The Distant Frontier

All of the planets are immersed in the hot gale that blows from the Sun. It moves past the planets and beyond the most distant comets to the very edge of the Solar System, creating the heliosphere, a vast region centered on the Sun and enclosed by the interstellar medium. Within the heliosphere, physical conditions are dominated, established, maintained, modified and governed by the Sun.

tte solar wind becomes increasingly rarefied as it spreads out into space. By the time it has reached the Earth's orbit, there are about 5 million protons and 5 million electrons per cubic meter in the solar wind, which is nearly a perfect vacuum by terrestrial standards. As it moves into a greater volume, the density of the solar wind decreases even further, as the inverse square of the distance from the Sun, and eventually blends with the gas between the stars.

How far does the Sun's influence extend, and where does it all end? Somewhere out there the solar wind becomes too dispersed to continue pushing with sufficient vigor against the interstellar medium, no longer dense or powerful enough to repel the ion-

FIG. 6.17 Edge ofthe Solar System Voyager 1 and 2 spacecraft, located at a distance of about 90 AU and 70 AU, approach the place where the Solar System ends and interstellar space begins. One AU is the mean distance between the Earth and the Sun, and the edge of the Solar System is located at roughly 100 times this distance. At the termination shock, the supersonic solar wind abruptly slows from an average speed of 400 kilometers per second to less than one quarter that speed. Beyond the termination shock is the heliosheath, a vast region where the turbulent and hot solar wind is compressed as it presses outward against the interstellar wind. The edge of the Solar System is found at the heliopause, where the pressure of the solar wind balances that of the interstellar medium. A bow shock likely forms as the interstellar wind approaches and is deflected around the heliosphere, forcing it into a teardrop-shaped structure with a long, comet-like tail. (Courtesy of JPL and NASA.)

ized matter and magnetic fields coursing between the stars, tte radius of this celestial standoff distance, in which the pressure of the solar wind falls to a value comparable to the interstellar pressure, has been estimated at about 100 AU, or one hundred times the mean distance between the Earth and the Sun.

Instruments aboard the twin Voyager 1 and 2 spacecraft, launched in 1977 and now cruising far beyond the outermost planets, are approaching this edge of the Solar System (Fig. 6.17). In 2005, scientists announced that Voyager 1 had crossed the termination shock, where the pressure of the interstellar gas slows the outward supersonic flow from the Sun, terminating the solar wind's power. At about 90 AU from the Sun, the Voyager 1 instruments recorded a sudden increase in the strength of the magnetic field carried by the solar wind, as expected when the solar wind slows down and its particles pile up at the termination shock.

Voyager 1 has therefore crossed into the vast, turbulent heliosheath, the region where the interstellar gas and solar wind start to mix. Both Voyager spacecraft are equipped with plutonium power sources expected to last until 2020. So Voyager 2, at about 70 AU in 2005, should record the termination shock in the future, and both spacecraft ought to eventually measure the heliopause, at the outer edge of the heliosheath. It is the place where the Solar System ends and interstellar space begins.

tte motion of the interstellar gas, with its own wind, compresses the heliosphere on one side, producing a non-spherical shape with an extended tail (Fig. 6.17). A bow shock is formed when the interstellar wind first encounters the heliosphere; just as a bow shock is created when the solar wind strikes the Earth's magnetosphere. And the graceful arc of a bow shock, created by an interstellar wind, has been detected around the young star LL Orionis (Fig. 6.18).

Closer to home, space physicists are concerned about the impact of powerful solar eruptions on the Earth's environment in space.

FIG. 6.18 Stellar bow shock A crescent-shaped bow shock is formed when the material in the fast wind from the bright, very young star, LL Ori (center) collides with the slow-moving gas in its vicinity, coming from the lower right. The stellar wind is a stream of charged particles moving rapidly outward from the star. It is a less energetic version of the solar wind that flows from the Sun. A second, fainter bow shock can be seen around a star near the upper right-hand corner of this image, taken from the Hubble Space Telescope. Both stars are located in the Orion Nebula; an intense star-forming region located about 1,500 light-years from the Earth. (Courtesy of NASA, the Hubble Heritage Team, STScI, and AURA.)

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