

Contact: Mary A. Hardin (818) 354-0344



      Twenty years after their launch and long after their planetary reconnaissance flybys were completed, both Voyager spacecraft are now gaining on another milestone -- crossing that invisible boundary that separates our solar system from interstellar space, the heliopause.

      Since 1989, when Voyager 2 encountered Neptune, both spacecraft have been studying the environment of space in the outer solar system. Science instruments on both spacecraft are sensing signals that scientists believe are coming from the heliopause -- the outer most edge of the Sun's magnetic field that the spacecraft must pass through before they reach interstellar space.

      "During their first two decades the Voyager spacecraft have had an unequaled journey of discovery. Today, even though Voyager 1 is now more than twice as far from the Sun as Neptune, their journey is only half over and more unique opportunities for discovery await the spacecraft as they head toward interstellar space," said Dr. Edward Stone, Voyager project scientist and director of NASA's Jet Propulsion Laboratory, Pasadena, CA. "The Voyagers owe their ability to operate at such great distances from the Sun to their nuclear electric power sources, which provide the electrical power they need to function."




      "Based on a radio emission event detected by the Voyager 1 and 2 plasma wave instruments in 1992, we estimate that the heliopause is located at110 to 160 AU from the Sun," said Dr. Donald A. Gurnett, principal investigator on the plasma wave subsystem at the University of Iowa. (One AU is equal to 150 million kilometers, or 93 million miles, or the distance from the Earth to the Sun.)


      Voyager 2 was launched first on Aug. 20, 1977 and Voyager 1 was launched a few weeks later on a faster trajectory on Sept. 5. Initially both spacecraft were only supposed to explore two planets -- Jupiter and Saturn. But the incredible success of those two first encounters and the good health of the spacecraft prompted NASA to extend Voyager 2's mission on to Uranus and Neptune. As the spacecraft flew across the solar system, remote-control reprogramming has given the Voyagers greater capabilities than they possessed when they left the Earth.




      Voyager 1 encountered Jupiter on March 5, 1979, and Saturn on November 12, 1980 and then, because its trajectory was designed to fly close to Saturn's large moon Titan, Voyager 1's path was bent northward by Saturn's gravity, sending the spacecraft out of the ecliptic plane, the plane in which all the planets except Pluto orbit the Sun. Voyager 2 arrived at Jupiter on July 9, 1979, and Saturn on August 25, 1981, and was then sent on to Uranus on January 25, 1986 and Neptune on August 25, 1989. Neptune's gravity bent Voyager 2's path southward, sending it out of the ecliptic plane as well and on toward interstellar space.

      Both spacecraft have enough electrical power and attitude control propellant to continue operating until about 2020, when the available electrical power will no longer support science instrument operation. Spacecraft electrical power is supplied by Radioisotope Thermoelectric Generators (RTGs) that provided approximately 470 watts power at launch. Due to the natural radioactive decay of the plutonium fuel source, the electrical energy provided by the RTGs is continually declining. At the beginning of 1997, the power generated by Voyager 1 had dropped to 334 watts and to 336 watts for Voyager 2. Both of these power levels represent better performance than had been predicted before launch.


      Voyager 1 is currently 10.1 billion kilometers (6.3 billion miles) from Earth, having traveled 11.9 billion kilometers (7.4 billion miles) since its launch. The Voyager 1 spacecraft is departing the solar system at a speed of 17.4 kilometers per second (39,000 miles per hour).

      Voyager 2 is currently 7.9 billion kilometers (4.9 billion miles) from Earth, having traveled 11.3 billion kilometers (6.9 billion miles) since its launch. The Voyager 2 spacecraft is departing the solar system at a speed of 15.9 kilometers per second (35,000 miles per hour).


NOTE TO EDITORS: A NASA TV video file featuring animation of the Voyager spacecraft will air today, Sept. 2 at 9 a.m., noon, 3 p.m., 6 p.m. and 9 p.m. Pacific time. NASA TV is located at GE-2, transponder 9C at 85 degrees West longitude, with vertical polarization. Frequency is on 3880.0 megahertz, with audio on 6.8 megahertz.

8/26/97 MAH