The Solar System and Its Place in the Galaxy 21
FIGURE 11 Artist’s concept of the major boundaries predicted for the heliosphere and the
trajectories of the twoVoyagerspacecraft.Voyager 1crossed the termination shock in 2004.
solar magnetic field. The ions stream away from the
cometary comae at high velocity in the antisunward direc-
tion. Structures in the tail are visible as a result of fluores-
cence by CO+and other ions.
At some distance from the Sun, far beyond the orbits
of the planets, the solar wind reaches a point where the
ram pressure from the wind is equal to the external pres-
sure from the local interstellar medium flowing past the
solar system. A termination shock will develop upstream
of that point, and the solar wind will be decelerated from
supersonic to subsonic. RecentlyVoyager 1detected the
termination shock at 94 AU. Beyond this distance is a re-
gion still dominated by the subsonic solar plasma, extending
out another 30–50 AU or more. The outer boundary of this
region is known as the heliopause and defines the limit be-
tween solar system–dominated plasma and the interstellar
medium. It is not currently known if the flow of interstellar
medium past the solar system is supersonic or subsonic. If it
is supersonic, then there must additionally be a bow shock
beyond the heliopause, where the interstellar medium en-
counters the obstacle presented by theheliosphere. A di-
agram of the major features of the heliosphere is shown in
Fig. 11.
TheVoyager 1and 2 spacecraft, which are currently leav-
ing the planetary region on hyperbolic trajectories, continue
to study the outermost regions of the heliosphere.Voyager 1
is currently at 100.4 AU (as of September 2006) andVoy-
ager 2is at 80.7 AU. TheVoyagerspacecraft are expected to
continue to send measurements until the year 2015, when
they are expected to be at about 130 and 106 AU from the
Sun, respectively.
To many planetary scientists, the heliopause defines
the boundary of the solar system because it marks the
changeover from a solar wind to an interstellar medium
dominated space. However, as already noted, the Sun’s grav-
itational sphere of influence extends out much farther, to
∼ 2 × 105 AU, and there are bodies in orbit around the
Sun at those distances. These include the Kuiper belt and
scattered disk, which may each extend out to∼ 103 AU (pos-
sibly even farther for the scattered disk), and the Oort cloud
which is populated to the limits of the Sun’s gravitational
field.