INTERPLANETARY DUST
The space between the planets is a vacuum by terrestrial standards, but it is not devoid of some interplanetary gas and small solid particles called interplanetary dust. The particulate matter consists of particles blown out from the sun's atmosphere by the solar wind, micrometeoric debris scattered by comets, anc~ perhaps less plentiful granular powder strewn about by asteroid and meteoroid collisions.
We have learned about interplanetary dust from several sources. One is the zodiacal light, which' is most easily observed in our Northern Hemisphere in spring after sundown in the west and in fall before dawn in the east. It appears as a faint pyramidal band of light tapering upward from the horizon along the line of the ecliptic. The spectrum of zodiacal light is a faint replica of the solar spectrum; it is produced by small particles lying in the plane of the planets' orbits that scatter solar photons in our direction.
Most direct evidence of interplanetary dust comes to us from spacecraft experiments. Electronic sensors on the skin of the spacecraft are arranged to count small dust particles as they strike the su rface. From the numbers of impacts it is estimated that the average spacing between interplanetary dust particles is many meters. The total mass of dust particles is estimated to be about 1020 grams, or about a hundred-millionth of the mass of the earth.
INTERPLANETARY GAS
Most interplanetary matter is in the form of an ionized gas comprising the solar wind. It consists of an almost continuous stream of particles, mostly protons and electrons, flowing out from the sun's corona. As the solar wind moves forward, it forms an expanding spiral pattern due to the sun's rotation, and its velocity increases until, several solar radii from the sun, it equals the speed of sound in the plasma. Its velocity continues to increase as it flows outward, much as rocket gases are accelerated to supersonic velocities in a rocket nozzle. Near the earth the solar wind reaches a velocity of about 400 kilometers per second. Beyond the earth its speed remains very nearly constant.
At earth's distance the wind's density is down to about five protons and five electrons per cubic centimeter on the average, but it can rise on occasion to 100 particles per cubic centimeter.
