he Voyager I spacecraft has moved into the solar system's final frontier, a vast area where the sun's influence gives way to interstellar space, NASA's Web site reports.
"Voyager has entered the final lap on its race to the edge of interstellar space, as it begins exploring the solar system's final frontier," said Edward Stone, Voyager project scientist at the California Institute of Technology in Pasadena, in a statement on the Web site Tuesday.
At 8.7 billion miles from the sun, Voyager I has entered the heliosheath, a region beyond termination shock -- the critical boundary that marks the transition from the solar system into interstellar space.
Contrary to popular belief, space is not an empty void. Rather, the solar system is awash in solar wind, charged gases that flow off the sun at supersonic speed. The wind travels at an average speed ranging from 300 to 700 kilometers per second (700,000 to 1.5 million mph).
At the termination shock boundary, the solar wind dissipates and begins to give way to the interstellar medium -- the gases that float in the void between stars.
Instruments aboard Voyager I able to measure the solar wind's speed suggest the probe "has passed through the termination shock into the slower, denser wind beyond," NASA's Web site says.
"Voyager's observations over the past few years show that the termination shock is far more complicated than anyone thought," said a statement from Eric Christian, discipline scientist for NASA's Sun-Solar System Connection research program in Washington.
Beyond the heliosheath lies the heliopause -- the boundary where the pressure of the solar wind and interstellar wind are in balance.
Scientists believe Voyager 1 will reach the heliopause in about 10 years.
Once Voyager I passes through the heliopause, it will be in interstellar space.
Voyager I and its sister ship, Voyager II, were launched in 1977 on a mission to explore the solar system. Voyager I passed Jupiter in March 1979 and Saturn in November 1980. In 1998, it became the most distant man-made object from Earth.
Voyager II, which observed Uranus and Neptune, is 6.5 billion miles from the sun, heading in the opposite direction of Voyager I and at a slightly slower speed.
The Voyager probes were equipped with three radioisotope thermoelectric generators to produce electrical power for the spacecraft systems and instruments.
Barring hardware failure, Voyager I and II boast enough power and communications capability to keep radioing back to Earth until 2020, NASA says