Voyagers 1 and 2 cruise along at a brisk 3.6 and 3.3 AU per year, respectively (1 AU is equivalent to the distance between the Earth and Sun, 93 million miles, or 148 million kilometers), but that trip to what is called the heliopause could take until 2017. That's a long wait.
Now here's the encouraging news: the probes could arrive at a preliminary boundary called the termination shock in 2005, Stone predicts. The termination shock, nearly 4 billion miles inside the heliopause, is where the solar wind first starts to slow down and reverse due to its first encounters with pressure from interstellar space.
With Voyager 1 now the further along at 82 AU, it will win the race between the twin spacecraft. Stone's reasoning was published in this week's issue of the journal Science.
What is the interstellar medium?
Arriving at the termination shock is more than a rat race. It will allow scientists to estimate the size of the heliosphere, and Stone says this measure will help answer truly fundamental questions.
"If we can determine how large the heliosphere is, we will then have a measure of what's outside in interstellar space. We have indications now, but we don't know accurately what's out there," he said.
Other blanks Voyager may help fill in could be things like the nature of the interstellar magnetic field and the density of interstellar material.
But Voyager was designed for planetary observations, so there it is unclear how much information can be gathered.
"Although Voyager is our best chance, it was not designed, not optimized, to study these things," said J.R. Jokipii, a Regents' professor at the University of Arizona's departments of planetary sciences and astronomy.
But both Stone and Jokipii, believe getting outside the heliosphere will give science new, worthwhile data on the interstellar media untainted by the Sun's influence.
"This will be an opportunity to study 'in situ' the process by which nature accelerates particles to very high velocities," Stone said.
Peak opportunity
The heliosphere is a comet-shaped (the entire disk is distended by the cosmic wind) bubble of charged particles carved out of interstellar space by the solar wind. The extent to which this wind blows fluctuates in an 11-year solar cycle. When the Sun is at peak activity, which is the case now, the size of the heliosphere is smallest.
"When the sun is quiescent, the wind is stronger," Stone said. "Solar activity just means there are more sun spots, which result in more magnetic activity. And magnetic activity is just a configuration that reduces what escapes from the Sun."
With the Sun currently peaking in solar activity, the heliosphere is contracted. This is a fortuitous for Voyager as it cruises toward the termination shock. But Stone says that if the shock is not located by 2005, it may be a few more years before it is reached because of the Sun's inevitable, heliospheric exhalation.
At launch in 1977, the Voyager mission was designed for a five-year mission to Jupiter and Saturn. An alignment of the four gas giants that comes only once every 175 years made it possible for Voyager to make a "Grand Tour" of these magnificent bodies, and then continue on.
Stone, who started on the mission in 1972 and stepped down recently as director of NASA's Jet Propulsion Laboratory, believes that luck and the rugged design of the craft that was needed to survive the radiation of Jupiter has helped both Voyagers endure.
"This has been a wonderful mission of exploration," he said, "and it is still continuing."
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