The pilot pulled up, and all we could see through wide-open eyes was sky. We zoomed straight up until the glider ran out of pizzazz, then the pilot tipped it over into another sheer drop. Again, squeezed faces, dizziness, otherworldliness. After two or three loop-the-loops, the thrill became dread: Would he ever stop? My daughter was laughing, but I thought I would pass out.

What was going on? What happens to us physiologically when we start "pulling G's," as pilots label what we were feeling? Why was the sensation most pronounced as we swooped out of a dive? Might the glider pilot, I wondered at the time, pass out himself?

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If you're sensitive to G forces, aka gravity forces, think twice before going up in a glider and asking the pilot if he knows any tricks. © Doug Berry/Corbis

"Fainting in the air"

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Before the advent of airplanes, which could accelerate the human body like nothing before, people rarely experienced G forces. So-called gravity forces first became a concern during World War I, when pilots began mysteriously losing consciousness during dogfights. As early as 1919, a doctor wrote up this strange phenomenon for the literature, calling it "fainting in the air."

With the development of faster and more maneuverable planes, G forces became more dangerous. Based on rates of survival (or lack thereof) during crashes, it became accepted wisdom that no pilot could withstand more than 18 G's, or 18 times the force of gravity at sea level. So cockpits were designed to withstand only 18 G's. Yet pilots sometimes walked away from crashes in which the G forces were calculated to have been much higher.

In the mid-1940s, an Air Force physician named John Stapp began to suspect that it was the mangling effects of a crash and not the G's that killed pilots. Hoping to improve cockpit safety, Stapp set out to determine just what humans could take in the way of G forces. He built a rocket-powered sled, the "Gee Whiz," which accelerated a tightly strapped-in body—initially a dummy but soon Stapp himself—to extraordinarily high speeds along a track before coming to an almost unimaginably abrupt stop.

By the late summer of 1948, Stapp had done 16 runs himself and withstood up to 35 G's. He lost dental fillings, cracked a few ribs, and twice broke a wrist, but he survived. Still he was not satisfied. Eager to know what pilots ejecting at high speed could endure in terms of sudden deceleration, Stapp built a new sled called "Sonic Wind" in the early 1950s.

Related Space Shuttle Rescue Scenarios The Final Eight Minutes All About G Forces John Stapp riding the "Sonic Wind" during a 421-mph ride in March 1954 Courtesy of the U.S. Air Force

On what became his final run, in December 1954, Stapp decided to pull out all the stops. Firing nine solid-fuel rockets, his sled accelerated to 632 miles per hour in five seconds, slamming him into two tons of wind pressure, then came to a stop in just over one second. A witness said it was "absolutely inconceivable anybody could go that fast, then just stop, and survive." But Stapp did—in fact, he went on to live another 45 years, dying quietly at home in 1999 at the age of 89—and he experienced a record-breaking 46.2 G's. For an instant, his 168-pound body had weighed over 7,700 pounds.

Stapp's efforts put him on the cover of Time , and he was called "The Fastest Man on Earth." More importantly, his work led to greatly improved safety in both planes and cars, and he gave us a much-improved understanding of human tolerance to G forces.

A matter of acceleration

Even before Stapp it was well-known that G forces have less to do with speed than with acceleration—the change in speed over time. If speed alone could cause the thrill that comes from feeling G forces, then simply driving on the highway would suffice.

There is a limit to what anyone can take. Princess Diana tragically proved that.

When most of us think of acceleration, we think of, say, a Jaguar doing 0 to 60 in six seconds. But acceleration is technically any change in the velocity of an object: speeding up, slowing down, and changing direction are all types of acceleration. That's why, on a rollercoaster, you feel G forces when you round tight bends and are thrown against the side of your seat (a change in direction) as much as when you plunge from the heights (accelerate) or grind to a halt (decelerate).

You feel the thrill, but don't black out, because the coaster's creators designed it to be well within the G-force tolerance of the average person. The amount of G forces that are tolerable differs by individual. But for all of us it depends on three factors: the direction in which the G forces are felt, the amount of G's involved, and how long those G's last.

Roller coasters are precisely calibrated so average people can enjoy the spine-tingling effects of G forces and few of the ill effects. © Michael Braun/istockphoto.com

Blood pressures

Depending on which way your body is oriented when it accelerates, you can feel G forces front-to-back, side-to-side, or head-to-toe. (Or, in each case, vice versa—for example, toe-to-head.) Each of us can tolerate the two horizontal axes a lot better than the vertical, or head-toe, axis. Facing forward in his seat on that final run, Stapp felt front-to-back G forces as he accelerated and back-to-front G forces as he decelerated, and as we've seen, he endured well over 10 times the G's my daughter and I encountered in the glider.

