The National Aeronautical and Space Administration's (NASA) unmanned spacecraft, Pioneer 10, was expected to leave the outer reaches of our solar system on June 13, 1983. (The division between the solar system and outer space is defined as the edge of the sheath of influence of the solar wind; this sheath is called the heliosphere.) On its way to space beyond our solar system, Pioneer 10 will pass close enough to Neptune and Uranus to obtain data which may be useful in determining whether or not “Planet X” exists. Planet X is the hypothetical cause of Neptune's and Uranus' perturbed motion, so far unexplained by earth observation. In addition, as it leaves the heliosphere, scientists hope Pioneer 10 will map the apex of the solar wind teardrop.