A spacecraft from Earth has left its cosmic backyard and
taken its first steps in interstellar space, NASA says.
“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, and as it enters interstellar space, it adds a new chapter in human scientific dreams and endeavors,” NASA science chief John Grunsfeld said in a statement. “Perhaps some future deep-space explorers will catch up with Voyager, our first interstellar envoy, and reflect on how this intrepid spacecraft helped enable their future.”
A long and historic journey
Voyager 1 launched on Sept. 5, 1977, about two weeks after
its twin, Voyager 2. Together, the two probes conducted a historic “grand tour”
of the outer planets, giving scientists some of their first up-close looks at
Jupiter, Saturn, Uranus, Neptune and the moons of these faraway worlds.
The duo completed its primary mission in 1989, and then kept
on flying toward the edge of the heliosphere, the huge bubble of charged
particles and magnetic fields that the sun puffs out around itself. Voyager 1
has now popped free of this bubble into the exotic and unexplored realm of
interstellar space, the scientists behind the latest study said.
They reached this historic conclusion with a little help
from the sun. A powerful solar eruption caused electrons in Voyager 1’s
location to vibrate significantly between April 9 and May 22 of this year. The
probe’s plasma wave instrument detected these oscillations, and researchers
used the measurements to figure out that Voyager 1’s surroundings contained
about 0.005 electrons per cubic inch (0.08 electrons per cubic centimeter).
That’s far higher than the density observed in the outer
regions of the heliosphere (roughly 0.0001 electrons per cubic inch, or 0.002
electrons per cubic cm) and very much in line with the 0.006 electrons per cubic
inch (0.10 electrons per cubic cm) or so expected in interstellar space.
“We literally jumped out of our seats when we saw these
oscillations in our data — they showed us that the spacecraft was in an
entirely new region, comparable to what was expected in interstellar space, and
totally different than in the solar bubble,” study lead author Don Gurnett of
the University of Iowa, the principal investigator of Voyager 1’s plasma wave
instrument, said in a statement.
It may seem surprising that electron density is higher beyond the solar system than in its extreme outer reaches. Interstellar space is, indeed, emptier than the regions in Earth’s neighborhood, but the density inside the solar bubble drops off dramatically at great distances from the sun, researchers said.
Magnetic-field mystery
Mission scientists have long pegged Voyager 1’s departure
from the heliosphere on the observation of three phenomena: a big drop in solar
particles, a dramatic jump in galactic cosmic rays and a shift in the
orientation of the surrounding magnetic field.
Voyager 1 has measured the first two changes, but not the
third; the magnetic field is stronger than it used to be in the probe’s
location, but it hasn’t changed direction.
This key point led NASA and the mission team to proceed with
caution. For example, they held off on making any big announcements, despite
several recent studies by outside researchers — including one published lastmonth — suggesting that Voyager 1 entered interstellar space in July or August
2012.
But the new electron-density measurements have convinced
Voyager mission scientists that the probe is, indeed, beyond the solar bubble.
After all, magnetic-field measurements were always regarded
as a proxy for observations of electron density, said Voyager chief scientist
Ed Stone, a physicist at the California Institute of Technology in Pasadena.
“The solar wind carries the solar magnetic field with it, and the interstellar wind carries the galactic magnetic field with it,” Stone, who is not an author of the new Science paper, told Space.com. “Once we got the plasma data itself in interstellar space, we knew we must have left the bubble.”
(Voyager 1 launched with an instrument designed to measure
plasma density directly, but it failed in 1980, forcing the team to get more
creative.)
Scientists need a better understanding of the complex
interface between the solar and galactic magnetic fields to figure out why
Voyager 1 hasn’t measured the predicted change in field direction, Stone said.
“What we need to do now is go back and look more carefully at
the models of that interaction,” he said.
Voyage of exploration
The Voyager mission has racked up a series of discoveries
over the last 36 years, revealing key insights about the giant planets and
their moons, as well as conditions at the edge of the solar system.
The spacecraft’s arrival in interstellar space could bring
many more exciting finds, the researchers said.
“Every day we look at data, we know we’re looking at data
that no one has seen before and is in a region where nothing has ever been before,”
Stone said. “I think we’re all looking forward to learning a lot in the years
ahead.”
Voyager 1 could keep beaming data home for a while, provided
nothing too important breaks down. The spacecraft’s declining power supply
won’t force engineers to shut off the first instrument until 2020, mission
scientists have said. All of Voyager 1’s science gear will probably stop
working by 2025.