Data from Voyager 1, now more than 11 billion miles from the sun, suggest the
spacecraft is closer to becoming the first human-made object to reach
interstellar space.

Research using Voyager 1 data and published in the
journal Science Thursday provides new detail on the last region the spacecraft
will cross before it leaves the heliosphere, or the bubble around our sun, and
enters interstellar space. Three papers describe how Voyager 1’s entry into a
region called the magnetic highway resulted in simultaneous observations of the
highest rate so far of charged particles from outside heliosphere and the
disappearance of charged particles from inside the heliosphere.

Scientists have seen two of the three signs of interstellar arrival they
expected to see: charged particles disappearing as they zoom out along the solar
magnetic field and cosmic rays from far outside zooming in. Scientists have not
yet seen the third sign, an abrupt change in the direction of the magnetic
field, which would indicate the presence of the interstellar magnetic field.

“This strange, last region before interstellar space is coming into
focus, thanks to Voyager 1, humankind’s most distant scout,” said Ed Stone,
Voyager project scientist at the California Institute of Technology in Pasadena.
“If you looked at the cosmic ray and energetic particle data in isolation, you
might think Voyager had reached interstellar space, but the team feels Voyager 1
has not yet gotten there because we are still within the domain of the sun’s
magnetic field.”

Scientists do not know exactly how far Voyager 1 has to
go to reach interstellar space. They estimate it could take several more months,
or even years, to get there. The heliosphere extends at least 8 billion miles
beyond all the planets in our solar system. It is dominated by the sun’s
magnetic field and an ionized wind expanding outward from the sun. Outside the
heliosphere, interstellar space is filled with matter from other stars and the
magnetic field present in the nearby region of the Milky Way.

Voyager 1
and its twin spacecraft, Voyager 2, were launched in 1977. They toured Jupiter,
Saturn, Uranus and Neptune before embarking on their interstellar mission in
1990. They now aim to leave the heliosphere. Measuring the size of the
heliosphere is part of the Voyagers’ mission.

The Science papers focus
on observations made from May to September 2012 by Voyager 1’s cosmic ray,
low-energy charged particle and magnetometer instruments, with some additional
charged particle data obtained through April of this year.

Voyager 2 is
about 9 billion miles from the sun and still inside the heliosphere. Voyager 1
was about 11 billion miles from the sun Aug. 25 when it reached the magnetic
highway, also known as the depletion region, and a connection to interstellar
space. This region allows charged particles to travel into and out of the
heliosphere along a smooth magnetic field line, instead of bouncing round in all
directions as if trapped on local roads. For the first time in this region,
scientists could detect low-energy cosmic rays that originate from dying stars.

“We saw a dramatic and rapid disappearance of the solar-originating
particles. They decreased in intensity by more than 1,000 times, as if there was
a huge vacuum pump at the entrance ramp onto the magnetic highway,” said
Stamatios Krimigis, the low-energy charged particle instrument’s principal
investigator at the Johns Hopkins University Applied Physics Laboratory in
Laurel, Md. “We have never witnessed such a decrease before, except when Voyager
1 exited the giant magnetosphere of Jupiter, some 34 years ago.”

Other
charged particle behavior observed by Voyager 1 also indicates the spacecraft
still is in a region of transition to the interstellar medium. While crossing
into the new region, the charged particles originating from the heliosphere that
decreased most quickly were those shooting straightest along solar magnetic
field lines. Particles moving perpendicular to the magnetic field did not
decrease as quickly. However, cosmic rays moving along the field lines in the
magnetic highway region were somewhat more populous than those moving
perpendicular to the field. In interstellar space, the direction of the moving
charged particles is not expected to matter.

In the span of about 24
hours, the magnetic field originating from the sun also began piling up, like
cars backed up on a freeway exit ramp. But scientists were able to quantify the
magnetic field barely changed direction — by no more than 2 degrees.

“A
day made such a difference in this region with the magnetic field suddenly
doubling and becoming extraordinarily smooth,” said Leonard Burlaga, the lead
author of one of the papers, and based at NASA’s Goddard Space Flight Center in
Greenbelt, Md. “But since there was no significant change in the magnetic field
direction, we’re still observing the field lines originating at the sun.”