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

More than two years after Voyager 2 looked Neptune's Great Dark Spot in the eye and darted past the frozen surface of its moon Triton, both Voyager spacecraft are continuing to return data about interplanetary space and some of our stellar neighbors near the edges of the Milky Way.

After the Voyager spacecraft flew by the four giant outer planets -- Jupiter, Saturn, Uranus and Neptune -- their mission might have been over. But, in fact, these 14-year-old twins are just beginning a new phase of their journey, called the Voyager Interstellar Mission (VIM).

As the Voyagers cruise gracefully in the solar wind, their fields, particles and waves instruments are studying the space around them while searching for the elusive heliopause -- the outer edge of our solar system.

The heliopause is the outermost boundary of the solar wind, where the interstellar medium restricts the outward flow of the solar wind and confines it within a magnetic bubble called the heliosphere. The solar wind is made up of electrically charged atomic particles, composed primarily of ionized hydrogen, that stream outward from the Sun. "The termination shock is the first signal that we are approaching the heliopause. It's the area where the solar wind starts slowing down," said Voyager Project Scientist and JPL's Director, Dr. Edward C. Stone. Mission scientists now anticipate that the spacecraft may cross the termination shock by the end of the century. Exactly where the heliopause is remains a mystery. Its long been thought to be located some 75 to 150 astronomical units (AU) from the Sun. (One AU is equal to 150 million kilometers (93 million miles), or the distance from the Earth to the Sun.) Any speculation about where the heliopause is or what it is like, has come only from computer models and theories. "Voyager 1 is likely to return the first direct evidence from the heliopause and what lies beyond it," Stone said.

Yet the Voyagers are not sitting idly by as they wait to cross over into interstellar space. Both spacecraft are involved in an extensive program of ultraviolet astronomy that allows them to study active galaxies, quasars and white dwarf stars, in ways unlike any other spacecraft or telescope in existence.

Voyager's ultraviolet spectrometers are the only way scientists can currently observe celestial objects in a unique region in the short end of the ultraviolet portion of the electromagnetic spectrum. "Voyager's instruments allow it to observe things at wavelengths that are for the most part unavailable to other spacecraft," said Dr. Jay Holberg, a member of Voyager's ultraviolet subsystem team.

The Voyagers have become space-based ultraviolet observatories and their unique location in the universe gives astronomers the best vantage point they have ever had for looking at celestial objects that emit ultraviolet radiation. "The light that Voyager is sensitive to has to be observed in outer space," said Holberg.

Voyager's ultraviolet instruments are best suited to study the two extreme phases of a star's life -- its birth and its death. Thus the Voyagers are currently gathering data on early blue stars as well as other white dwarf stars nearing the end of their lifetime. "Voyager is helping us get a better handle on exactly how much energy these hot stars emit," Holberg said.

Now that Voyager's primary mission of exploring the outer planets is over, there are fewer constraints on the science team when it comes to programming the spacecrafts' observations. "We can sit on these things for very long periods of time and watch these stars go through their phases," Holberg said.

Stars can be very active, but also unpredictable. "We don't know when a star will do something. If we want to sit there and wait, we can do it in the hopes that the star will go through an outburst and Voyager will be there to observe it," he continued. Voyager can now stare at an object for days and even weeks at a time to thoroughly map it and the region around it.

Since the beginning of the interstellar mission, the Voyager project has been conducting a guest observer program which allows astronomers from around the world to make proposals and apply for time to use the Voyager ultraviolet spectrometer in much the same way that astronomers apply for time at ground-based observatories. This program enables scientists to make simultaneous observations of the same object using Voyager and ground-based telescopes.

The cameras on the spacecraft have been turned off and the ultraviolet instrument is the only experiment on the scan platform that is still functioning. Voyager scientists expect to continue to receive data from the ultraviolet spectrometers at least until the year 2000. At that time, there will not be enough electrical power for the heaters to keep the ultraviolet instrument warm enough to operate.

Yet there are several other fields and particle instruments that can continue to send back data as long as the spacecraft stay alive. They include: the cosmic ray subsystem, the low-energy charged particle instrument, the magnetometer, the plasma subsystem, the plasma wave subsystem and the planetary radio astronomy instrument. Barring any catastrophic events, JPL should be able to retrieve this information for at least the next 20 and perhaps even the next 30 years.

"In exploring the four outer planets, Voyager has already had an epic journey of discovery. Even so, their journey is less than half over with more discoveries awaiting the first contact with interstellar space," Stone said. "The Voyagers revealed how limited our imaginations really were about our solar system, and I expect that as they continue toward interstellar space, they will again surprise us with unimagined discoveries of this never-before-visited place which awaits us beyond our planetary neighborhood."

Voyager 1 is now 7 billion kilometers (4.3 billion miles) from Earth, traveling at a heliocentric velocity of 63,800 km/hr (39,700 mph). Voyager 2, traveling in the opposite direction from its twin, is 5.3 billion kilometers (3.3 billion miles) from Earth with a heliocentric velocity of 59,200 km/hr (36,800 mph).

The Voyager Interstellar Mission is managed by JPL and sponsored by NASA's Office of Space Science and Applications, Washington, DC.