Even outside the solar system, space is not truly empty but is filled with waves of electromagnetic radiation and with the interstellar wind. Scientists have theorized for years that the solar wind will reach a point, far past Pluto, where it is so cool and weak that it can no longer expand against the pressure of the interstellar wind. The surface that bounds the solar wind is called the heliopause.

The heliopause was thought to be located at 110—160 AU from the Sun, based on models that used guesses for the pressure of the interstellar wind (that pressure is unknown). Now it has been found that the shock where the solar wind strikes the interstellar wind creates a hiss in radio frequencies.

The intrepid Voyager 1, as it moved farther and farther from Earth, heard this hiss. Based on the radio sounds, it is now estimated that the heliopause is 90—120 AU from the Sun. After traveling through space for more than 26 years, Voyager 1 reached 90 AU from the Sun on November 5, 2003 (ca. 8.3 billion miles, or 13.5 billion kilometers). Voyager 1 is the only spacecraft to have made measurements in the solar wind at such a great distance, and it is the most distant man-made object in the Universe. Voyager 2 is close behind, also on its way out of the solar system.

Voyager 1 is moving away from the Sun at about 3.6 AU per year, 35° out of the ecliptic plane (the plane of Earth's orbit) to the north, in the general direction of the solar apex (the direction of the Sun's motion relative to nearby stars). Voyager 2 is also escaping the solar system, at a speed of about 3.3 AU per year, 48° out of the ecliptic plane to the south. Each spacecraft is expected to be able to communicate with Earth until about 2020. On NASA's excellent Voyager mission Web site (http://voyager.jpl.nasa.gov) weekly reports on each spacecraft are posted. During the first week of 2004, each spacecraft had about 66 pounds (30 kg) of propellant left. Voyager 1 was 90.1 AU from the Sun (8.3 billion miles, or 13.5 billion kilometers), and Voyager 2 was 71.8 AU from the Sun (6.6 billion miles, or 10.7 billion kilometers). A year later, Voyager 1 had attained a distance of 94.3 AU and Voyager 2 75.4 AU. Communications with the spacecraft are accomplished using the worldwide Deep Space Network, an international coalition of giant radar dishes.The spacecraft are now so far away that the dishes 110 feet (34 m) in diameter cannot reach them, and only the three largest, 230-foot (70-m) dishes can make the transmissions. These huge dishes are located 120° apart around the world—in Canberra, Australia; near Madrid, Spain; and near Barstow, California—to make continuous radio communication possible as the Earth rotates.Transmissions take about 10 hours to reach the spacecraft, even though the transmissions are traveling at the speed of light, and success requires that the dish be pointed exactly at the spacecraft, billions of kilometers away in space.The success of the Voyager missions is truly one of mankind's most astonishing and exciting accomplishments.

Soon the Voyager crafts are expected to come to the termination shock, which is the point inside the solar system that the solar wind is radically slowed by the external pressure of the heliopause (see the figure below). At the termination shock, the solar wind slows from

Different parts of the interaction between the Sun's magnetic field and the interstellar wind are shown schematically: the solar wind termination shock, heliopause, and the bow shock.

supersonic to subsonic speeds (from about 276 to 70 miles per second, or 440 to 110 kilometers per second), the direction of the solar wind changes and may become chaotic, and the magnetic field also changes directions.

The spacecraft are expected to encounter powerful, short-wavelength magnetic field disruptions, of increasing amplitude toward the termination shock. Ions should be accelerated violently in this compressed, oscillating magnetic field. Some researchers argue that Voyager 1 has already reported these conditions and that it passed into the termination shock at 85 AU, on August 1, 2002, though these findings are in contention, since subsequently the solar wind around Voyager 1 seems to have returned to supersonic speeds and smoothness. Could this have been some sort of pretermination shock or a local termination storm? Though the data analysis is ongoing, the best estimates for the distance from the Sun to the termination shock is between 80 and 90 AU, and so some scientists believe that Voyager 1 has encountered the termination shock and is in the process of leaving the solar system, while Voyager 2 will begin the process soon.

Between the exits of Voyagers 1 and 2 and the close inspection of the SOHO mission and other solar orbiters, humankind now has instruments inspecting the Sun over its entire range of influence. Because scientists are now watching solar activity continuously, coronal mass ejections and other events that affect the Earth are seen long before they strike the Earth's magnetosphere. Local news stations are even connecting to this information source and adding space weather to their forecasts; some stations in Massachusetts and elsewhere now warn about impending electrical disturbances and possibly lively auroral displays. The careful analysis of the increasing quantities of high-quality solar data is rapidly driving knowledge about the Sun forward.

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