<P> The volume of interplanetary space is a nearly total vacuum, with a mean free path of about one astronomical unit at the orbital distance of the Earth . However, this space is not completely empty, and is sparsely filled with cosmic rays, which include ionized atomic nuclei and various subatomic particles . There is also gas, plasma and dust, small meteors, and several dozen types of organic molecules discovered to date by microwave spectroscopy . A cloud of interplanetary dust is visible at night as a faint band called the zodiacal light . </P> <P> Interplanetary space contains the magnetic field generated by the Sun . There are also magnetospheres generated by planets such as Jupiter, Saturn, Mercury and the Earth that have their own magnetic fields . These are shaped by the influence of the solar wind into the approximation of a teardrop shape, with the long tail extending outward behind the planet . These magnetic fields can trap particles from the solar wind and other sources, creating belts of charged particles such as the Van Allen radiation belts . Planets without magnetic fields, such as Mars, have their atmospheres gradually eroded by the solar wind . </P> <P> Interstellar space is the physical space within a galaxy beyond the influence of each star on the plasma . The contents of interstellar space are called the interstellar medium . Approximately 70% of the mass of the interstellar medium consists of lone hydrogen atoms; most of the remainder consists of helium atoms . This is enriched with trace amounts of heavier atoms formed through stellar nucleosynthesis . These atoms are ejected into the interstellar medium by stellar winds or when evolved stars begin to shed their outer envelopes such as during the formation of a planetary nebula . The cataclysmic explosion of a supernova generates an expanding shock wave consisting of ejected materials . The density of matter in the interstellar medium can vary considerably: the average is around 10 particles per m, but cold molecular clouds can hold 10--10 per m . </P> <P> A number of molecules exist in interstellar space, as can tiny 0.1 μm dust particles . The tally of molecules discovered through radio astronomy is steadily increasing at the rate of about four new species per year . Large regions of higher density matter known as molecular clouds allow chemical reactions to occur, including the formation of organic polyatomic species . Much of this chemistry is driven by collisions . Energetic cosmic rays penetrate the cold, dense clouds and ionize hydrogen and helium, resulting, for example, in the trihydrogen cation . An ionized helium atom can then split relatively abundant carbon monoxide to produce ionized carbon, which in turn can lead to organic chemical reactions . </P>

The vast majority of human activity takes place in just one atmospheric layer. which one