But vertical forces are another matter, and it has everything to do with blood pressure. At sea level, or 1 G, we require 22 millimeters of mercury blood pressure to pump sufficient blood up the foot or so distance from our hearts to our brains. In 2 G's, we need twice that pressure, in 3 G's, three times, and so on. Most of us would pass out with head-to-toe G forces of just 4 or 5 because our hearts can't summon the necessary pressure. Blood pools in our lower extremities, and our brains fail to get enough oxygen.

Fighter pilots can handle greater head-to-toe G forces—up to 8 or 9 G's—and for longer periods by wearing anti-G suits. These specialized outfits use air bladders to constrict the legs and abdomen during high G's to keep blood in the upper body. Fighter pilots can further increase their G-tolerance by training in centrifuges, which create artificial G's, and by learning specialized breathing and muscle-tensing techniques.

All of us, fighter pilots included, can handle only far lower toe-to-head, or negative, G forces. Facing a mere -2 or -3 G's, many of us would lose consciousness as too much blood rushed to our heads.

Spinning at high speed, NASA's 20-G research centrifuge at California's Ames Research Center can simulate up to 20 times the normal force of gravity we feel at sea level. Courtesy NASA

Too much and too long

Magnitude and duration are as critical as direction. While John Stapp showed that people can withstand much higher G forces than had long been thought, there is a limit to what anyone can take. Princess Diana tragically proved that. Experts estimate that, in the car accident that killed her, the G forces on her chest were about 70 G's (and 100 G's on her head). That acceleration was enough to tear the pulmonary artery in her heart, an injury almost impossible to survive. If Diana had been wearing a seatbelt, the G forces would have been in the neighborhood of 35 G's, and she may have lived.

Astronauts in orbit are still subject to about 95 percent of the gravity we feel on Earth.

Diana's death notwithstanding, Stapp proved that people can often survive high G forces for very brief periods. We're all familiar with this to a certain degree. According to a 1994 article in the journal Spine , the average sneeze creates G forces of 2.9, a slap on the back 4.1, and a plop down into a chair 10.1. If you jump from three feet up and land stiff-legged, write the authors of the book Physics of the Body , you'll feel about 100 G's momentarily.

We suffer no ill-effects from these everyday events because they're so brief. The trouble starts when G forces linger. That's why I began feeling worse with each dive the glider made. It's also why, during launches of the space shuttle, controllers keep the acceleration low—no greater than what generates about 3 G's—so as not to unduly stress the astronauts.

Zero G's

Of course, once the shuttle goes into orbit, astronauts no longer feel G forces. They're in a zero-G environment, right?

Well, not exactly. There's no such thing as zero G's. Even the two Pioneer spacecraft, launched in the 1970s and now the most distant man-made objects, experience a tug of one 10-millionth of a G from the solar system they've now left. Astronauts in orbit are still subject to about 95 percent of the gravity we feel on Earth. It's just that they're in a constant free fall. They're falling towards Earth, but their speed—up to 25 times the speed of sound—means that the planet is falling away from them just as fast. Better to say they're in a microgravity, or weightless, environment.

Even though the force of gravity is still very much in effect, astronauts in orbit do not feel it because they're in a constant free fall. Here, astronaut Ed White during the first U.S. spacewalk in 1965. Courtesy NASA

Weightlessness may be a gas, but it comes at a cost, because our bodies are used to a 1-G environment. Each of us here on Earth is actually accelerating towards the center of the planet at roughly 32 feet per second squared. We don't feel we're accelerating because the ground holds us in place. But without that customary pressure, our bodies take a beating. Over time, our cell walls collapse, our muscles atrophy, our bones decalcify. (The opposite happens in hypergravity: A 2001 study found that Australian fighter pilots who routinely felt G forces of 2 to 6 experienced, over the course of a year, an 11 percent increase in the bone mineral content and density of their spinal columns.)

These health effects of microgravity are of concern to NASA as it contemplates sending astronauts to Mars, a trip that could take three months one-way. On the way there, astronauts would need a centrifuge or other means to create artificial gravity to ensure that any "small step for a man" onto the Red Planet didn't result in a broken ankle. Visionaries are already wondering whether people born in potential future colonies on Mars (38 percent of Earth's surface gravity) or the moon (17 percent) could ever safely come to Earth.

Give me gravity

Coming safely to Earth was just what my glider-riding daughter and I began to wish for in the worst way. (Later she admitted to feeling increasingly queasy, adding, "I felt like my whole body was collapsing.") Fortunately, after four figure-eights, the pilot tired of his sport and leveled off, and we returned to the airport without further ado. One G never felt so welcome—good old 32 feet per second squared.