The closer Voyager 1 gets to interstellar space 35 years after leaving Earth, the more surprises it is springing on mission scientists as they repeat a question familiar to any parent who has taken a child on a long road trip: Are we there yet?
Features that, according to theory, Voyager should be detecting by now if it's close to the boundary between the sun's sphere of influence and interstellar space haven't appeared, according to a new study appearing in Thursday's issue of the journal Nature.
But subsequent data taken over the past few months and yet to be published also show activity no theorist predicted and could hint at the beginning of Voyager's breakout.
"We all have the sense that something big is imminent," says Stamatios Krimigis, a researcher at the Johns Hopkins University's Applied Physics Laboratory in Laurel, Md., and the lead scientist on one of Voyager's still-operating instruments.
But, he adds, "If I were a theorist I would feel pretty humble right about now."
Expecting the unexpected is the bread and butter of exploration, and the edge of the solar system is as good an illustration of that as any place NASA has sent a spacecraft.
Launched in 1977, Voyager 1 and Voyager 2 have long since finished their revolutionary tour of the outer planets. Voyager 1 flew by Saturn in 1980, then headed for interstellar space. Voyager 2 left Neptune in 1989 on a similar mission, but in a different direction. At 11.3 billion miles from Earth, Voyager 1 has traveled the farthest.
The hope is that one or both craft will have crossed the boundary between a bubble the sun's solar wind sets up around the solar system and the galactic wind before 2025. The solar wind is a constant stream of charged particles the sun hurls into space in all directions at a million miles an hour. The 2025 deadline is set by the crafts' plutonium power supplies. That's the year the power sources no longer will provide enough energy to run the instruments needed to study the solar and galactic winds and the processes in the transition zone between them.
Inside the bubble "the wind is from the sun, and outside the wind is from the explosion of supernovae 5, 10, and 15 million years ago," said Ed Stone, the mission's project scientist and former director of NASA's Jet Propulsion Laboratory in Pasadena, Calif., during a panel discussion of the twin crafts' mission Tuesday.
The challenge in knowing how big the sun's bubble is stems from the fact "that we haven't gotten outside yet," Dr. Stone says. One experiment Voyager 1 ran allowed researchers to estimate that the boundary between the bubble and interstellar space – the heliopause – fell somewhere between 117 and 177 astronomical units (AU), or 117 to 177 times the distance from the sun to Earth.
"We're now at 122, but I hope it's not 177," Stone says – a distance that would rule out taking measurements, since at the craft's current speed of about 3.6 AU a year, it would reach 177 AU two years after the remaining instruments fall silent for lack of power.
Voyager 1 currently is traveling through the heliosheath, a region between the termination shock – a sudden slowing of the solar wind as it feels the effects of the oncoming galactic wind – and the heliopause.
Last year, the team reported that the solar wind's speed outward had virtually reached zero, as expected for the heliosheath. Theory also suggested that these charged particles also should be deflected north and south with greater intensity as Voyager 1 approaches the outside edge of the sun's bubble where the bubble meets the galactic head wind.
Over the past year, Voyager 1 has dutifully been shifting its orientation so the instrument involved – the Low-Energy Charged Particle detector – can take readings in different directions.
In a paper published Thursday in the journal Nature, the team, which includes Dr. Krimigis, reports that it finds no north-south flow. It's unclear whether this means the craft hasn't traveled far enough yet to detect such flows signaling a final transition to interstellar space, or whether theories miss the mark on what happens inside the heliosheath.
Whatever the explanation, Voyager 1 seems to have hit the sun's version of the doldrums – a region where there is turbulence, but the solar wind has no outward flow nor any flow suggesting deflection.
The data the team used was collected through last February.
Since February, however, Voyager 1's instruments have picked up unexpected swings in the relative abundance of relatively low-energy charged particles originating in the sun and trying to escape, and higher-energy particles coming from the cosmos outside the bubble. And the swings have been appearing more frequently and in some cases with increasing intensity.
Where changes in Voyager data occurred over weeks or months, "now they happen on a day-by-day basis," Krimigis says. "We could be in and out of these ups and downs in the data for another month or another three years, for all we know."
The relative abundance of the low-energy insiders trying to get out and the higher energy outsiders trying to get in represent a key indicator of how close Voyager 1 is to breaking free of the sun's bubble, Stone says.
A third benchmark, and change in the orientation of the magnetic field the solar wind carries versus the field the galactic wind carries will be the final clue – although one that will be fiendishly difficult to measure, he adds.
For now, Voyager 1 appears to have entered a transition region with some kind of connection to the environment outside the sun's bubble.
With the sudden appearance and disappearance of particles from the outside, "We don't know whether these are filaments connected to the outside, or whether we're dancing along the edge of a new region which is connected to the outside," he says. "This is all exploration. There is no model that has predicted this kind of detailed variation as we approach the heliosphere."