(Updates with details, clarifies Voyager detection of plasma)
By Irene Klotz
CAPE CANAVERAL, Fla., Sept 12 (Reuters) - Scientists have been debating for more than a year whether NASA's 36-year-old Voyager 1 spacecraft has left the solar system and become the first human-made object to reach interstellar space.
By a fluke measurement, they now know definitively it has.
"We made it," lead Voyager scientist Edward Stone, from the California Institute of Technology, told reporters on Thursday.
The key piece of evidence came by chance when a pair of solar flares blasted charged particles in Voyager's direction in 2011 and 2012. It took a year for the particles to reach the spacecraft, providing information that could be used to determine how dense the plasma was in Voyager's location.
Plasma consists of charged particles and is more prevalent in the extreme cold of interstellar space than in the hot bubble of solar wind that permeates the solar system.
Voyager 1, now 13 billion miles (21 billion km) from Earth, could not make the measurement directly because its plasma detector stopped working more than 30 years ago.
"This was basically a lucky gift from the sun," Stone said.
Extrapolating from the measurements, scientists believe Voyager actually left the solar system in August 2012. That summer, the spacecraft radioed back another tantalizing piece of information, showing a huge spike in the number of galactic cosmic rays from outside the solar system and a corresponding decrease in particles emanating from the sun.
Scientists had been reluctant to conclude last year that Voyager had reached interstellar space because it was still picking up magnetic field measurements that were very similar to the sun's magnetic field.
Computer models had predicted a significant shift in the interstellar magnetic field's alignment.
"The magnetic field is still something that puzzles us considerably," said physicist Gary Zank, with the University of Alabama in Huntsville.
Scientists now believe the interstellar magnetic field is somehow draped around and twisted by the heliosphere, the bubble of space under the sun's influence.
Understanding how that happens is just one of the questions the Voyager team will attempt to figure out while the probe still has power. Voyager 1, and a sister spacecraft Voyager 2, use heat released by the natural decay of radioactive plutonium to generate electrical power for their instruments.
After 2020, scientists expect they will have to start turning off instruments, until around 2025 when the probes will be completely out of power and fall silent.
Voyager 2, which is heading out of the solar system in another direction, has five to seven more years before it reaches interstellar space, said Donald Gurnett, a longtime Voyager scientist at the University of Iowa.
"We're in a truly alien environment," Zank said. "What Voyager is going to discover truly beggars the imagination."
The two Voyager probes, which were both launched in 1977 to study the outer planets of the solar system, contain gold phonographic records etched with music, greetings, sounds and images from Earth. The project was spearheaded by astronomer Carl Sagan, who died in 1996.
With Voyager 1 having left the solar system, the next time it will encounter a star is in 40,000 years, when it flies about 1.7 light years away from a star in the constellation Camelopardalis called AC +79 3888. The spacecraft is traveling nearly 1 million miles (1.6 million km) a day.
"Voyager has once again joined the ranks of the great human journeys of exploration," Gurnett said. "This is the first journey into interstellar space."
NASA's twin Pioneer spacecraft, launched in the 1970s, also are leaving the solar system, but they have run out of power to relay information back to Earth.
The research is published in this week's journal Science. (Editing by Jane Sutton and Peter Cooney)