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Voyager 1 cruising along magnetic highway

Posted: 5 December 2012

The Voyager 1 spacecraft, sailing through the unexplored frontier of the solar system, has detected a new region of space at the enigmatic boundary between the sun's sphere of influence and the interstellar medium, scientists said Monday.

Artist's concept depicting the Voyager 1 spacecraft exploring a new region at the edge of the solar system. Credit: NASA/JPL-Caltech
More than 35 years since launching from Earth, the plutonium-powered Voyager 1 probe has flown past Jupiter and Saturn and is now pioneering science at the edge of the heliosphere, a teardrop-shaped bubble blown out by the solar wind.

Beyond the heliosphere lies a vacuous expanse known as interstellar space, where the solar wind stops and material expelled from exploding stars hold reign.

Voyager 1, the most distant human-made object, is the first spacecraft to explore the boundary region.

In July and August, scientists noticed intriguing data coming from Voyager 1's particle counters as the craft flew more than 11 billion miles from Earth.

"Voyager has discovered a new region of the heliosphere that we had not realized was there," said Ed Stone, Voyager project scientist at NASA's Jet Propulsion Laboratory.

The instruments registered dramatic, temporary changes in the levels of cosmic rays and low-energy particles two times in late July and mid-August.

On Aug. 25, Voyager 1 detector sensed a permanent rise in high-energy cosmic rays, just as the probe's telescopes a sharp drop in low-energy particles coming from inside the heliosphere.

Scientists believed cosmic rays, which originate from outside the solar system, would not penetrate the heliopause, the border where the heliosphere and interstellar space meet. And researchers thought low-energy particles from the solar system would be constrained inside the heliosphere.

"If we had only looked at particle data alone, we would have said we're out. Goodbye, solar system," said Stamatios Krimigis, principal investigator of the low-energy charged particle instrument, based at the Johns Hopkins University Applied Physics Laboratory.

But scientists instituted a third test to check whether Voyager 1 had crossed the heliopause and left the solar system.

Inside the heliosphere, the magnetic field is oriented in an east-west direction due to the spinning of the sun. Outside, scientists say, evidence points to the magnetic field being in a north-south direction.

So far, Voyager 1 has not recorded a change in magnetic field direction, according to Leonard Burlaga, a Voyager magnetometer team member based at NASA's Goddard Space Flight Center.

This artist's concept shows how NASA's Voyager 1 spacecraft is bathed in solar wind from the southern hemisphere flowing northward. It also depicts Voyager 1's location relative to the heliosphere and interstellar space. Credit: NASA/JPL-Caltech
But Burlaga said Voyager 1's magnetometer indicates the craft is in a much more intense magnetic environment than before the summer.

"We are in a magnetic region unlike any we've been in before - about 10 times more intense than before the termination shock - but the magnetic field data show no indication we're in interstellar space," Burlaga said. "The magnetic field data turned out to be the key to pinpointing when we crossed the termination shock. And we expect these data will tell us when we first reach interstellar space."

Voyager 1 passed the termination shock in December 2004, entering a region called the heliosheath, in which the million-mile-per-hour solar wind slowed and became turbulent. From December 2004 until the summer of 2012, the environment around Voyager 1 was consistent.

Researchers say Voyager 1 is now in a region where the sun's magnetic field lines are connected to interstellar magnetic field lines. The connection creates an avenue between the solar system and the space outside, allowing low-energy particles from inside the heliosphere to stream out and allows cosmic rays from interstellar space to pass inside.

Scientists call the connection a magnetic highway because the magnetic field lines allow particles to freely flow in and out of the heliosphere.

Stone said it is impossible to predict exactly when Voyager 1 will leave the solar system.

"It could take several more months or take several more years, but we believe this may be the very last layer between us and interstellar space," Stone said.

Voyager 1, along with a twin craft named Voyager 2, launched in 1977 to tour the solar system's outer planets. Both probes are now on trajectories leaving the solar system.

Voyager 2, flying in a different direction than its sister craft, is now about 9 billion miles away and will reach interstellar space several years after Voyager 1.

"In 1977, no one knew how large the heliosphere was, and no one knew how long the spacecraft would last," Stone said. "We're very lucky that there seems to be a compatibility between our mission lifetime and the size of the heliosphere."

Suzanne Dodd, Voyager's project manager, said about 12 engineers and support personnel work on the mission full-time at JPL. Another dozen researchers are on the Voyager science team.

The Voyager probes are powered by the radioactive decay of plutonium-238. A power generator converts heat from the plutonium's decay into electricity.

The power source will be sufficient to operate all of spacecraft's science instruments until around 2020, then controllers will begin to switch off the sensors one-by-one. By 2025, there be no electricity for any of Voyager's instruments, according to Stone, who has been with the project since launch.

But scientists are confident the Voyager probes will last long enough to leave the heliosphere and taste interstellar space.

"We could well be quite surprised once we get outside the bubble," Stone said.