Hess Discovers Cosmic Rays Summary

  • Last updated on November 10, 2022

Victor Franz Hess pioneered dangerous balloon experiments that revealed that air ionization increases with altitude, indicating the existence of cosmic rays originating outside Earth and its solar system.

Summary of Event

As the twentieth century began, a series of discoveries of surprising radiations marked a new direction in physics. In 1895, Wilhelm Conrad Röntgen had created X rays within an evacuated tube in his laboratory, and then Antoine-Henri Becquerel discovered radioactive rays emitted from elements in the earth; this discovery led Marie and Pierre Curie to discover radium. Victor Franz Hess undertook a search for the sources of radioactivity, and this led him, instead, to discover a new and more puzzling set of radiations—cosmic rays—that had their origins outside Earth and beyond the Sun. Cosmic rays Radioactivity Air ionization Ionization [kw]Hess Discovers Cosmic Rays (Aug. 7 and 12, 1912) [kw]Cosmic Rays, Hess Discovers (Aug. 7 and 12, 1912) [kw]Rays, Hess Discovers Cosmic (Aug. 7 and 12, 1912) Cosmic rays Radioactivity Air ionization Ionization [g]Austria;Aug. 7 and 12, 1912: Hess Discovers Cosmic Rays[03160] [c]Science and technology;Aug. 7 and 12, 1912: Hess Discovers Cosmic Rays[03160] [c]Physics;Aug. 7 and 12, 1912: Hess Discovers Cosmic Rays[03160] Hess, Victor Franz Exner, Franz Millikan, Robert Andrews Anderson, Carl David

In 1910, Hess received his Ph.D. from the University of Graz. He committed himself to understanding a set of strange atmospheric charging effects. As an assistant at the Institute of Radium Research of the Vienna Academy of Science, he joined a group working on atmospheric electricity under the leadership of Franz Exner and Ergon von Schweidler. Hess performed his experiments during ten dangerous balloon flights that ultimately proved the extraterrestrial origin of cosmic rays.

Victor Franz Hess.

(The Nobel Foundation)

At the beginning of the twentieth century, scientists were presented with a puzzling fact: A tightly sealed electroscope slowly developed an electric charge even when all sources of electrical leakage were carefully eliminated. Scientists wondered if the strange charging was merely a nuisance or if it pointed to some new science awaiting discovery. The charging diminished slightly when the electroscope was covered with thick shielding. If the charging was real, a very penetrating radiation might produce it by colliding with the trapped air inside the sealed electroscope and releasing electrons. The charging was as strong over the oceans as over the land; therefore, radioactivity from rocks could not be the source of the penetrating rays. As the land and oceans had been eliminated as the source, scientists reasoned that perhaps the rays came from the skies. They made measurements high up on the Eiffel Tower and in primitive balloon flights and found little change in the charging. If the sky was the source, then the source was to be found at quite a high altitude.

In 1910, Hess undertook study of this problem and decided on a strategy to attain greater heights. The electroscope was the basic instrument used to study the strange charging. Electrons were removed carefully from the air before a small amount was sealed in the electroscope. Initially, the sealed air was an excellent insulator, but in time conduction appeared, pointing to the presence of free electron charges in the electroscope and the charging effect. Hess designed special electroscopes with the aid of the Vienna Academy of Science and enlisted the help of the Austrian Aeroclub. Hess had calculated that a height greater than one and one-half times that of the Eiffel Tower would be needed to begin to show a real dependence of charging on altitude and that much greater altitudes would be necessary to distinguish a different sky source from ground radiation. He requested that the Aeroclub provide him with high-altitude balloon flights. The electroscopes were designed for very low temperatures and high pressures. The Aeroclub agreed. Hess gathered daring companions and, in 1911, undertook his series of dangerous balloon experiments.

Hess began his next-to-last flight with two companions on August 7, 1912, from the vicinity of Aussig, Austria. He monitored his three electroscopes, one companion navigated, and the other checked altitude and temperature. After rising for two and one-half hours, the balloon drifted between 4,000 and 5,000 meters (approximately 13,100-16,400 feet) for one hour more, covering 200 kilometers (about 124 miles) to Pieskew, Germany, near Berlin. At the highest altitude, Hess’s electroscopes were charging four times faster than at ground level. He believed the radiations responsible for the charging effects were coming from outside Earth.

Hess made his next ascent to determine whether the extraterrestrial rays originated in the Sun. He scheduled the balloon flight for August 12 to coincide with a solar eclipse. He took the balloon to 3,000 meters (about 9,800 feet) and found no reduction in the intensity of the rays as the Sun disappeared from view. This eliminated the Sun as a major source of the cosmic rays. The August 12 flight marked the end of the historic set of the ten flights, half of which were carried out at night. Hess summarized his conclusions in a paper published that November: “The results of my observations are best explained by the assumption that a radiation of very great penetrating power enters our atmosphere from above.”

The radiation that Hess found decreased with distance near Earth’s surface but began to increase at about 150 meters (492 feet) above Earth. At an altitude of about 5 kilometers (3.1 miles), the radiation causing the electroscope discharge was several times stronger than at Earth’s surface. The radiation was the same both day and night and was unaffected by the solar eclipse. There was less air at higher altitudes to block the rays. The rays were cosmic.

Confirmation by other scientists came slowly. Werner Kolhörster Kolhörster, Werner found that the unknown rays were twelve times as intense at an altitude of 10 kilometers (6.2 miles) in a balloon as on Earth’s surface. Nevertheless, general acceptance was still slow. Many scientists could not envision rays coming from outer space. In 1913, Hess founded the meteorological station at Hoch Obir to continue his radiation studies, where he worked until World War I intervened. After the war, Hess traveled to the United States and set up the research laboratory of the U.S. Radium Corporation in New York. Also, he served as a consultant to the U.S. Bureau of Mines and established contacts with American scientists. Two years after his 1923 return to Austria, Hess acquired his own laboratory at Graz University. In 1925, Robert Andrews Millikan acknowledged full acceptance of Hess’s pioneering efforts by labeling the unknown radiation “cosmic rays.”

