Where Does Outer Space Begin?

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.

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7 Responses to Where Does Outer Space Begin?

  1. Robert Jans says:

    I’m not sure your analysis of the flip-up-tail is correct by saying that there still are enough air particles to make the flip-up-tail effective. I believe that Spaceship one or two goes about straight up, comes to a halt by shutdown of the engines and falls back under gravity pull from earth. At an altitude of 100 km the gravity pull may be less but still very much there. The falling creates the full sensation of weightlessness. Then, when coming lower and lower, more air is encountered to make the flip-up-tail ever more effective. It does not need to be effective at 100 km altitude: gravity does its work. How come that satellites don’t fall down? They have high forward speeds creating a centrifugal force which balances the gravity pull. The “tourist spaceship” has no forward speed: it goes up, stops and comes down.

  2. Thomas Boyle says:

    Robert Jans is right. If you stood on a tower 100km tall, you’d find you weighed 97% as much as you do on the Earth’s surface – gravity is very present up there. The reason for the weightlessness is the lack of atmosphere: with neither engine to push, nor air to drag, the machine is in freefall and the occupants experience the sensation of weightlessness. The flip-up tail comes into effect during the return, but doesn’t do much of anything when it’s first “flipped up”.

    It’s true, there’s still some air there. Low Earth Orbit is rarely lower than 300km because of air drag (Sputnik was at about 200km). The International Space Station is at about 400km. The Hubble Space Telescope is at 600km. The GPS system satellites are at 20,000 km. Geosynchronous orbit is out at nearly 36,000 km.

  3. Roger Halstead says:

    I think lack of atmosphere at that altitude has little to do with weightlessness. Ever do a Hammerhead? Weightless A parabolic arc in a high performance plane? Weightless. The lack of atmosphere, or rather the presence of a very thin atmosphere presents very little drag drag so terminal velocity is very high which does allow for a longer duration of the weightless experience.

    Straight up followed by straight down at any flying altitude produces produces the sensation of being weightless.

    Private Enterprise has reached the point of our early space program and although expensive they have done it at far less cost to the taxpayer. Private enterprise again does things at less cost than the government. I wonder how they will stack up for the next steps? There is indeed a chance that private enterprise will beat the governments to interplanetary travel and offer a less expensive route to space research.

    I Wish them well.

  4. DEL says:

    Weighlessness has nothing to do with either “space,” altitude or vacuum. It has to do with the total lack of surface forces acting on the subject feeling it. (“Free fall.”)

    A person standing on the ground experiences weight because the reaction force from the ground, operating through his/her feet, cancels the gravitational downward acceleration that would otherwise occur. That reaction force is mechanically distributed to all body particles in proportion to their masses, so that every one of them is in equilibrium and prevented from falling—every particle “feels” its own weight.

    Imagine now that the ground is suddenly pulled away from under the person’s feet, and consider only the brief moment before a significant drop velocity develops that would induce a noticable drag force. The reaction force that previously gave rise to the weight phenomenon suddenly disappears, and with it its descendant particle forces— every particle’s weight vanishes at once. Even non-airman earthlings can easily imagine being in that situation and appreciate what weightlessness must feel like, without having to pay $200,000 for the sensation. Kingdom [of space] comes [to earth]!

    All that until the drop velocity builds up and brings about a drag force that gradually restores weight. (In the vacuum of space that would never happen.) Equilibrium is obtained when the total drag force reaches the value of the falling object’s total weight. That’s what gives a free-fall parachute jumper his/her “terminal velocity.”

    Attributing the weightlessness phenomenon to the space environment is a common error among lay people innocent of either flight, space, or 17-th Century mechanics. Finding it in this blog is eyebrow-raising, to say the least.

  5. charles says:

    I don’t think Rutan designed SS2. He’s on several videos of presentations he’s given over the last several years saying the last airplane he designed (before Bipod) was SS1. I heard him say that as well at a few talks at Airventure, as well. He’s working up an ultralight airplane now, I think. I would think he would have claimed design credit on SS2 and WK2 if he had designed them. He’s on several videos giving design credit for the original White Knight to one of the Scaled engineers.

    The feather didn’t have any effect until AFTER SpaceShipOne had been in free fall for many seconds BACK into enough atmosphere to generate enough force to “center” the descent, as seen on the Black Sky DVDs from Discovery Channel.

  6. Brad says:

    Yes, the “tail up”, actually called the “feather” by Burt/Scaled, absolutely happens out of the atmosphere. Burt has detailed that many times. He even said that SS1 was actually still going upward while they feathered, and with the lack of atmosphere, the body has no attitude change due to the absence of any (or micro amounts of) dynamic pressure. That sets it up for the plunge back, of which he describes as “effortless reentry”.

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