Voyager's prolonged journey into interstellar space took another dramatic turn when the intrepid space probe last summer passed into a bizarre and unanticipated cosmic hallway between the bubble of space under the sun's influence and whatever lies beyond.
On the celestial highway since September 1977, the Voyager 1 probe soared past Jupiter and Saturn in 1979 and 1980, respectively, then ended up an a path that led toward interstellar space. Eventually, the spacecraft will get there, but exactly when that will happen -- and what else it may encounter before then -- is anybody's guess.
"The results of the measurements from Voyager have been surprising us not just since last August, but for about the last 2.5 years," astronomer Stamatios Krimigis, with Johns Hopkinds University's Applied Physics Laboratory, told Discovery News.
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Scientists thought Voyager 1 had finally passed beyond the heliosheath, the outermost region of space touched by the solar wind, a stream of charged particles continuously flowing the sun. On Aug. 25, 2012, Voyager suddenly found itself in an uncharted region of space, marked by the abrupt disappearance of particles from the sun and the sudden rise of particles emanating from interstellar space.
"As far as we could tell there was absolutely no solar material in the vicinity of the spacecraft and there hasn't been since then. At the same time, the cosmic rays coming from outside the system started to increase. We all thought at the time that, by God, we were probably out of the solar system," Krimigis said.
But there were two other puzzling bits of data that didn't fit that scenario.
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The first mystery was why the magnetic field Voyager measured was still aligned like the sun's -- and even more perplexing, why the magnetic field suddenly strengthened.
Scientists had expected to see a different magnetic orientation once Voyager was in interstellar space.
The second conundrum was why the cosmic ray particles were not evenly distributed. The thinking was -- and is -- that cosmic rays, which emanate from distant supernova explosions all over the galaxy, should be uniformly spread out in every direction in interstellar space.
The best scientists can conclude is that Voyager is in some sort of foyer where particles from inside and outside the solar system can easily flow, but which is not quite yet in interstellar space. The rather unpoetic name they came up for this zone is the "heliosheath depletion region."
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"What we have is kind of a hybrid. The magnetic field still seems to be the solar magnetic field, not the interstellar magnetic field, so how do you define interstellar medium if that's the case? If you really need to finally reach the case where both the magnetic field and the plasma are from other stars, then we're still not there," lead project scientist Ed Stone, with the California Institute of Technology in Pasadena, told Discovery News.
Scientists have no idea how much longer it will take Voyager to reach the next and presumably last leg of its journey into interstellar space, but the proverbial clock is ticking.
The spacecraft, which is powered by the slow decay of radioactive plutonium, will begin running out of power for its science instruments in 2020.
"By then, we would have shut off everything we can shut off other than the instruments and will have to turn off the first instrument. As time goes on, each year there are four watts less available, we'll have to turn off the second instrument," Stone said.
By 2025, Voyager, which was originally designed to last just five years, will be completely shut down.
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Voyager is now about 122 times farther from the sun than Earth. At that distance it takes radio signals from Earth traveling at the speed of light 17 hours to reach the spacecraft.
A sister spacecraft called Voyager 2 is taking a different path toward interstellar space and has not yet encountered the panoply of twists and turns on the solar system's exit ramp -- and it never may.
"Voyager 2 has seen exactly what the models predicted we would see, unlike Voyager 1, which didn't," Stone said.
The region where the heliosheath and interstellar space connect, where Voyager 1 is located, may be a local phenomenon, he added.
The research appears in this week's Science.