--- title: "Hess found stronger radiation above the clouds" description: "How did a balloon flight show that some radiation reaches us from space?" canonical_url: "https://sandboxphysics.com/stories/astronomy-gravity/hess-balloon-radiation/" language: "en" page_type: "story" alternate_languages: zh-CN: "https://sandboxphysics.com/zh/stories/astronomy-gravity/hess-balloon-radiation/" last_modified: "2026-10-24" --- [Stars and new windows on the sky](https://sandboxphysics.com/stories/astronomy-gravity/#stellar) Astronomy · Cosmology · Gravity # Hess found stronger radiation above the clouds Sandbox Physics · 2026-10-24 How did a balloon flight show that some radiation reaches us from space? 1. Charge a sealed detector 2. Compare discharge at different heights 3. Check the rise with several instruments 4. Infer radiation arriving from above The decisive high flight was in August 1912. Hess received the Nobel Prize in 1936. ## The signal grew as the ground fell away On 7 August 1912, Victor Hess rose above five kilometres in a hydrogen balloon over central Europe. The instrument beside him was losing electric charge faster than it had on the ground. If the radiation came entirely from radioactive material in the soil, climbing farther from the soil should have weakened it. Instead, the strongest radiation appeared high above it. Hess concluded that a very penetrating radiation was entering the atmosphere from above. The result answered a puzzle that had followed the discovery of radioactivity. A carefully insulated, charged instrument would still slowly discharge. Radiation could knock electrons from air molecules, leaving electrically charged fragments that carried charge through the gas. This process is called ionization. The air inside the instrument was acting as a radiation detector, even though the individual particles causing the effect remained invisible. Sources: [1] [3] ## A detector that could survive the climb Hess measured the discharge with a Wulf electrometer. Two fine, charged fibres repelled each other; as charge escaped, their separation changed. Looking through an eyepiece gave a reading of that change. The question was how quickly the separation changed during a timed interval. Comparing those rates at different heights made the ascent an experiment. The balloon carried more than one instrument. Two had thick walls and sealed chambers, keeping the air inside at essentially its launch density while the surrounding atmosphere thinned. A third, thin-walled instrument admitted less penetrating radiation and followed the outside pressure. Those differences mattered. A changing quantity of gas in the detector could otherwise alter the discharge rate and imitate a change in radiation. Hess also checked charge leakage through the insulation and made control observations before flights. He paid attention to the balloon basket and ballast as possible nearby radioactive sources. Even the eyepiece had been improved so that refocusing on the fibres would not noticeably change the magnification. The high-altitude reading depended on all these small precautions. Sources: [1] ## The first part of the climb was misleading Close to the ground, the measured ionization could fall. That was what the terrestrial explanation predicted: air was absorbing part of the radiation emitted by the crust. The surprise came farther up, where the readings rose again. A short ascent could therefore miss the decisive part of the curve. Hess's earlier flights had been limited by weather and by how much lift the available gas could supply. The August ascent finally reached roughly 5.3 kilometres. In the highest observing interval, one sealed instrument recorded an ionization rate of 40.7 ions per cubic centimetre per second, compared with a usual ground value around 16. The other sealed instrument also rose strongly. The small volume in that unit is about the size of a sugar cube: the measurement counted how many ion pairs radiation could create there each second. Agreement between instruments made the rise much harder to blame on a single faulty electrometer. The highest altitude and the average height of an observing interval are different quantities. During the highest hour, the balloon ranged from about 4.4 to 5.35 kilometres, with a mean altitude near 4.8 kilometres. The reading of 40.7 belongs to that whole interval. Hess placed the times, heights and instrument readings in the same table so that readers could follow the rise through the flight. Sources: [1] [2] ## The Sun was an obvious suspect Radiation from above naturally suggested the Sun. Hess had already observed during a solar eclipse in the spring of 1912. The penetrating signal did not noticeably weaken as the Moon covered part of the Sun. Night flights also failed to reveal a large day–night difference. Those observations argued against a simple picture in which the whole signal was sunlight-like radiation travelling straight from the Sun. The balloon experiment established the direction of the explanation before it established the identity of the incoming radiation. Hess could see the ionization produced inside his chambers. He could not use that reading alone to sort the primary radiation into protons, nuclei or other particles. His 1936 Nobel lecture still discussed major unanswered questions about its origin. The discovery had made a new question measurable. Sources: [1] [2] ## The atmosphere became part of the detector Later experiments showed that most primary cosmic rays are fast atomic nuclei. When they strike nuclei high in the atmosphere, collisions create further particles. Some of those secondaries reach the ground and pass through our surroundings. A ground-level detector therefore sees the result of a journey through the atmosphere as well as the original radiation from space. That distinction helps explain the lasting value of Hess's method. Changing altitude changes how much atmosphere lies above an instrument. The atmosphere can absorb particles and create new ones, so a vertical series of measurements reveals processes that one reading at ground level cannot. Hess's 1912 ascent opened this route; the 1936 prize recognized his discovery of cosmic radiation. The charged fibres had turned an unexpected leak of electricity into evidence about the sky. Sources: [3] [4] ## Sources and reading notes 1. [Hess (1912), Measurements of the Penetrating Radiation during Seven Balloon Flights; translated primary report](https://arxiv.org/html/1808.02927v2) Read the instrument descriptions, the seventh-flight table and the conclusions. The table separates absolute and relative altitude. 2. [Hess (1936), Unsolved problems in physics: tasks for the immediate future in cosmic ray studies](https://www.nobelprize.org/uploads/2018/06/hess-lecture.pdf) The opening revisits the rise with altitude; the lecture then explains how much remained unknown about cosmic-ray origin. 3. [CERN, Cosmic rays: particles from outer space](https://home.cern/science/physics/cosmic-rays-particles-outer-space/) Use the primary-particle and atmospheric-secondary distinction to connect Hess's ionization to the modern particle picture. 4. [Nobel Prize in Physics 1936, official summary](https://www.nobelprize.org/prizes/physics/1936/summary/) Hess's cosmic-radiation contribution and Anderson's positron contribution received separate halves of the prize. 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