On a cold April morning in 1875, three men ascended into the sky above Paris in a balloon, hoping to push the boundaries of human flight. Their journey was a scientific expedition, an attempt to explore the upper atmosphere and venture into a realm where few human beings had ever been. But as their balloon climbed higher, the air grew thinner, the temperature plunged, and the simple act of breathing became a desperate struggle. What began as a daring experiment in the conquest of the skies would end in one of the most horrifying tragedies in the early history of aviation.

Depiction of Théodore Sivel, Gaston Tissandier and Joseph Crocé-Spinelli in Zénith's fatal ascent of 15 April 1875. "La Nature", 1 May 1875

The balloon, named Zénith, carried its crew into a hostile world for which 19th-century science had only begun to prepare. At ground level, the atmosphere seemed limitless. But at altitude, it revealed itself as an unforgiving environment where the human body could fail with terrifying speed. The Zénith expedition exposed the dangers of high-altitude flight, and left behind a grim lesson for generations of aeronauts who dreamed of going higher.

The aeronauts of the Zénith were Gaston Tissandier, a French scientific writer and balloonist; Joseph Crocé-Spinelli, a scientist and journalist; and Théodore Sivel, an experienced sailor and aeronaut.

Gaston Tissandier, born in Paris in 1843, was a scientist, author and enthusiastic aeronaut. He had a particular interest in meteorology and the scientific study of the atmosphere. His brother, Albert Tissandier, was also involved in aeronautics, and the two helped popularise ballooning as a means of scientific investigation.

Joseph Crocé-Spinelli, born in 1845, shared this interest in atmospheric research. He had participated in earlier high-altitude experiments and understood the importance of studying the composition and properties of the air. Théodore Sivel, born in 1834, brought considerable ballooning experience to the expedition. His practical knowledge complemented the scientific interests of his companions. It was under his guidance that Zénith was constructed.

Barely a month before the fatal flight, Sivel along with zoologist and physiologist Paul Bert, Joseph Crocé-Spinelli, Albert Tissandier, Albert's brother Gaston Tissandier, and Claude Jobert, successfully completed a long-duration flight lasting 24 hours and 40 minutes.

Encouraged by the success of the flight and the positive coverage in the press, the Société Française de Navigation Aérienne (SFNA) announced that it would organize a high-altitude ascent using the aptly named Zénith, a 3,000 cubic meter balloon owned by Sivel.

At approximately 11:35 a.m. on 15 April, Zénith lifted off from the gasworks at La Villette. They carried an array of instruments with them to study the atmosphere, including an aneroid barometer, a hygrometer and an electroscope. They also carried an oxygen-supply apparatus. Three small reservoirs containing a mixture rich in oxygen were attached to the balloon's basket. Rubber tubes and mouthpieces allowed the men to inhale the gas when required.

The crew understood that oxygen deprivation was one of the greatest obstacles to high-altitude ballooning. At lower elevations, the atmosphere supplied sufficient oxygen for normal breathing. As a balloon climbed, however, atmospheric pressure fell, making it increasingly difficult for the lungs to transfer enough oxygen into the bloodstream.

Diagram of the balloon's ascent from Paris to Ciron. Credit: Wikimedia Commons

The men had prepared for the dangers as best they could with the knowledge and technology available. What they could not fully anticipate was how rapidly hypoxia, the condition caused by an inadequate supply of oxygen to the body, could impair their judgement, coordination and consciousness.

By around one o'clock, according to Tissandier's later account, they had passed 5,000 metres. The atmospheric pressure had fallen substantially, and the temperature outside was below freezing. Their instruments were working, and they continued to monitor their physical condition.

As they climbed higher, however, the men began to experience symptoms of oxygen deprivation. At 7,000 meters, Tissandier recorded that Crocé-Spinelli and Sivel had become pale. He himself felt weak and tried inhaling the oxygen they carried, which briefly improved his condition. Sivel then turned to Tissandier and asked whether he would throw out some ballast in order to ascend higher. Tissandier, feeling lethargic because of the altitude and the freezing temperature of −10 Celsius, agreed to the proposal. Sivel proceeded to cut three bags of ballast and the balloon shifted to a quick ascent.

By the time that they reached 7,500 meters, Tissandier could barely move. He described feeling no pain at all, but ‘an inner joy’ and indifference to the increasingly dangerous situation. The lack of oxygen made him delirious. Increasingly debilitated, Tissandier's memory of the events from hereon became even hazier. He remembered trying to reach for the oxygen but unable to lift his arm. Then, looking at the barometer, he saw the pressure at 280 mm and tried to yell out that they had breached 8,000 meters, but the words did not come out from his mouth and he fell inert at about 1.30 p.m.

Sketch depicting Crocé-Spinelli and Sivel, having lost consciousness due to a lack of oxygen, and Tissandier, in the basket of the Zénith on 15 April 1875. Credit: Wikimedia Commons

At 2.08 p.m., Tissandier woke up and saw the balloon descending at a perilous speed. Gathering all the strength he could muster, he managed to cut off a bag of ballast to re-establish the equilibrium before passing out again. After a while he was shaken awake by Crocé-Spinelli, who asked to throw out even more ballast. Fading in and out, Tissandier saw Crocé-Spinelli throw out the respirator, blankets and more ballast, which caused the balloon to rise again.

Tissandier drifted in and out of consciousness. The effects of oxygen deprivation had left him confused and physically incapacitated, and the sequence of events could only be pieced together from his fragmentary memories.

When he regained consciousness again, the balloon was descending at terrifying speed. He tried to rouse his companions by calling their names and shaking them, but neither man responded. Sivel's face had darkened, and blood was visible around his mouth. Crocé-Spinelli, too, appeared lifeless.

The Zénith finally landed at about 4.00 p.m. in the plains near Ciron, 250 km from Paris. Local residents rushed to help the surviving aeronaut. Tissandier was severely weakened, bruised and suffering from the effects of his ordeal. His companions, however, could not be saved. Both Crocé-Spinelli and Sivel had died during the flight.

The ballloon lands at Ciron. Credit: Wikimedia Commons

The deaths of Crocé-Spinelli and Sivel raised urgent questions about the safety of high-altitude ballooning. The crew had carried oxygen, understood the dangers of ascending too far and possessed considerable experience. Yet their precautions had proved inadequate.

The central problem was the physiological effect of low atmospheric pressure. At high altitude, the reduced pressure makes it harder for oxygen to pass from the lungs into the blood. The resulting oxygen deprivation can cause headaches, confusion, impaired coordination, loss of consciousness and, in severe cases, death.

One particularly dangerous feature of hypoxia is that it can undermine a person's ability to recognise the danger. Someone who is becoming severely oxygen-deprived may lose the capacity to make sensible decisions or carry out the actions necessary to s...

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