Voyager at the edge

The Voyager 1 spacecraft, and its partner, Voyager 2, are approaching the edge of the Sun's protective bubble.

Spacecraft finds unexpected calm at the boundary of Sun’s bubble.
Seventeen and a half billion kilometres from Earth, mankind’s most distant probe seems to be on the edge of interstellar space.
The Voyager 1 spacecraft is at the limit of the ‘heliosheath’, where particles streaming from the Sun clash with the gases of the galaxy. Contrary to scientists’ expectation of a sharp, violent edge, the boundary seems to be a tepid place, where the solar wind mingles with extrasolar particles.
“We’re in this mixed-up region where the Sun still has some influence,” says Stamatios Krimigis, a physicist at the Applied Physics Laboratory of Johns Hopkins University in Laurel, Maryland. “It’s certainly not what we thought.”
The new findings, reported by Krimigis and his colleagues this week in Nature, are the latest of many during the spacecraft’s long journey1. Launched in 1977, Voyager 1 photographed active volcanoes on the moon Io on its way past Jupiter in 1979. The following year, it confirmed the existence of three new moons orbiting Saturn. In one of its final photographs, transmitted in 1990, Earth appears as a grainy speck bathed in the rainbow rays of the Sun.

Crossing the unknown

Since then, NASA scientists have shut down six of its ten instruments, and it is so far away that transmissions now take more than 16 hours to reach Earth. But Voyager’s work continues. It is now travelling out of the heliosphere, the bubble of space filled by the Sun’s wind. In late 2004, Voyager 1 crossed the ‘termination shock’, the boundary beyond which the solar wind’s influence begins to wane. And this year researchers were expecting it to meet another boundary — one at which the solar wind sharply reverses direction, signalling the beginning of interstellar space.
Instead, Krimigis says, measurements of low-energy charged particles show that the solar wind has gradually slowed to zero and is mingling with interstellar gases. Theories failed to predict this mixed-up environment, and Krimigis says it may even be possible that this is, in fact, what interstellar space looks like. “We may have crossed and don’t know it, because nobody has a model that describes what we’re seeing,” he says.
The blowing of far-flung interstellar gases may seem inconsequential to those of us closer to the Sun, but the details do matter, says Voyager’s chief scientist Ed Stone at the California Institute of Technology in Pasadena. The Sun is currently flying through debris from several nearby supernovae. Streams of particles and the magnetic fields produced by our star are shielding us from the some of the interstellar radiation from the blasts, he says. “The size of this bubble is important.”
Voyager should be able to provide more answers in the coming years. The spacecraft’s plutonium power plant will allow it to operate smoothly until at least 2020, and “we will continue to be taking data”, says Krimigis. Even after its signal fades, the spacecraft’s journey will continue; it should pass the constellation Camelopardalis in around 40,000 years.


1. Stamatios M. Krimigis, Edmond C. Roelof, Robert B. Decker & Matthew E. Hill
Voyager 1 has been in the reservoir of energetic ions and electrons that constitutes the heliosheath since it crossed the solar wind termination shock on 16 December 2004 at a distance from the Sun of 94 astronomical units (1 AU = 1.5 × 108 km). It is now ~22 AU past the termination shock crossing. The bulk velocity of the plasma in the radial–transverse plane has been determined using measurements of the anisotropy of the convected energetic ion distribution. Here we report that the radial component of the velocity has been decreasing almost linearly over the past three years, from ~70 km s−1 to ~0 km s−1, where it has remained for the past eight months. It now seems that Voyager 1 has entered a finite transition layer of zero-radial-velocity plasma flow, indicating that the spacecraft may be close to the heliopause, the border between the heliosheath and the interstellar plasma. The existence of a flow transition layer in the heliosheath contradicts current predictions—generally assumed by conceptual models—of a sharp discontinuity at the heliopause……

A Big Surprise from the Edge of the Solar System

NASA’s Voyager probes are truly going where no one has gone before. Gliding silently toward the stars, 9 billion miles from Earth, they are beaming back news from the most distant, unexplored reaches of the solar system.
Mission scientists say the probes have just sent back some very big news indeed.
It’s bubbly out there.

