This article recounts the discovery and investigation of space radiation—from early
observations with electroscopes to modern satellite measurements. It describes how
scientists uncovered the origin, nature, and impact of ionizing radiation from space
on both living beings and spacecraft.
An electroscope is a scientific instrument used to detect the presence of an electric
charge. When charged, the leaves of an electroscope diverge due to electrostatic
repulsion. However, after a period of time the charge gradually dissipates and the
leaves close again. Early researchers wondered why the charge was lost. They initially
suspected that imperfect insulation or the type of gas inside might be responsible.
Despite improvements in insulation and testing with different gases, the charge still
dissipated quickly.
By the late 19th century, scientists discovered ionizing radiation. They suspected
that the ions produced when the air is ionized were neutralizing the electroscope’s
charge. Indeed, when an electroscope was placed inside a lead housing, its rate of
discharge was reduced. This finding led to a new question: what is the source of this
ionizing radiation?
Scientists originally thought that radiation was emitted solely by radioactive
materials in the ground. To test this, electroscopes were taken to high altitudes—up
high in the mountains and even atop the Eiffel Tower—in the hope that the discharge
would slow with increasing altitude.
A breakthrough came on 7 August 1912. An expedition led by Austrian physicist
Victor F. Hess lifted electroscopes in a balloon to altitudes exceeding 5000 meters.
The experiment revealed that at 600 meters the ionization was lower than at ground
level, but beyond that altitude the ionization increased dramatically—with
measurements at 5000 meters showing ionization levels four times greater than on the
ground. Hess thus concluded that the radiation originates “from above.” His results were
soon confirmed by other researchers; for example, W. Kohlhörster later found that at
9200 meters the ionization was 30 times greater than at sea level.
To further test the hypothesis, Robert Millikan conducted experiments by submerging
electroscopes in two different lakes:
Millikan’s reasoning was that 2050 meters of air should absorb the same amount of
radiation as 1.8 meters of water. In his experiment, electroscopes submerged in
Arrowhead Lake (at a depth 1.8 m greater than in Lake Muir) showed the same level of
ionization. From this, Millikan concluded that the radiation truly comes from above
rather than from the atmosphere between the lakes. He coined the term “space
radiation” for this phenomenon.
Erich Regener, a German physicist, carried out extensive measurements of ionization
both at high altitudes and deep underwater. His work provided detailed characteristics
of space radiation intensity as a function of altitude and water depth. Combined with
Millikan’s results, these studies revealed that space radiation is a ubiquitous
phenomenon whose intensity is independent of weather and arrives from every
direction.
In 1929, the invention of the Geiger–Müller (G–M) tube revolutionized radiation
detection. This simple and reliable instrument enabled scientists to detect
short-term fluctuations in cosmic radiation—something that was not possible with
long-term electroscope observations.
Using a setup of two G–M tubes separated by a thin gold plate (an excellent particle
absorber), Kohlhörster and Bothe demonstrated that coincident detections in both tubes
were possible. This experiment suggested that a charged particle could pass through
one tube, traverse the gold barrier, and then be detected by the other tube. Later,
Bruno Rossi showed that cosmic radiation can penetrate up to 1 meter of lead,
indicating that its energy far exceeds that of known types of radiation (β radiation,
for instance, can penetrate only a few millimeters of lead).
Rossi continued his experiments by arranging three G–M tubes in a triangle inside a
lead housing. With an open top, coincident detections across all tubes were rare;
however, when the top was closed, coincidences became frequent. This observation
provided evidence that cosmic radiation interacts with matter, producing secondary
radiation. Moreover, combining G–M tubes with a Wilson cloud chamber allowed
scientists to visually track the paths of cosmic rays.
In 1933, Blackett and Occhialini published research based on the observation of
particle tracks in a Wilson chamber. They discovered “particle showers”—cascades of
particles created when high-energy cosmic rays interact with matter. Because it is
highly improbable that a single cosmic particle could reach Earth’s surface without
interacting in the atmosphere, these studies raised questions about the composition
of both primary and secondary radiation. Their work eventually led to the discovery
of new elementary particles, such as the positron.
Earth’s magnetic field, though weak, extends widely and exerts a force on moving
charged particles. Scientists expected that this magnetic field would deflect space
radiation particles toward the magnetic poles, leading to different ionization rates
at different latitudes—a phenomenon known as the latitude effect.
