Voyager 1 cruises ‘highway’ at solar system’s edge

JOHNS HOPKINS / CALTECH (US) — NASA’s Voyager 1 spacecraft, soon to become the first human-built object in interstellar space, is now cruising along an unexpected “magnetic highway” on the outside edge of our solar system.

Scientists think that the area on the outskirts of the sun’s influence is the final obstacle Voyager has to negotiate before finally—at least 35 years after launch—crossing over into the void between the stars.

“We believe this is the last leg of our journey to interstellar space,” says Edward Stone, a Voyager project scientist based at the California Institute of Technology (Caltech). “Our best guess is that it’s likely just a few months up to a couple years away. The new region isn’t what we expected, but we’ve come to expect the unexpected from Voyager.”

The “magnetic highway” moniker refers to a connection in the area between our sun’s magnetic field lines and interstellar magnetic field lines. That connection allows lower-energy charged particles that originate inside our heliosphere—the bubble of charged particles the sun blows around itself—to zoom out. It also permits higher-energy particles from outside to stream in.

Before entering this region, the sun’s charged particles bounced around in all directions, as if trapped on local roads inside the heliosphere. When Voyager found the highway, scientists operating its Johns Hopkins-built low-energy charged particle detector wondered if the probe had already ventured into interstellar space. Data indicating that the direction of the magnetic field lines has not changed leads the Voyager team, however, to conclude that this region is still inside the solar bubble.

“If we were judging by the charged-particle data alone, I would have thought we were outside the heliosphere,” says Stamatios Krimigis of Johns Hopkins University’s Applied Physics Laboratory and principal investigator for the low-energy charged particle instrument.

“In fact,” he says, “our instrument has seen the low-energy particles taking the exit ramp toward interstellar space. But we need to look at what all the instruments are telling us, and only time will tell whether our interpretations about this frontier are correct. One thing is certain: None of the theoretical models predicted any of Voyager’s observations over the past 10 years, so there is no guidance on what to expect.”

Since December 2004, when Voyager 1 crossed a shockwave known as the “termination shock”, the spacecraft has been exploring the heliosphere’s outer layer, called the heliosheath. Here, the solar wind—the stream of charged particles from the sun—abruptly slowed down from supersonic speeds and became turbulent. Voyager 1’s environment was consistent for about five and a half years, but then the spacecraft detected that the outward speed of the solar wind slowed to zero. The intensity of the magnetic field also began to increase.

Around May 14, LECP measured a sudden 5 percent increase in cosmic rays—high-energy particles coming into the solar system from elsewhere in the galaxy—followed by a similar increase on July 28.

This second increase was accompanied by a decrease (by a factor of five) in low-energy particles, but this lasted only for four days. A few days later, the same up-and-down exchange occurred, but on Aug. 25 the instrument recorded an even larger increase in cosmic rays—bringing the total increase since the end of March to about 30 percent.

Voyager 1 and its twin spacecraft, Voyager 2, were launched 16 days apart in 1977. Between them, they have visited Jupiter, Saturn, Uranus, and Neptune. Voyager 1 is the most distant manmade object, about 11 billion miles (18.5 billion kilometers) from the sun. Voyager 2 is about 9 billion miles (15 billion kilometers) out. While Voyager 2 has seen some gradual changes in charged particles, they are very different from those seen by Voyager 1. Scientists do not think Voyager 2 has reached the magnetic freeway.

“The solar wind measurements speak to the unique abilities of the LECP detector, designed at APL nearly four decades ago,” Krimigis says. “Where a device with no moving parts would have been safer—lessening the chance a part would break in space—our team took the risk to include a stepper motor that rotates the instrument 45 degrees every 192 seconds, allowing it to gather data in all directions and pick up something as dynamic as the solar wind. A device designed to work for 500,000 ‘steps’ and four years has been working for 35 years and well past 6 million steps.”

The new results were described at the American Geophysical Union meeting in San Francisco.

The Voyager spacecraft were built and are operated by the Jet Propulsion Laboratory, a division of Caltech. The LECP instrument was designed and built at APL with NASA funding. The Voyager missions are a part of the NASA Heliophysics System Observatory, sponsored by the Heliophysics Division of the Science Mission Directorate in Washington

Source: Johns Hopkins University