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Year in Space Calendar
  

Space Topics: Voyager

The Interstellar Mission

When Voyager 2 passed by Neptune in 1989, the mission had run out of planets to visit.  But both spacecraft were on courses that would eventually escape the solar system entirely.  In 1990, the Voyager mission was extended again, and the planetary explorers turned into interstellar explorers, and the Voyager Interstellar Mission began.

While the Voyagers were beyond the orbits of all the planets, they were still and are still well within the realm of the Sun’s magnetic field or heliosphere. Often described as a “bubble” blown up by a supersonic, solar wind of electrically-charged particles coming from the Sun’s atmosphere, the heliosphere extends far beyond the orbit of the outermost planet, which sometimes is Pluto and sometimes Neptune, and out to the farthest reaches of the Sun’s magnetic field.

Scientists believe that the heliosphere’s boundary or heliopause – the region between the heliosphere and interstellar space, where the spacecraft are headed now -- fluctuates with the varying strengths of the solar wind during the 11-year cycle of the Sun’s maximum and minimum atmospheric flux. Beyond the heliosphere is interstellar space, where particles from exploded stars and other materials float and soar.

Voyagers at the edge of the solar system
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Voyagers at the edge of the solar system
Our solar system exists inside a heliosphere, a bubble created by the outward flow of the solar wind. The region that separates our system from interstellar space is the heliopause. In between these is the termination shock, where the solar wind slows from supersonic to subsonic speeds. Credit: JPL / NASA

The Voyager spacecraft have four instruments that can measure the solar wind: the plasma subsystem; the magnetometer, which measures the magnetic fields carried out into interplanetary space by the solar wind; the planetary radio astronomy subsystem; and the ultraviolet spectrometer subsystem. Where the mission once commanded personnel numbering around 600, only 10 scientists and engineers now man the mission. Continuing funding cuts may further reduce staff, leaving those who remain to figure new and perhaps riskier ways to carry on.

On December 17, 2004, Voyager 1 reached the “termination shock,” the beginning of the heliopause. (As the solar wind approaches the heliopause it must slow down from millions to 400,000 kilometers per hour (250,000 miles per hour); this abrupt change is the termination shock.) Five to 20 years after reaching the termination shock, Voyager 1 should reach the heliopause.  But it’s a race against time. If they make it -- and we still have contact before their power is spent -- they could return data not only from the heliopause, but perhaps taste the interstellar space environment to reveal some of the particles from other stars. It would be the crowning achievement of a remarkable voyage.