A Mountain of Myths about Space Launch

Space is high vacuum. Zero lift for wings or balloons. The legal boundary of 100 km (the Kármán line ) is convenient but entirely arbitrary; it is twice as high as any atmospheric vehicle can fly or float, and half as high as any practical satellite can orbit. Tourist suborbitals to 100 km (like SpaceShipOne ) are not practical spacecraft, merely dangerous and ostentatious displays of wealth, somewhat like climbing Mount Everest but without the exercise or the strenuous accomplishment.

Real space is orbit, and the lowest practical orbit is 200 km. Even that is far too low for a large, high-drag object like ISS, which requires frequent reboost to compensate for air drag, even though the air is 2 parts per trillion of the density at sea level. That is a difficult number to grasp - a volume of "ISS vacuum" as big as AT&T Stadium in Texas (104 million cubic feet, 3 million cubic meters) would fit in 7 cubic centimeters, half a tablespoon, if compressed to sea level density.

A 400 kilometer drop from Space Station altitude, zero orbital velocity

If you were suddenly transported to 400 km altitude (in a space suit) and dropped from there, you would not orbit the Earth, You would fall down, your vertical speed increasing rapidly. After 260 seconds, you would plunge through the 100 km altitude Kármán line at 2330 meters per second, 8 times the speed of sound at that altitude, but you would notice nothing, a 0.3 Pascal pressure force added to the 100 kPa (100,000 Pascal( air pressure in your suit.

Things get frisky during your final 40 second plunge deep into the atmosphere, approaching a terminal velocity of 2700 m/s. At 7.1 kilometers altitude (higher than all mountain peaks in the Western Hemisphere), the air density is about half of sea level. However, the wind pressure beneath you is 40 times air pressure, about 400 tonnes per square meter; you would be smashed to jelly, though you would not burn up like a (vastly faster) small meteor.

• Actually not, you slam on the brakes at a higher altitude. You will stop accelerating and slow down when the drag pressure equals gravity. If your effective drag area is 1 square meter and you and your suit weigh 100 kilograms, then 1000 Pascals of pressure will start slowing you down at an atmospheric density around 170 ppm of surface, around 60 kilometers up. Air density (and drag pressure) doubles every two seconds, so within 10 more seconds of downward plunge, your suit will be ripped off by turbulence, and you will be crushed by gee forces.

Meteors arrive at 15 to 30 thousand meters per second. Orbital velocity is around 8000 meters per second, and air drag force is proportional to velocity squared. So, entering at orbital velocity is about 9 times the drag force of a 400 kilometer plunge, and vastly more than encountered by SpaceShipOne, though 4 to 14 times lower than a meteor. Meteors (and reentering spacecraft) slow down higher in the atmosphere; the smallest meteorites may burn up, but most hit the ground at bullet speed, with a charred surface and a frozen (by deep space) center. As we tragically learned in the catastrophic entry of Columbia, vehicles are ripped to shreds in the middle atmosphere (70 km for Columbia, air density 70 ppm of surface density), but most fragments impact the ground intact.

• Launch Loop (2017 design) vehicles reach orbital velocity at 100 km altitude, 0.5 ppm of surface density, so the drag pressure is 140 times lower. Passenger vehicles may fail to enter orbit (they require an additional small rocket burn half an orbit later, to circularize the orbit at destination altitude), and must be capable of reentry, but these will be heat-shield and parachute reentries into the ocean, perhaps 20 degrees west of the launch loop for vehicles intended for low earth orbit destinations, and 100 degrees west for GEO-bound vehicles.

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