Most authorities set the beginning of space at 50 miles above the Earth's surface. However, space-equivalent conditions are encountered at lower altitudes. The region, called the aeropause, is located between 10 and 19 miles in altitude.
We looked at Joe Kittinger's amazing freefall from a ballon gondola at an altitude of 19 miles in 1960. Now let's look at what happens when an astronaut has to bail-out from a rocket plane at 56 miles (300,000 feet) above the Earth.
Why choose 300,000 feet?
Well, within the next decade, commercial suborbital ballistic flights up to 300,000 feet will be conducted by Sir Richard Branson's Virgin Galactic Corp. from New Mexico's Spaceport America, the world's first commercial spaceport.
An estimated 500 passengers a year - paying $200,000 for one roundtrip ticket - will be flying aboard the company's X-15-like aerospace plane. So what would happen if an accident occurred at 300,000 feet and Virgin's high-paying space tourists had to bail out?
Oops, as far as we know, Virgin has no provisions for getting its wealthy astronaut-passengers back to Earth "flying" 56 miles up at Mach 10 (7,500 mph).
Beyond a certain safe bail-out altitude, even NASA space-shuttle astronauts are helpless as we witnessed with the Challenger and Columbia accidents.
But let's consider a late 21st century suborbital passenger rocketplane heading from the imaginary Montreal Spaceport, Que., to Cape Canaveral, Fla. Suddenly, at 300,000 feet above Boston, Mass., the pilot declares an in-flight emergency; it's time for crew and passengers to bail-out. Can they do it safely?
The following scenario is based on calculations by veteran space-expert Roy A. Gallant.
If a suborbital rocket passenger - equipped with a yet-to-be invented ceramic-coated, pressurized reentry suit with parachute - bails-out at 300,000 feet above Boston, his body is nearly weightless for about a minute - at Mach 10, 90 percent of the Earth's gravitational force is overcome so the 150 lb. passenger weighs only 15 lbs.
But after just two and half minutes - now 220 miles away from the bail-out point - the passenger has increased in weight and is falling at 2,000 mph. So in less than three minutes he reaches 140,000 feet, almost halfway to the ground! The air here is dense enough to slow him to Mach 5 (3,700 mph) with a g-load of 7gs (that means seven Earth gravities). Aerospace researchers define g-load as "the numerical ratio of an applied force to the gravitational force on the Earth's surface."
Within 30 seconds, the astronaut falls another 50,000 feet but now the thickening atmosphere slows his plunge down to 185 mph.
At 15,000 feet, he can deploy the backpack parachute to land safely.
When he touches down near New York City, he is nearly 250 miles beyond where he bailed out of the rocket plane. Total elapsed time from 56 miles up to ground landing: 12 minutes!
In the summer of 2009, French aviator Michel Fournier will attempt to best Joe Kittinger's freefall from the edge-of-space. Fournier will be lofted to the upper stratosphere in a giant helium balloon from western Canada. At 131,000 feet, the Frenchman will jump ship and use a unique airfoil to return to terra firma. Stayed tuned; we'll cover this upcoming historic aerospace event here.
Lou Varricchio, M.Sc., a former NASA science writer, is the Vermont's NASA/JPL solar system ambassador. You can order his recent space book, "Inconstant Moon", online or at Monroe Street Books in Middlebury.