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What Does the Edge of the Solar System Look Like? Ask Voyager.

Voyager is reaching the edge of the solar system. This is no Christopher Columbus false call, mind you. Eight billion miles from the Sun, Voyager's 26-year journey has made it the farthest-reaching spacecraft in the solar system and it is about to set a new record. All of the planets and objects within our solar system are surrounded by a sort-of bubble created by supersonic wind from the Sun. The spacecraft that first laid its robotic eyes on Jupiter and Saturn, is about to burst through that bubble, or may have already, according to some scientists.

still image of heliosphere

This still shows the locations of Voyager 1 & 2 Voyager 1 is traveling faster, reaching the termination shock sooner. The dramatic orange border to the left represents the bow shock, a theoretical area created as interstellar gas runs into the solar atmosphere. The location of the termination shock, or the boundary into the area where interstellar gas and solar wind start to mix, has been a mystery to scientists because it moves with regard to the power of the solar wind. Click on the image for movie. Also avail: color print-resolution & other movie formats or black & white high res. Credit: NASA

"There has to be a sense of wonder in that this is the first manmade object that is touching interstellar space and this has never happened," said Dr. Tom Krimigis of the Applied Physics Laboratory (APL), Johns Hopkins University. "We're getting out of the protective cocoon of the Sun and this clearly marks a milestone in humanity's knowledge of our environment."

It makes sense that the Voyager mission, consisting of two identical spacecraft launched in the heady robotic exploration era of the 1970s should be the farthest traveled manmade objects ever, nearly four billion miles from Pluto's orbit. Voyager 1 provided the first views of volcanoes outside of Earth on one of Jupiter's moon, Io, as well as close-ups of Saturn's rings and evidence of a ring around Jupiter in 1979. Voyager 2 provided the very first look at Uranus and Neptune in 1986 and 1989, respectively. Also significant, both are appointed planetary ambassadors - they each carry a 12-inch gold-plated copper disk containing sounds and images selected to portray the culture and diversity of Earth and meant to be played by an alien form of life that may encounter Voyager.

Where's the Edge?

Christopher Columbus had to sail the high seas to prove the world was round; how are scientists so certain a bubble blown by the solar wind surrounds us? Scientists call the murky region to the left of the Voyager trajectory in the image the heliosheath, because everything within it is influenced by the Sun ('helio' in Greek).

Map of our galaxy

What's out there at the boundaries of our solar system? Starting out at a view of our Milky Way galaxy, the orange gas in the animation represents the interstellar medium. The bow shock is created because the heliosphere is moving through like a boat through the water, crashing through the interstellar gases. The bow shock in front of the moving heliosphere is similar to the one observed by the Hubble Space Telescope. Click for animation. Credit: NASA

The heliopause is the last boundary of that region where interstellar space, or matter from other stars, takes over as the ruler of the roost. Voyager isn't expected to reach the area for another 20 years. Entering the fluid region known as the heliosheath, past the theoretical boundary of the 'termination shock,' however, marks the first step toward that ultimate goal. The heliosheath represents a mixing bowl-region in which smaller amounts of solar wind mix with gas from outside our solar system.

NASA Scientist Dr. Eric Christian imagines the theoretical boundary looks a lot like the water that bounces off a plate in the sink. It's not a defined straight (or round) line, but more fluid so the boundary moves a little bit. This movement has been a problem for researchers. It's very hard to know when Voyager has crossed the line - in fact, two science teams are lining up right now with different interpretations of unusual readings in their data stream from the spacecraft.

heliosphere surrounding planets artificial eclipse of Sun showing CME expanding into space

Left: video of water running on a plate: the first border formed is like the termination shock and the water between the shock and the rim is the heliosheath. Water runs out toward the rim and then rushes back, similar to solar wind and indeed the reason scientists know the boundaries exist. Like the heliopause, once the water runs off the rim, it's out in the sink, or in this case, interstellar space. Credit: NASA/ESA. Right: solar wind raging into space as seen by the spacecraft SOHO after the solar storms of October 28. Credit: NASA/ESA. The European Space Agency has more on solar wind.

And then there are the implications of this discovery. Voyager scientists are about to re-write a lot of textbooks and update a lot of theories. As Dr. Krimigis said, "the models that predict what the Sun and Voyager do are very crude."

Uncharted Territory

Both Dr. Frank McDonald of the University of Maryland and Dr. Krimigis have spent the better part of their careers tracking the fascinating adventure that has been the Voyager mission. And they represent the two opposing sides to this new controversy. In August 2002, Voyager scientists saw data that they had never seen before. Namely a large amount of low-energy particles flowing away from the Sun. The APL group interpreted this as the energy from the Sun reaching a dead end in this particular area.

"All models and theories have been telling us that's where the edge of the solar system begins to feel the pressure from the interstellar space," said Dr. Krimigis. "When you're sitting on the beach, depending on the wind, the waves can become very strong and blow over you, and at other times you're perfectly dry. Solar wind blows hot and cold - when hot, it [washes over Voyager] and when it's cold it retreats."

Voyager Spacecraft

The Voyager 1 and 2 spacecraft are identical with different flight paths. Voyager 2 was actually launched first, on August 20, 1977; Voyager 1 was launched September 5. Both are about 8 billion miles from the Sun, but Voyager 1 is traveling at a speed of 3.6 AU per year while Voyager 2 is speeding along at about 3.3 AU per year. One 'AU' equals the distance between the Sun and Earth, or 93 million miles. Credit: NASA

This strange data appeared from August through February, convincing the APL team that for six months Voyager had entered into the heliosheath. In February, it was as if the heliosheath retreated like water moving back toward the ocean at the beach, and Voyager was back within the solar wind. Meanwhile Dr. McDonald's team analyzed data from their instruments and concluded that they had merely rubbed against the boundary of the termination shock, but had not entered it. While both groups are uncertain what happened for those six months, they are both convinced that Voyager is currently back within the bubble of the solar wind, and will be crossing that termination shock either again in the next year, or for the first time.

"We say we're just in the suburbs approaching the termination shock. They would say we're downtown, we're there," said Dr. McDonald.

A Great Educated Guess

Scientists are basing their theories on examples of this termination shock phenomenon they have seen before. The Hubble Space Telescope snapped a picture of a bow shock created as the wind from a star more powerful than that of our Sun's collided with a young star in the nearby Orion Nebula. As the fast stellar wind ran into slow moving gas, a shock front was formed, like the wave created when a boat moves through water.

HST image of L.L. Orionis Nebula

The Hubble Space Telescope imaged this view in February 1995. The arcing, graceful structure is actually a bow shock about half a light-year across, created from the wind from the star L.L. Orionis colliding with the Orion Nebula flow. For more information on this image, see HubbleSite. Click on the image for a very large version. Credit: NASA, The Hubble Heritage Team (STScI/AURA)

Unfortunately, the instrument on Voyager 1 that measures solar wind has stopped working. A few of the remaining instruments have tried to pick up the slack, but that makes for a bit more guesswork and legwork on the part of scientists to prove their case. The excitement resulting from the controversy is inspiring the science community and the many people who continue to be fascinated by Voyager's journey. It also provides a great example of the uncertainty scientists deal with on a daily basis as they create theories, and then modify them again and again.

In the meantime, as Dr. McDonald put it, "We keep making new discoveries, going places no one has ever been before."

And isn't that the point of exploration?

More Voyager Resources

JPL Voyager Home Page

Voyager's Golden Record / Flash Feature

The History & Science of Voyager

Voyager Press Release & Images

Rachel A. Weintraub
NASA Goddard Space Flight Center