The space between the planets of the solar system is not an empty void. It is permeated by thinly scattered matter called the interplanetary medium. The material components of the interplanetary medium consist of neutral hydrogen, the solar wind (discussed in detail in the Sun section), cosmic rays, and dust particles.
Extremely small amounts of neutral (nonionized) hydrogen have been detected throughout much of interplanetary space. At the distance of Earth's orbit from the Sun, for example, the concentration of neutral hydrogen is about one atom per 100 cubic cm (6 cubic inches).
Some of the neutral hydrogen that enters the solar system from interstellar space is ionized by sunlight and by charge exchange with the plasma of the solar wind emanating from the Sun.
Those cosmic rays detected in the vicinity of Earth comprise high-speed, high-energy atomic nuclei and electrons. Among the nuclei, the most abundant are hydrogen nuclei (protons; 90 percent) and helium nuclei (alpha particles; 9 percent). Nuclei outnumber electrons about 50 to 1. A minority of cosmic rays are produced in the Sun, especially at times of increased solar activity. The origin of those coming from outside the solar system—called galactic cosmic rays—remains to be conclusively identified, but they are thought to be produced in stellar processes such as supernova explosions.
Relatively small amounts of dust particles, often called micrometeoroids, exist in the solar system, most of which appear to be orbiting the Sun in or near the plane of the solar system. This dust consists of small grains, generally less than a few hundred micrometres in size and composed of silicate minerals and glassy nodules but sometimes including sulfides, metals, other minerals, and carbonaceous material, in orbit around the Sun. Spacecraft have detected these particles nearly as far out in space as the orbit of Uranus, which indicates that the entire solar system is immersed in a disk of dust, centred on the ecliptic plane.
The existence of interplanetary dust particles was first deduced from observations of zodiacal light, a glowing band visible in the night sky that comprises sunlight scattered by the dust. The zodiacal light is seen in the west after twilight and in the east before dawn, being easily visible in the tropics where the plane of the zodiac, or ecliptic, is approximately vertical. In mid-northern latitudes it is best seen in the evening in February and March and in the morning in September and October. The zodiacal light can be followed visually along the ecliptic from a point 30° from the Sun to about 90°. Photometric measurements indicate that the band continues to the region opposite the Sun where a slight enhancement called the gegenschein, or counterglow, is visible. There is some zodiacal light in all parts of the sky.
Every object in the solar system can produce dust by outgassing, cratering, volcanism, or other processes. Most interplanetary dust is believed to come from the surface erosion and collisions of asteroids, as well as from comets, which give off gas and dust when they travel near the Sun.
The orbits of interplanetary dust particles are easily altered by interaction with the light and charged particles (solar wind) that emanate from the Sun. The smallest particles, less than 0.5 micrometre (0.00002 inch) in size, are blown out of the solar system. Drag effects from sunlight and the solar wind cause larger particles to spiral toward the Sun, some on paths that intercept planets or their moons.
Considered in the context of their collisions with other objects in space, interplanetary dust particles are frequently called micrometeoroids. Because of their high speed (in the tens of kilometres per second), micrometeoroids as small as a few hundred micrometres in size pose a significant collision hazard to spacecraft and their payloads. An impact can, for example, puncture a vital component or create a transient cloud of ions that can short-circuit an electrical system. Consequently, protection against micro-meteoroid impacts has become a necessary element of space hardware design. Components of the Earth-orbiting International Space Station use a "dust bumper," or Whipple shield (named for its inventor, the American astronomer Fred Whipple), to guard against damage from micrometeoroids and orbiting debris. Spacesuits intended for extravehicular activity, which is activity that takes place outside a module or space station, also incorporate micrometeoroid protection in their outer layers.
Analyses of micrometeoroid pitting on Earth-orbiting satellites indicate that about 30,000 tons a year of interplanetary dust strike Earth's upper atmosphere, mostly particles between 50 |am and 1 mm (0.002-0.04 inch) in size. Particles from space larger than a few hundred micrometres—i.e., meteoroids—are heated so severely during deceleration in the atmosphere that they vaporize, producing a glowing meteor trail. Smaller particles experience less severe heating and survive, eventually settling to Earth's surface. When found in Earth's atmosphere or on its surface, they are often referred to as micrometeorites or cosmic dust particles.
Using high-altitude research aircraft, the U.S. National Aeronautics and Space Administration (NASA) has collected cosmic dust particles directly from Earth's stratosphere, where the concentration of terrestrial dust is low. Particles larger than 50 |am are relatively uncommon there, however, which makes their collection by aircraft impractical. These larger particles have been collected in sediment that has been filtered from large volumes of melted polar ice. Spacecraft missions have been developed to retrieve dust particles directly from space. The U.S. Stardust spacecraft, launched in 1999, flew past Comet Wild 2 in early 2004, collecting particles from its coma (the cloud around a comet formed by evaporating ice) for return to Earth. In 2003 Japan's space agency launched its Hayabusa spacecraft to return small amounts of surface material, comprising fragments and dust, from the near-Earth asteroid Itokawa for laboratory analysis.
Some cosmic dust particles gathered from the stratosphere are the least altered samples of early solar system dust that have been studied in the laboratory. They provide clues to the temperature, pressure, and chemical composition of the nebular cloud from which the solar system condensed 4.6 billion years ago. The continuous accretion of micrometeorites on early Earth may have contributed organic compounds that were important for the development of life. A few micrometeorites are thought to contain preserved interstellar grains—samples of matter from outside the solar system. Spacecraft sample-return missions to comets and asteroids should provide scientists on Earth the opportunity to study even better-preserved material from the birth of the solar system.
There is also a nonmaterial component to the interplanetary medium. The magnetic field lines that are carried outward from the Sun by the solar wind remain attached to the Sun's surface. Because of the Sun's rotation, the lines are drawn into a spiral structure. Closely associated with the interplanetary magnetic field are electric forces that act to attract or repel charged particles.
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