The remarkable twin Voyager spacecraft continue to explore the outer reaches of the solar system decades after they completed their surveys of the Outer Planets. Launched in 1977 (September 5 for Voyager 1 (V1) and August 20 for Voyager 2 (V2), whose trajectory took it past Jupiter after Voyager 1), the spacecraft pair made many fundamental discoveries as they flew past Jupiter (March 1979 for V1, July 1979 for V2) and Saturn (November 1980 for V1, August 1981 for V2). The path of Voyager 2 past Saturn was targeted so that it continued within the plane of the solar system, allowing it to become the first spacecraft to visit Uranus (January 1986) and Neptune (August 1989). Following the Neptune encounter, both spacecraft started a new phase of exploration under the intriguing title of the Voyager Interstellar Mission.
Five instruments continue to collect important measurements of magnetic fields, plasmas, and charged particles as both spacecraft explore different portions of the solar system beyond the orbits of the planets. Voyager 1 is now more than 118 astronomical units (one AU is equal to the average orbital distance of Earth from the Sun) distant from the sun, traveling at a speed (relative to the sun) of 17.1 kilometers per second (10.6 miles per second). Voyager 2 is now more than 96 AU from the sun, traveling at a speed of 15.5 kilometers per second (9.6 miles per second). Both spacecraft are moving considerably faster than Pioneers 10 and 11, two earlier spacecraft that became the first robotic visitors to fly past Jupiter and Saturn in the mid-70s.
As seen in the night sky at Earth, Voyager 1 is within the confines of the constellation Ophiuchus, only slightly above the celestial equator; no telescope can see it, but radio contact is expected to be maintained for at least the next ten years. Voyager 2 is within the bounds of the constellation Telescopium (which somehow sounds quite appropriate) in the far southern night sky.
Both spacecraft have already passed something called the Termination Shock† (December 2004 for V1, August 2007 for V2), where the solar wind slows as it starts to interact with the particles and fields present between the stars. It is expected that both spacecraft will encounter the Heliopause, where the solar wind ceases as true interstellar space begins, from 10 to 20 years after crossing the Termination Shock. Theories exist for what should be present in interstellar space, but the Voyagers will become the first man-made objects to go beyond the influences of the Sun, hopefully returning the first measurements of what it is like out there. Each spacecraft is carrying a metal record with encoded sounds and sights from Earth, along with the needle needed to read the recordings, and simplified instructions for where the spacecraft came from, in case they are eventually discovered by intelligent extra-terrestrials.
Jim Zimbelman is a geologist in the Center for Earth and Planetary Studies at the National Air and Space Museum.
† The sun ejects a continuous stream of charged particles (electrons, protons, etc) that is collectively termed the solar wind. The particles are traveling extremely fast and are dense enough to form a very tenuous atmosphere; the heliosphere represents the volume of space where the effects of the solar wind dominate over those of particles in interstellar space. The solar wind particles are moving very much faster than the local speed of sound represented by their low volume density. When the particles begin to interact with interstellar particles and fields (the interaction can be either physically running into other particles or experiencing an electromagnetic force resulting from a charged particle moving within a magnetic field), then they start to slow down. The point at which they become subsonic (rather than their normal hypersonic speed) is the Termination Shock.