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Voyager 1 and 2: The endless journey continues
POSTED 6 SEPTEMBER 2007

Shocking: You are terminated!
Within 10 years, Voyager 1 is set to become the first artificial object to exit the region influenced by our sun, when it finally enters the vast emptiness of interstellar space. Leaving the sun's environs, as defined by the area touched by the solar wind, is proving as convoluted as a plot-rich Shakespeare play.

Cross-section of the solar system shows position of two Voyagers and places named above
End of the line: The solar wind slows abruptly at the "termination shock," then abruptly ceases at the "heliopause," where interstellar space begins. The giant bubble called the "heliosphere" is the region affected by the solar wind. Graphic: NASA

The solar wind is a stream of particles (mainly electrons and protons) that zip away from the sun, and astronomers call the region it reaches the "heliosphere." The solar wind suddenly slows at the inside border of the thick boundary of the heliosphere, called the "termination shock."

Voyager 1 passed the termination shock in 2004 and should pass through the "heliopause" within 10 years. That's where the solar wind finally peters out and interstellar space begins.

 Having finished with the planets, the Voyagers are now exploring the edge of the solar system.Both Voyagers have already played a key role in understanding the waning influence of the sun, says Krimigis of Johns Hopkins. "Voyager established the dimensions of the heliosphere as we know it today. When we launched in 1977, people thought the termination shock could not be much farther than Saturn, at about a billion miles, but it has turned out to be 10 times that distance."

Voyager 1 is in a limbo occupied by no other human object, he adds. "It's like we crossed one side (termination shock) of a river, and now we are traveling across on our boat to get to the other side (heliopause)."

After 30 years of service, Voyager 1 is likely to continue working for another 10 years, but scientists are still crossing their fingers. The little spaceships that could are a long way from the car-fix, and no follow-on missions to the edge of the solar system are in the works.

Bubble trouble
When the solar wind finally crashes into the transition zone, energy is released, and some of that is transferred to particles that zoom back to Earth, where they are trapped by the planet's magnetic field.

These "anomalous cosmic rays" reach Earth carrying far less energy than the "galactic cosmic rays" that arrive at blistering speeds from sources in various galaxies. Yet like all "visitors" from space, cosmic rays can reveal information about their point of origin. And thus studies of the transition zone can explain conditions here on Earth, says Frank McDonald, a senior research scientist at the University of Maryland who is working on Voyager data from the termination shock. He says Voyager 1 found only about 10 percent of the expected number of anomalous cosmic rays at the shock, a discrepancy that may be due to a massive solar eruption that in 2003 spread huge numbers of protons and electrons through the solar system. This flood of particles reached the edge of the solar system when Voyager 1 did, and "the termination shock got wiped clean."

In other words, a region 15 billion kilometers from the sun still responds to the star's changing moods. With luck, McDonald says, the "solar weather" will be different when Voyager 2 reaches the shock. By stitching together data from the two far-flying sister satellites, scientists could draw a better picture of an unimaginably remote and empty region. (Empty? Stone tells us five cubic centimeters of the interstellar medium contains about one hydrogen atom. For comparison, at sea level, a cubic centimeter of nitrogen, the major constituent of air, contains 5,000,000,000,000,000,000 (5 * 1019) atoms.)

Stars in deep space appear caught in a sand storm
As the heliosphere plows through interstellar space, a bow shock forms, much as ripples form in front of a boulder in a stream. The Hubble Space Telescope took this image in 1995. The arcing, graceful bow shock is about half a light-year across -- that's three trillion miles. Photo: Hubble Heritage Team (STScI/AURA)

Termination station
Stone says radiation measurements indicate that Voyager 2 could arrive at the termination shock "any day." If so, then the heliosphere must bigger in the north, above the solar system.

And because the heliopause is positioned where the outward pressure of the solar wind balances inward pressure from the interstellar magnetic field, that would mean the magnetic field is pushing stronger in the south. Voyager 2's data "will give us an idea how strong that field is," Stone says.

Analysis of Voyager data also shows that the magnetic field beyond the solar system is not parallel to the galaxy's dominant magnetic field. The fact may seem unimaginably obscure and trivial, but it would help explain the history of our cosmic neighborhood.

A cockeyed magnetic field just beyond the solar system makes sense, says Stone, "because the nearby interstellar medium is very chaotic. It was created by the explosions of a lot of supernovas 20 to 30 million years ago."

Supernovas are giant star explosions that seed the universe with elements heavier than helium, so ancient supernovas were the source of almost all of the elements on Earth. A more recent explosion that apparently occurred within the last 100,000 years left a cloud "that determined our local environment," says Stone, and made it subtly different than other interstellar regions.

What about Voyager's pretty planet pictures?

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