What are cosmic rays?

Cosmic rays are invisible, high-energy particles that constantly bombard Earth from all directions. Most cosmic rays are protons moving at extremely high speeds, but they can be atomic nuclei of any known element. They enter Earth's atmosphere at velocities of 90 percent the speed of light or more.

Am I getting hit by cosmic rays?

Everyone is being struck by cosmic rays all the time—probably about several each second. Ordinarily, the number of cosmic rays that strike you have no negative effect on your health. Even though the energy of these particles is very high, the number of them striking you is relatively low. If you went beyond Earth's magnetosphere, though, your health might be at risk. On Earth's surface, the magnetosphere acts as a shield against cosmic rays by redirecting them toward Earth's magnetic poles. Thousands of miles up, however, the cosmic ray flux on your body would be much higher, and thus cause potentially more damage to your body's cells and systems.

Who first discovered cosmic rays?

The Austrian-American astronomer Victor Franz Hess (1883-1964) became interested in a mysterious radiation that scientists had found in the ground and in Earth's atmosphere. This radiation could change the electric charge on an electroscope—a device used to detect electromagnetic activity—even when placed in a sealed container. Hess thought that the radiation was coming from underground and that at high altitudes it would no longer be detectable. To test this idea, in 1912 Hess took a series of high-altitude, hot-air balloon flights with an electroscope aboard. He made ten trips at night, and one during a solar eclipse, just to be sure the Sun was not the source of the radiation. To his surprise, Hess found that the higher he went, the stronger the radiation became. This discovery led Hess to conclude that this radiation was coming from outer space. For his work on understanding cosmic rays, Hess received the Nobel Prize in physics in 1936.

How were cosmic rays shown to be charged particles?

In 1925 American physicist Robert A. Millikan (1868-1953) lowered an electroscope deep into a lake and detected the same kind of powerful radiation that Victor Franz Hess had found in his balloon experiments. He was the first to call this radiation cosmic rays, but he did not know what they were made of. In 1932, the American physicist Arthur Holly Compton (1892-1962) measured cosmic-ray radiation at many points on Earth's surface and found that it was more intense at higher latitudes (toward the North and South Poles) than at lower latitudes (toward the equator). He concluded that Earth's magnetic field was affecting the cosmic rays, deflecting them away from the equator and toward Earth's magnetic field. Since electromagnetism was now shown to affect the rays, it was clear that cosmic rays had to be electrically charged particles.

Where do cosmic rays come from?

A continuous stream of electrically charged particles flows from the Sun; this flow is called the solar wind. It makes sense that some fraction of cosmic rays originate from the Sun, but the Sun alone cannot account for the total flux of cosmic rays onto Earth's surface. The source for the rest of these cosmic rays remains mysterious. Distant supernova explosions could account for some of them; another possibility is that many cosmic rays are charged particles that have been accelerated to enormous speeds by interstellar magnetic fields.

What is a meteorite?

A meteorite is a large particle from outer space that lands on Earth. They range in size from a grain of sand on up. About 30,000 meteorites have been recovered in recorded history; about 600 of them are made primarily of metal, and the rest are made primarily of rock.

What is a meteor?

A meteor is a particle from outer space that enters Earth's atmosphere, but does not land on Earth. Instead, the particle burns up in the atmosphere, leaving a short-lived, glowing trail that traces part of its path through the sky. Like meteorites, meteors can range from the size of a grain of sand on up; most of the time, though, a meteor larger than about the size of a baseball will reach Earth, in which case we call it a meteorite.

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