,000 feet—an aircraft was an aircraft, the FAI declared. Above that height and you were in space.
SpaceShipTwo and the other suborbital space projects use the 100 kilometer altitude definition as the beginning of space. At that altitude gravity is micro, as they say, so people and objects are nearly weightless. And it is certainly high enough to see the curvature of the earth horizon. But you would need to be at least twice as high and traveling much faster to enter orbit. And even in low earth orbit of a few hundred miles instead of 100 kilometers there are enough particles left to drag down a space craft pretty quickly.
The fact that SpaceShipTwo’s “flip-up” tail works to slow the craft demonstrates that it is still flying in some amount of atmosphere. If there were no atmospheric molecules left the flipped up tail would have nothing to drag against and it would be ineffective just as the blunt end of a space capsule does nothing to slow it until it descends back into the atmosphere.
We are all watching the exploits of SpaceShipTwo just as we did the flights of the much smaller SpaceShipOne and cheering for it to succeed employing unique techniques that make private space flight possible. Is 100 km really the edge of space? Who can say, but it sure is one heck of a lot higher than any passenger carrying craft not paid for by taxpayers has ever gone. And that is one for the FAI record books.