WASHINGTON - Never before has a human-built spacecraft traveled so far. NASA's Voyager 1 probe has now left the solar system and is wandering the galaxy, U.S. scientists said Thursday.
The spacecraft was launched in 1977 on a mission to explore the outer planets of our solar system and to possibly journey into the unknown depths of outer space.
"This is the first time that humanity has been able to step outside of the cradle of the solar system to explore the larger galaxy," Marc Swisdak, an astrophysicist at the University of Maryland, told AFP.
The precise position of Voyager has been fiercely debated in the past year, because scientists have not known exactly what it would look like when the spacecraft crossed the boundary of the solar system -- and the tool on board that was meant to detect the change broke long ago.
However, U.S. space agency scientists now agree that Voyager is officially outside the protective bubble known as the heliosphere that extends at least eight billion miles beyond all the planets in our solar system, and has entered a cold, dark region known as interstellar space.
Their findings -- which describe the conditions that show Voyager actually left the solar system in August 2012 -- are published in the U.S. journal Science.
"Voyager has boldly gone where no probe has gone before marking one of the most significant technological achievements in the annals of the history of science," said a statement by John Grunsfeld, NASA's associate administrator for the Science Mission Directorate.
The twin spacecraft, Voyager 1 and Voyager 2, were launched in 1977 on a primary mission to explore Jupiter and Saturn.
They discovered new details about the nature of Saturn's rings and found volcanoes on Jupiter's moon Io.
Voyager 2 traveled on to Uranus and Neptune, before the duo's mission was extended to explore the outer limits of the Sun's influence.
Voyager 1 -- with Voyager 2 a few years behind in its travels to the edge of the solar system -- sent back data to scientists on Earth on August 25 last year, showing an abrupt drop in energetic charged particles, or cosmic rays, that are produced inside the heliosphere.
Scientists expected that the direction of the magnetic field in space would reverse at the barrier known as the heliopause.
The Voyager 1 magnetometer did not show this change, leading scientists to be extra cautious about declaring whether or not the spacecraft had left the solar system.
However, an analysis of data from Voyager's plasma wave science instrument between April 9 and May 22 this year showed the spacecraft was in a region with an electron density of about 0.08 per cubic centimeter.
Astrophysicists have projected that the density of electrons in interstellar space would be between 0.05 and 0.22 per cubic centimeter, placing Voyager squarely in that range.
"Now that we have new, key data, we believe this is humankind's historic leap into interstellar space," said Ed Stone, Voyager project scientist based at the California Institute of Technology, Pasadena.
"The Voyager team needed time to analyze those observations and make sense of them. But we can now answer the question we've all been asking: 'Are we there yet?' Yes, we are."
While the Voyager team has reached a consensus, not all are convinced.
"I don't think it's a certainty Voyager is outside now," space physicist David McComas of the Southwest Research Institute in San Antonio, Texas told Science magazine.
"It may well have crossed," he said. "But without a magnetic field direction change, I don't know what to make of it."
The spacecraft is expected to keep cruising for now, though the radioisotope thermo-electric generators that power it are beginning to run down.
Voyager's instruments will have to shut down permanently in 2025, Science reported. NASA spends $5 million per year to operate the twin spacecraft.
"Even though it took 36 years, it's just an amazing thing to me," said co-author Bill Kurth, of the University of Iowa.
"I think the Voyager mission is a much grander voyage of humankind than anyone had dreamed."
-- Kerry Sheridan, Jean-Louis Santini, AFP
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