Our solar system moves through the interstellar medium of gases and plasma (ionized gas) and creates a magnetic bubble that
we call the heliosphere. (Image from NASA/Walt Feimer.) As the wind of plasma emanating from our Sun (solar wind) pushes
outward it is abruptly slowed down and its density increases at the so-called termination shock at a distance around 80–100 astronomical units (AU) from the Sun (Earth–Sun distance is 1 AU by definition). Outside the termination shock is a turbulent region
called the heliosheath bounded by the heliopause where pressure balance with the interstellar medium is achieved. Outside the
heliopause, an outermost bow shock is believed to exist where the incoming interstellar flow is slowed, heated, and diverted. To
put the gigantic scales of the heliosphere in proportion, the orbit of Pluto is 40 AU—less than half the distance to the termination
shock, and Earth’s orbit is a mere dot in the center. In December 2004, APL scientists reported evidence for Voyager 1 crossing the
termination shock at approximately 94 AU (Decker et al., “Voyager 1 in the Foreshock, Termination Shock, and Heliosheath,”
Science [2005]). The increased intensity of ions and electrons, together with a dramatic drop in solar wind flow and an increase in
the magnetic field, was the convincing evidence needed for confirming that Voyager 1 had at last crossed into the uncharted territories toward interstellar space.