<P> Interplanetary space, the space around the Sun and planets of the Solar System, is the region dominated by the interplanetary medium, which extends out to the heliopause where the influence of the galactic environment starts to dominate over the magnetic field and particle flux from the Sun . Interplanetary space is defined by the solar wind, a continuous stream of charged particles emanating from the Sun that creates a very tenuous atmosphere (the heliosphere) for billions of kilometers into space . This wind has a particle density of 5--10 protons / cm and is moving at a velocity of 350--400 km / s (780,000--890,000 mph). The distance and strength of the heliopause varies depending on the activity level of the solar wind . The discovery since 1995 of extrasolar planets means that other stars must possess their own interplanetary media . </P> <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 magnetic particles such as the Van Allen radiation belt . 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 will generate 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>

International space station is in which atmospheric layer