CAPE CANAVERAL, Florida (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
"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
"The magnetic field is still something that
puzzles us considerably," said physicist Gary
Zank, with the University of Alabama in
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.
'TRULY ALIEN ENVIRONMENT'
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
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