Joe Kittinger and Einstein’s Equivalence Principle: The First Human in Space


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Joe Kittinger’s Great Leap: Image courtesy of USAF

Lord, take care of me now,” Joe silently prayed. USAF Captain and test pilot Joseph Kittinger had just ridden a gigantic helium balloon from the Earth’s surface to nearly 20 miles above the New Mexico desert floor. Then he jumped — out of his cramped gondola, literally into thin air.

Kittinger’s Historic Leap

“Try anything” Joe made his jump on August 16, 1960 — nearly eight months before Russian cosmonaut Yuri Gagarin’s pioneering space-flight — making Kittinger the first human in space.

Before jumping, Joe’s balloon had risen to a staggering 102,800 feet altitude, some three times higher than commercial jets fly. The temperature at that height was 110 degrees below zero F, the air 99% vacuum, the pressure so low that his 35-foot-wide balloon expanded to 200 feet in width. His thin pressure suit passed its first test in space — preventing him from freezing to death due to intense cold, and keeping his blood from boiling due to the extremely low pressure of space.

It’s so black overhead,” Joe later recalled. “I was struck by the beauty of it. But I was also struck by how hostile it is.” (Our atmosphere scatters sunlight, giving it its blue appearance. With 99% of the air gone, the sky was a black deeper than coal. And Kittinger couldn’t see the stars, because his pupils had contracted from the sun’s glare.)

Cold Warrior

In 1960, the cold war was raging. A few months before Joe’s jump, the Soviets had shot down a high-flying U-2 aircraft over Central Russia. Wily Soviet Premier Nikita Khrushchev let the world believe the pilot hadn’t survived. U.S. President Dwight Eisenhower announced his cover story: it was a weather plane which had drifted off course. Then, Khrushchev produced the pilot; USAF Captain Francis Gary Powers, as well as the remains of the plane’s reconnaissance camera. The embarrassed Eisenhower had to fess up — it was a spy plane.

Kittinger next to the Excelsior gondola: Image courtesy of USAF

When Powers was later asked how high he was flying, he quipped, “not high enough.” Such aircraft could reportedly fly at altitudes up to 70,000 feet, but the U.S. needed to fly its spy planes even higher to prevent being shot down by Soviet missiles. What if the pilot had to eject in an emergency? How would he survive the severe cold and near-vacuum at the edge of space?

So 150-pound Joe Kittinger volunteered for the ultra-high altitude balloon experiment called Project Excelsior. Within a few seconds after jumping, Joe reached a staggering free-fall speed of 714 miles per hour, exceeding the speed of sound (at this altitude). He was in free-fall for over four and a half minutes.

Then, some 14,000 feet above the desert, Kittinger felt his first parachute open. The never-before-tested chute had worked. “Thank, you, God, thank you,” he said into his voice recorder. He landed safely on the desert floor.

Einstein’s Equivalence Principle at Work

Once in free-fall, Joe looked at the altimeter on his wrist — it was unwinding rapidly. “I rolled over and looked up, and there was the balloon just roaring into space,” Kittinger later recalled. “(But for me) there was no sense of speed.” From Joe’s point of view (his frame of reference), he was at rest – with the balloon above accelerating away from him and the Earth below accelerating towards him.

Kittinger in free-fall above Earth: Image courtesy of USAF.

Joe also felt weightless during free-fall, just as Einstein had predicted in 1907. With his eyes closed, Joe couldn’t tell whether he was falling to the Earth or floating in outer space with no gravity at all.

Joe experienced the core principle behind Einstein’s theory of general relativity — the Equivalence Principle (EP): acceleration and gravity produce equivalent physical effects. Per the EP, free-fall in a gravitational field is equivalent to free-float in zero gravity. Why? Because the acceleration of free-fall cancels the effects of gravity.

Highest Skydive Ever: The Next Jump

Joe Kittinger holds the world free-fall altitude record to this day. He remains the only person to break the sound barrier without an aircraft and live to tell about it.

In late summer or early fall of this year, daredevil Felix Baumgartner will attempt to break Kittinger’s record. He plans to jump from a stratospheric balloon some 120,000 feet above Roswell, New Mexico. Joe Kittinger, now 84 years old, is helping Baumgartner prepare for the event.


Rumerman, J. Joseph Kittinger. U.S. Centennial of Flight Commission. Accessed February 19, 2012. 

Mohazzabi, P. Why Do We Feel Weightless in Free Fall? THE PHYSICS TEACHER. Vol 44, April 2006.

National Cold War Exhibition. Biography of Francis Gary Powers. Accessed February 19, 2012. 

Clash, J. C. One Giant Step. Forbes. Accessed February 19, 2012. 

Reichhardt, T. First Up? Air & Space MagazineAccessed February 19, 2012. 

Atkinson, N. Skydiver prepares for record-setting freefall from edge of space. Universe Today. Accessed February 19, 2012. 

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