This artist's concept shows NASA's Voyager spacecraft against a backdrop of stars.
This artist's concept shows NASA's Voyager spacecraft against a backdrop of stars. › larger image

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."

The Sun emits a steady flow of electrically charged particles called the solar wind. As the solar wind expands supersonically into space, it creates a magnetized bubble around the Sun, called the heliosphere. Eventually, the solar wind encounters the electrically charged particles and magnetic field in the interstellar gas. The boundary created between the solar wind and interstellar gas is the heliopause. Before the spacecraft reach the heliopause, they will pass through the termination shock -- the zone in which the solar wind abruptly slows down from supersonic to subsonic speed.

Reaching the termination shock and heliopause will be major milestones for the spacecraft because no one has been there before and the Voyagers will gather the first direct evidence of their structure. Encountering the termination shock and heliopause has been a long-sought goal for many space physicists, and exactly where these two boundaries are located and what they are like still remains a mystery.

"Based on current data from the Voyager cosmic ray subsystem, we are predicting the termination shock to be in the range of 62 to 90 astronomical units (AU) from the Sun. Most 'consensus' estimates are currently converging on about 85 AU. Voyager 1 is currently at about 67 AU and moving outwards at 3.5 AU per year, so I would expect crossing the termination shock sometime before the end of 2003," said Dr. Alan Cummings, a co- investigator on the cosmic ray subsystem at the California Institute of Technology.

"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.)

"The low-energy charged particle instruments on the two spacecraft continue to detect ions and electrons accelerated at the Sun and at huge shock waves, tens of AU in radius, that are driven outward through the solar wind. During the past five years, we have observed marked variations in this ion population, but have yet to see clear evidence of the termination shock. We should always keep in mind that our theories may be incomplete and the shock may be a lot farther out than we think," said Dr. Stamatios M. Krimigis, principal investigator for the low energy charged particle subsystem at The Johns Hopkins University Applied Physics Laboratory.

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.

There are four other science instruments that are still functioning and collecting data as part of the Voyager Interstellar Mission. The plasma subsystem measures the protons in the solar wind. "Our instrument has recently observed a slow, year-long increase in the speed of the solar wind which peaked in late 1996, and we are now observing a slow decrease in solar wind velocity," said Dr. John Richardson, of the Massachusetts Institute of Technology, principal investigator on the plasma subsystem. "We think the velocity peak coincided with the recent solar minimum. As we approach the solar maximum in 2000 the solar wind pressure should decrease, which will result in the termination shock and heliopause moving inward towards the Voyager spacecraft."

The magnetometer on board the Voyagers measures the magnetic fields that are carried out into interplanetary space by the solar wind. The Voyagers are currently measuring the weakest interplanetary magnetic fields ever detected and those magnetic fields being measured are responsive to charged particles that cannot be detected directly by any other instruments on the spacecraft, according to Dr. Norman Ness, principal investigator on the magnetometer subsystem at the Bartol Research Institute, University of Delaware.

Other science instruments still collecting data include the planetary radio astronomy subsystem and the ultraviolet spectrometer subsystem.

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.

The Voyagers are now so far from home that it takes nine hours for a radio signal traveling at the speed of light to reach the spacecraft. Science data are returned to Earth in real-time to the 34-meter Deep Space Network (DSN) antennas located in California, Australia and Spain. Voyager 1 will pass the Pioneer 10 spacecraft in January 1998 to become the most distant human- made object in our solar system.

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).

JPL, a division of the California Institute of Technology, manages the Voyager Interstellar Mission for NASA's Office of Space Science, Washington, D. C.