Where does the Sun's influence end? Nobody is sure. Out past the orbits of Neptune [ http://antwrp.gsfc.nasa.gov/apod/neptune.html ] and Pluto [ http://www.nineplanets.org/pluto.html ] extends a region named the heliosphere [ http://science.msfc.nasa.gov/ssl/pad/solar/heliosph.htm ] where the Sun's magnetic field [ http://science.msfc.nasa.gov/ssl/pad/solar/the_key.htm ] and particles from the Solar Wind [ http://science.msfc.nasa.gov/ssl/pad/solar/sun_wind.htm ] continue to dominate. The surface where the Solar Wind [ http://antwrp.gsfc.nasa.gov/apod/ap000318.html ] drops below sound speed [ http://antwrp.gsfc.nasa.gov/apod/ap010221.html ] is called the termination shock [ http://web.mit.edu/space/www/voyager/voyager_science/helio.review/axford.suess.html#Distance ] and is depicted as the inner oval in the above computer-generated illustration [ http://www.cs.indiana.edu/~soljourn/ ]. It is thought that this surface occurs as close as 75-90 AU [ http://neo.jpl.nasa.gov/glossary/au.html ] -- so close that a Pioneer [ http://spaceprojects.arc.nasa.gov/Space_Projects/pioneer/PNhome.html ] or Voyager [ http://voyager.jpl.nasa.gov/ ] spacecraft may soon glide through [ http://www.srl.caltech.edu/personnel/ace/recentpub/JGR_96/pap_preprint.html ] it as they exit the Solar System [ http://antwrp.gsfc.nasa.gov/apod/ap020214.html ] at about 3 AU/year. The actual contact sheet between the Sun's ions [ http://www-istp.gsfc.nasa.gov/Education/wplasma.html ] and the Galaxy's ions is called the heliopause [ http://www-pw.physics.uiowa.edu/plasma-wave/voyager/heliopr.html ] and is thought to occur at about 110 AU. It is depicted above as the middle surface. The Sun's heliopause [ http://web.mit.edu/space/www/voyager/voyager_science/helio.review/axford.suess.html ] moves through the local interstellar medium [ http://antwrp.gsfc.nasa.gov/apod/ap020210.html ] much as a boat moves on water, pushing a bow shock [ http://antwrp.gsfc.nasa.gov/apod/ap001017.html ] out in front, thought to occur near 230 AU.