NASA's intrepid Voyager 1 space probe has begun its journey to the stars and is now exploring the farthest reaches of the Sun's influence where the solar wind strangely interacts with interstellar space, agency officials formally announced on Tuesday.

Credit: NASA/JPL

At the 2005 Joint Assembly Meeting of the American Geophysical Union in New Orleans, scientists revealed new findings that confirm the passage of the Voyager 1 spacecraft into an uncharted region of the far outer solar system, where the magnetic field intensifies and the solar wind is drastically slowed and becomes super-heated.

"Voyager has entered the final lap in its race to interstellar space as it begins exploring the solar system's final frontier," said Voyager project scientist Dr. Edward Stone of the California Institute of Technology.

Voyager 1 and its twin Voyager 2 were both launched aboard Titan rockets from Cape Canaveral, Florida, in 1977 to embark on a "grand tour" to visit the outer planets. Both are now approaching their 28th birthday and continue on extended missions to beam data back to Earth as it leaves the solar system headed for interstellar space.

Officials say Voyager 1 crossed what is known as the termination shock around December 16 of last year to enter the heliosheath- a place unlike any ever visited in the solar system. The boundary crossed in December marks a point where charged particles constantly emitted from the Sun called the solar wind slow down from hundreds of miles per second to subsonic speeds. This is due to pressure from the interstellar wind, or the gas blowing between stars likely resulting from ancient nearby supernovae.

Evidence of the transition was not received until the next day because the large Deep Space Network antennas were not scheduled to be in contact with Voyager 1 on December 16. Also a factor was the length of time it takes communications to travel one-way to and from the probe- now over 13 hours. However, all indications say the passage likely occurred December 16.

This shock also causes the temperature of the solar wind to significantly increase from about 200,000 degrees to over a million degrees due to the increased density of the particles once inside the heliosheath, which extends out to the heliopause and bow shock - the last official boundaries encountered before entering interstellar space. Dr. Stone likened the shocks to the waves generated ahead of the bow of a moving ship as the water is pushed forward and compacted.

Affects from the solar wind are felt across the vast expanse of the solar system known as the heliosphere, which scientists hypothesize is shaped much like a comet with a long tail due to the Sun's path through the Milky Way galaxy.

"The solar wind creates a bubble (the heliosphere) around the Sun, and near the edges of the bubble is a place where the solar wind piles up as it encounters the interstellar wind," Dr. Stone explained.

This artist's concept shows the locations of Voyagers 1 and 2. Voyager 1 is traveling a lot and has crossed into the heliosheath, the region where interstellar gas and solar wind start to mix. Credit: NASA/Walt Feimer

The entire heliosphere is impacted by the 11-year solar cycle, and Dr. Stone said Voyager team members believe the boundaries marking the edge of the solar system actually expand and contract as the Sun's activity level changes. The crossing of the termination shock could have actually been more due to the shrinking of the heliosphere than the outward motion of Voyager 1, he said.

Voyager 1 made the historic passage into the heliosheath at a distance 94 times that which lies between the Earth and Sun, or about 8.7 billion miles away.

Scientists point to tell-tale signs such as magnetic field changes and changes in the strength of particle energies measured since December as evidence the entrance into the heliosheath has occurred.

Two magnetometers aboard the Voyager 1 probe have been used throughout the mission as it flew past Jupiter and Saturn and as it continued its journey into the unknown. Measurements from the instruments indicate the magnetic field has been compressed and has increased by a factor of about two-and-a-half.

Particle energies recorded by the plasma wave instrument aboard Voyager 1 noted the strength of the particle beams was much more uniform than before, giving the ground team even more reason to believe the spacecraft is now in the heliosheath. A large burst of plasma wave noise was also observed and downlinked in telemetry data.

Officials had observed other substantial changes in science data throughout 2002 and 2003 as the intensity of charged particles within the solar wind spiked to high levels, but there was no increase in the strength of the magnetic field that would occur when the solar wind dramatically slows, marking the entrance into the heliosheath.

"The missing element that is there this time is the compression of the magnetic field," Dr. Stone explained.

It is believed that sistership Voyager 2 will cross the termination shock within the next three to five years, allowing scientists to once again be glued to incoming telemetry to try and learn more about this last leg of the trip out of the solar system. Voyager 2 is currently at a distance from the Sun of about 76 astronomical units, or around 6.5 billion miles.

"Voyager's observations over the past few years show that the termination shock is far more complicated than anyone thought," said Dr. Eric Christian, a discipline scientist with NASA's Sun-Solar System research initiative.

Before reaching the heliopause and passing into interstellar space, the Voyagers must complete a trip through the turbulent heliosheath that is projected to last about ten years. "The thickness (of the heliosheath) is unknown, and that is one of the things we intend to discover," Dr. Stone said. "We have a new region to explore."

The Voyager spacecraft are still in relatively good health with many of their instruments soldiering on in this new harsh environment almost 28 years after launch. Electrical power for the probes are produced by three radioisotope thermoelectric generators on each craft, which are projected to last until about 2020.

NASA is considering putting the $4.5 million per year Voyager project on the chopping block in the coming year in a cost-cutting move to prepare for the retirement of the space shuttle and the development of the next-generation Crew Exploration Vehicle, part of the agency's presidentially-mandated Vision for Space Exploration.

The Planetary Society reported Tuesday that a review of the project has been moved up from next year to November to help facilitate attempts to make sure the Voyager program is put into the NASA budget over the coming years.

"This is a wonderful opportunity to reach interstellar space, and we hope we can keep the spacecraft operating through the year 2020," Dr. Stone concluded.

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