Confusion over Voyager's whereabouts has a lot to do with the failure of a specific piece of equipment known as the plasma science experiment, or PLS. The device, which was developed at
Cool plasma, the product of stars that exploded millions of years ago, populates interstellar space. It has a high density of about 100,000 electrons per cubic yard of space, Gurnett said.
Super-heated plasma, like the solar wind that flows from our sun, fills the heliosphere. It is much less dense, only about 1,000 electrons per cubic meter.
A functioning PLS would have been able to sense the density change as Voyager exited the heliosphere.
"The instrument failed in 1980, so the spacecraft is sort of instrument-challenged," Gurnett said. "That's one of the major failures we've had. There really aren't that many."
Voyager does have two functional plasma wave antennas that stretch from its base and form a wide "V." The antennas detect the vibration of excited plasma particles and convert that motion into an audible ringing that is stored on the 8-track tape.
The frequency of the ring is associated with a specific density of plasma. The higher the frequency, the denser the plasma.
The only trouble is, something has to excite the plasma to get it to "ring" -- something like a large solar flare or coronal mass ejection. Waiting for such a solar event can take years. And when it does occur, it can take as long as a year for the shock wave to reach Voyager.
Fortunately for Voyager scientists, the antennas picked up two long-lasting oscillations in the past year -- one in October and
"It was key evidence," Stone said. "We really needed to measure plasma to know if we were inside or outside the heliosphere. Everything else is more of a proxy."
Gurnett and his colleagues arrived at the crossing date of
Plasma density was increasing in a linear fashion as Voyager moved farther into interstellar space. The frequency measured in the fall of 2012 was 2.2 kilohertz, and by the spring of 2013 it had risen to 2.6 kilohertz.
Previous research told Gurnett that the frequency at the crossing point should be 2 kilohertz. By plotting each point on a line, he was able to arrive at a date.
It's no small coincidence that it was the exact same date given in March by
Swisdak, who was not involved in the most recent study, said the new evidence looked "fairly conclusive."
"Density measurement is not quite a smoking gun, but it's pretty close to it," he said. "It's pretty clear that we've crossed some sort of boundary."
Scientists plan to continue scrutinizing data from Voyager and refining models to better understand the area the spacecraft is traveling though.
"Where we are now is a very disturbed region; we expect someday we'll get into a region that will be very boring and undisturbed," Krimigis said. "We have a lot of understanding to work on what exactly it is we're seeing, how we can model it, and how we can rewrite the books about what the border with the galaxies is supposed to be."
Scientists are hoping that many gaps in their understanding will be filled in by Voyager 2. The sister spacecraft, which was also launched in 1977, is nearing the edge of the heliosphere via a different path and is expected to encounter interstellar space sometime within the next several years.
Unlike Voyager 1 however, Voyager 2 has a fully functioning plasma science instrument and has been sending back density readings throughout its journey.
"I think it's going to teach us even more," Stone said. "We've entered a new era of exploration."
(c)2013 The Baltimore Sun
Visit The Baltimore Sun at www.baltimoresun.com
Distributed by MCT Information Services