Hess enlisted international aid from the Rockefeller Foundation, the Vienna Academy of Science, and the Prussian Academy of Science, as well as the Emergency Society for German Sciences, to begin the now-famous Hafelekar Spitze Observatory for cosmic-ray research near Innsbruck, Austria. He found a small daily variation in the intensity of cosmic rays, which pointed to a small fractional contribution to the overall cosmic-ray intensity of the Sun. His accumulated data indicated the origin of cosmic rays to be beyond our galaxy.


With the recognition that cosmic rays were real, physicists became aware that they had an extraordinary tool for investigating not only outer space but also the innermost parts of matter. Evidence accumulated that cosmic rays possess extraordinary energy, and small particles require large energies for their production. Carl David Anderson was one of the first to put these extreme energies to use to produce a new particle that had existed only in the dreams of theorists.

Physicists often measure radiation energy in units of volts. This unit measures the energy of an electron or a proton, which has the same value of charge as the electron. For example, visible light requires a particle energy of several volts to generate the light, while at 50,000 volts, the particle could generate penetrating X rays.

Anderson knew that cosmic rays possess extraordinary energies, although he did not know the full extent of these energies. He enlisted the rays in tracking down the positron, Positrons the positively charged twin of the tiny electron. He photographed a vapor-filled vessel located in a large magnetic field while exposing the contents to Hess’s ever-present cosmic rays. His discovery of the positron came in a cloud-chamber picture of the collision debris left when the antiparticle was hit by a 500-million-volt cosmic ray. No other instrument available at that time could generate such energies.

The vital connection between Hess’s work and the discovery of the positron was made clear to the public by Anderson. He praised Hess’s work on cosmic rays and stated, “All of our knowledge of interstellar space must for some time continue to proceed from further exploration of the cosmic ray.”

A wide range of new experimental techniques point to supernovas, Supernovas which occur about every hundred years in Earth’s galaxy. A supernova, the explosion of a massive star at the end of its life, throws off energetic particles, mainly protons, which are the core or nucleus of hydrogen, the lightest atom, and alpha particles, which are the nucleus of the next lightest atom, helium. There is also a small component of many heavier atomic nuclei. All these particles are born with very high energies and can acquire more energy in their long journey within galaxies. Gamma rays, which are very energetic X rays, are formed also by the cosmic rays when they encounter magnetic fields in space, near or far from a supernova. The cosmic rays generated by supernovas have almost unimaginable energies and pervade the galaxy throughout its life, raining down on any planets within.

Other cosmic rays may originate in the neutron star that is often born in the remains of a supernova explosion if the neutron star has a companion star from which it can draw matter. Whatever the exact source, the cosmic rays, particles and gammas, are space relics that can tell scientists much about the unknowns astronomers encounter in their journeys if they can be deciphered with ever-improving equipment. Thus the discoveries and puzzles prompted by the identification of cosmic rays continued into the twenty-first century. Cosmic rays Radioactivity Air ionization Ionization

Further Reading
  • citation-type="booksimple"

    xlink:type="simple">Auger, Pierre. What Are Cosmic Rays? Translated by Maurice M. Shapiro. Rev. ed. Chicago: University of Chicago Press, 1945. Popular book by a well-known authority presents fascinating investigations of cosmic rays. The physics is clearly and simply described. Discusses Hess’s contributions in chapter 1.
  • citation-type="booksimple"

    xlink:type="simple">Grieder, Peter K. F. Cosmic Rays at Earth: Researcher’s Reference Manual and Data Book. New York: Elsevier Science, 2001. Reference manual intended for use by scientists involved in cosmic-ray research includes background material that may be useful to lay readers with some science background.
  • citation-type="booksimple"

    xlink:type="simple">Hess, Victor F. The Electrical Conductivity of the Atmosphere and Its Causes. Translated by L. W. Codd. New York: D. Van Nostrand, 1928. Hess’s own work may interest readers with science background.
  • citation-type="booksimple"

    xlink:type="simple">Hillas, A. M. Cosmic Rays. New York: Pergamon Press, 1972. Presents Hess’s pioneering efforts and, most interesting, an excerpt of his 1912 paper that reviews his balloon research results. Aimed at readers with physics background, but chapter 1 presents a review without mathematics. Includes original papers by colleagues of Hess.
  • citation-type="booksimple"

    xlink:type="simple">Motz, Lloyd, and Jefferson Hane Weaver. The Story of Astronomy. New York: Plenum, 1995. Presents the history of astronomy from ancient times to the end of the twentieth century. Chapter 18 includes discussion of Hess’s work and the discovery of cosmic rays. Features bibliography and index.
  • citation-type="booksimple"

    xlink:type="simple">Pasachoff, J. M. Contemporary Astronomy. Philadelphia: W. B. Saunders, 1977. Undergraduate text in astronomy requires basic knowledge of mathematics and physics. Provides a general review of the universe, the stars, and the solar system. Chapter 8 includes a section on cosmic rays, their nature, and their probable origins in supernovas.
  • citation-type="booksimple"

    xlink:type="simple">Rossi, Bruno. Cosmic Rays. New York: McGraw-Hill, 1964. Readable and authoritative book by a well-known physicist who worked with cosmic rays. Chapter 1 gives fascinating details on Hess’s measurements and ballooning experiments. Although dated, a good introduction to the study of cosmic rays.

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