Another prediction was the west–east asymmetry. Norwegian scientist Carl
Størmer, while studying the aurora borealis, developed a mathematical model of
charged particle motion in Earth’s magnetic field. His model indicated that
positively charged particles should predominantly arrive from the west, while
negatively charged particles should come mainly from the east.
In addition to using Wilson chambers, researchers began employing radiation-sensitive
emulsions to detect ionizing particles. One challenge was understanding the “particle
showers” observed in cosmic radiation. Discoveries such as the muon (1936) and the pi
meson (1947) followed. Because muons (which account for much of the penetrating
radiation at sea level) have very short lifetimes, they could not have traveled directly
from space. This implied that many particles detected at lower altitudes were produced
in the Earth’s atmosphere when primary cosmic rays interacted with atmospheric nuclei.
To study primary radiation directly, automated balloons equipped with emulsion
detectors were launched to altitudes above 20,000 meters, and research stations were
set up on high mountains worldwide. Analysis of these emulsions revealed that at high
altitudes most particles are protons and helium nuclei, with high-energy electrons
and heavy nucleons also present.
In 1934, Rossi conducted experiments with horizontally separated G–M tubes.
Occasionally, these detectors registered simultaneous (coincident) particle detections;
Rossi also noted that the rate of coincidences increased with altitude. Then, in
1938, Pierre Auger carried out systematic studies and found that even when
detectors were separated by about 75 meters, they still detected coincident events.
These coincidences were due to a large group of secondary particles—an air shower—
generated by a single high-energy primary cosmic ray interacting with the atmosphere.
In such events, the primary particle collides with atmospheric nuclei and triggers a
chain reaction, producing a cascade of secondary particles that reach the ground
almost simultaneously. Researchers estimated that the energy of the rare primary
particles responsible for extensive air showers can be as high as 3×10^20 eV, while
current human-made accelerators (such as the LHC) reach only about 7×10^12 eV.
In 1937, American physicist Scott E. Forbush observed that following a solar flare
the level of cosmic radiation detected on Earth decreased. This phenomenon, later
called the Forbush Decrease, unfolds as follows:
Satellite instruments revealed that Earth is encircled by belts of high-energy particles
trapped by its magnetic field. James Van Allen, who designed the instruments and analyzed
the Explorer 1 data, lent his name to these belts. The inner belt is composed mainly of
protons, while the outer belt consists primarily of electrons. Due to their intense
radiation, these belts pose a threat to spacecraft electronics.
Based on Carl Størmer’s research, scientists had predicted that charged particles might be
trapped by Earth’s magnetosphere even before the Space Age. The USA later conducted
high-altitude nuclear tests (such as the Argus and Hardtack series in 1958 and the
powerful Starfish Prime test in 1962) to determine if artificial radiation belts could be
created. These experiments confirmed that it is indeed possible to produce belts of
electrons that contaminate the space environment around Earth.
In the mid-20th century, Ludwig Biermann proposed that comet tails are formed by a stream
of particles continuously emitted by the Sun. Eugene Parker later explained that the
Sun heats its corona, causing it to expand and release its outer layers into space. These
particles, known as the solar wind, are responsible for shaping comet tails. The
theory was confirmed by the Mariner 2 mission in 1962.
Their measurements greatly enhanced our understanding of solar cosmic rays, the Sun’s
plasma and magnetic fields, and the dynamics of particles in space.
Space radiation is a critical concern for both astronauts and spacecraft electronics. To
predict the absorbed dose of ionizing radiation (which ultimately limits mission duration),
it is essential to understand the intensity of radiation in different regions of space.
Based on data from numerous satellite missions, NASA developed empirical models to estimate
the flux of trapped particles in the Van Allen belts. The most widely known models are
AE8 (for electrons) and AP8 (for protons). Despite being based on experiments from
the 1960s and 1970s, these models remain in common use for mission planning.
Following the launch of the U.S. Orbiting Geophysical Observatories (OGO) in 1964,
scientists began collecting detailed data on Earth’s magnetosphere. Their improved models
revealed that over the South Atlantic there exists a region where the magnetic field is
significantly weaker. This weakness allows cosmic particles to penetrate more deeply into
the atmosphere.
Within the South Atlantic Anomaly (SAA), cosmic particles can descend to altitudes as low as
200 km, posing a serious threat to spacecraft electronics. For example, the small British
satellite UoSat3—launched in 1989—experienced numerous electronic anomalies during its
orbit. A subsequent map of these events showed that 75% occurred while the satellite was
passing through the SAA.
