<P> 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). Interplanetary space 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 . The distance and strength of the heliopause varies depending on the activity level of the solar wind . </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 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 each star has upon the encompassed 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 that further enrich the medium . 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>

Who studies outer space and the celestial bodies beyond earth