The heliopause is the boundary between a hot ionized gas flowing outward from the Sun, called the solar wind, and the relatively cool gas in the interstellar medium, according to Donald Gurnett, University of Iowa professor of physics and principal investigator for the plasma wave instruments on Voyagers 1 and 2.

In 1993 Gurnett and his colleagues reported the first direct evidence of the distance to the heliopause in the form of a powerful low-frequency radio burst. They theorized that a strong radio burst detected in 1992 by Voyagers 1 and 2 was generated near the heliopause by an interaction with an outward propagating shock wave produced by a series of strong solar flares in 1991. From the speed and travel time of the shock waves they estimated that the heliopause lies somewhere between 10.8 and 16.5 billion miles from the Sun, still well beyond Voyager's great distance.

"Our objective is to penetrate into the interstellar medium," says Gurnett. "It may take another 10 to 20 years, but I think there is a good chance that the spacecraft will still be operating when we get there."

"The Iowa instrument could observe a strong burst of plasma waves from a shock wave within the next five years, which will mark Voyager's entry into a very broad transition region between the solar wind and the heliopause," says William Kurth, who is a co-investigator for the University of Iowa instrument.

The location of the heliopause is uncertain because scientists believe it periodically swells and contracts, depending upon changes in the solar wind. The heliopause is thought to constantly change shape -- just as a cloud of smoke surrounding a Fourth-of-July sparkler is shaped by the wind.

Interestingly, Gurnett also notes that he and his research group have now designed and constructed instruments that have flown farther from the Sun (aboard Voyager 1) and closer to the Sun (aboard Helios 2 in 1976) than any other space research group.

Voyager 1, launched Sept. 5, 1977, completed fly-bys of both Jupiter and Saturn. A sister spacecraft, Voyager 2 was launched Aug. 20, 1977 on a flight path that took it to encounters with Jupiter, Saturn, Uranus and Neptune. At present Voyager 2 is about 5.1 billion miles from the sun. Pioneer 10, launched March 2, 1972 and the first spacecraft to explore Jupiter, officially ended its mission on March 31, 1997, however NASA intermittently receives science data from Pioneer as part of a training program for flight controllers of the Lunar Prospector spacecraft now orbiting the moon, and UI physicist Dr. James A. Van Allen receives a fresh batch of data gathered by his Pioneer cosmic ray detector every two weeks.