After completing their primary planetary exploration missions, several NASA spacecraft are now traveling through the vast reaches of space over which the sun's extended atmosphere and magnetic field dominate. This region, called the heliosphere, is probably many times the diameter of the orbit of the outermost planet of the solar system, Pluto. The outer boundary of the heliosphere is formed when the interstellar gases and magnetic fields exert more pressure then the gases and magnetic field associated with the sun, forming the vast cavity of the heliosphere within. One of these spacecraft will most likely be the first spacecraft to go beyond the heliosphere and become the first interstellar traveler.