Magnetic bubbles at the edge of the solar system are aboout 100 million miles wide--similar to the distance between

According to computer models, the bubbles are large, about 100 million miles wide, so it would take the speedy probes weeks to cross just one of them. Voyager 1 entered the “foam-zone” around 2007, and Voyager 2 followed about a year later. At first researchers didn’t understand what the Voyagers were sensing–but now they have a good idea.
“The sun’s magnetic field extends all the way to the edge of the solar system,” explains Opher. “Because the sun spins, its magnetic field becomes twisted and wrinkled, a bit like a ballerina’s skirt. Far, far away from the sun, where the Voyagers are now, the folds of the skirt bunch up.”
When a magnetic field gets severely folded like this, interesting things can happen. Lines of magnetic force criss-cross, and “reconnect”. (Magnetic reconnection is the same energetic process underlying solar flares.) The crowded folds of the skirt reorganize themselves, sometimes explosively, into foamy magnetic bubbles.
“We never expected to find such a foam at the edge of the solar system, but there it is!” says Opher’s colleague, University of Maryland physicist Jim Drake.
Theories dating back to the 1950s had predicted a very different scenario: The distant magnetic field of the sun was supposed to curve around in relatively graceful arcs, eventually folding back to rejoin the sun. The actual bubbles appear to be self-contained and substantially disconnected from the broader solar magnetic field.
Energetic particle sensor readings suggest that the Voyagers are occasionally dipping in and out of the foam—so there might be regions where the old ideas still hold. But there is no question that old models alone cannot explain what the Voyagers have found.
Says Drake: “We are still trying to wrap our minds around the implications of these findings.”
The structure of the sun’s distant magnetic field—foam vs. no-foam—is of acute scientific importance because it defines how we interact with the rest of the galaxy. Researchers call the region where the Voyagers are now “the heliosheath.” It is essentially the border crossing between the Solar System and the rest of the Milky Way. Lots of things try to get across—interstellar clouds, knots of galactic magnetism, cosmic rays and so on. Will these intruders encounter a riot of bubbly magnetism (the new view) or graceful lines of magnetic force leading back to the sun (the old view)?

Old and new views of the heliosheath. Red and blue spirals are the gracefully curving magnetic field lines of orthodox models. New data from Voyager add a magnetic froth (inset) to the mix.

The case of cosmic rays is illustrative. Galactic cosmic rays are subatomic particles accelerated to near-light speed by distant black holes and supernova explosions. When these microscopic cannonballs try to enter the solar system, they have to fight through the sun’s magnetic field to reach the inner planets.
“The magnetic bubbles appear to be our first line of defense against cosmic rays,” points out Opher. “We haven’t figured out yet if this is a good thing or not.”
On one hand, the bubbles would seem to be a very porous shield, allowing many cosmic rays through the gaps. On the other hand, cosmic rays could get trapped inside the bubbles, which would make the froth a very good shield indeed.
So far, much of the evidence for the bubbles comes from the Voyager energetic particle and flow measurements. Proof can also be obtained from the Voyager magnetic field observations and some of this data is also very suggestive. However, because the magnetic field is so weak, the data takes much longer to analyze with the appropriate care. Thus, unraveling the magnetic signatures of bubbles in the Voyager data is ongoing.
“We’ll probably discover which is correct as the Voyagers proceed deeper into the froth and learn more about its organization,” says Opher. “This is just the beginning, and I predict more surprises ahead.”

Read also:
1. Voyagers ride ‘magnetic bubbles’
2. Voyager at the edge

Voyager Set to Enter Interstellar Space

This labeled artist's concept shows NASA's two Voyager spacecraft exploring a turbulent region of space known as the heliosheath, the outer shell of the bubble of charged particles around our sun. After more than 33 years of travel, the two Voyager spacecraft will soon reach interstellar space, which is the space between stars. Credit: NASA/JPL-Caltech

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