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Topic: [ncsm-members] General Interest: Voyager 1 Leaves Solar System
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Jerry P. Becker

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Registered: 12/3/04
[ncsm-members] General Interest: Voyager 1 Leaves Solar System
Posted: Sep 20, 2013 7:16 PM
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From Scientific American, Thursday, September 12, 2013. See
[NOTE: See interesting comments on this article at the end.]

Voyager 1 Leaves the Solar System-for Real This Time

After much debate over the murky boundary of interstellar space, a
solar eruption gives scientists the evidence to say Voyager 1 has
finally crossed it

By Clara Moskowitz

Voyager 1 was starting to get a reputation as the spacecraft that
cried wolf, after scientists repeatedly claimed it was leaving the
solar system, only to change their minds and say it wasn't quite
there yet. Now researchers say new evidence shows Voyager really has
departed the sun's sphere of influence and become the first man-made
object to reach interstellar space.

Voyager 1, launched in 1977, is speeding away from us, traveling
about 3.5 times the Earth-sun distance every year. The probe is now
about 18.2 billion kilometers from Earth, farther away than anything
human beings have sent into space. (Its sister spacecraft, Voyager 2,
launched more than a month earlier, took a more circuitous path from
Earth and is now about 14.8 billion kilometers away.)

Scientists have long been expecting Voyager 1 to exit the bubble of
space containing particles from the sun, called the heliosphere, and
enter a region where particles are much more plentiful and come from
ancient explosions of other stars. But because Voyager 1 has lost its
ability to measure this particle plasma, there was no easy way to
tell when the transition had occurred.

A boon came from an eruption on the sun in March 2012, which sent
waves of solar material out into space. When this ejection reached
Voyager 1 13 months later in April 2013, it set the local plasma
vibrating. Voyager 1's plasma wave instrument (separate from the
defunct plasma particle detector) was able to measure the pitch of
these vibrations, which in turn reflected the density of plasma
around the spacecraft. The results show that Voyager 1 is surrounded
by plasma more than 40 times denser than it encountered earlier, when
it was in the heliosphere. "Because there's no other possible
conclusion, I think we're forced to and obliged to conclude that
we're truly in the interstellar medium," physicist Gary Zank of the
University of Alabama in Huntsville said during a press conference

Based on abrupt changes in the apparent plasma density around the
spacecraft, the researchers were even able to pinpoint August 25,
2012 as the most likely date that Voyager 1 left the solar system,
crossing the heliopause, the boundary between the heliosphere and the
interstellar medium.

The "little spacecraft that could," in the words of project manager
Suzanne Dodd of the NASA Jet Propulsion Laboratory in Pasadena,
Calif., is getting old in years and is technologically weak: the
average smartphone has thousands of times more memory than the
spacecraft. Yet most of Voyager 1's instruments still work, and they
continue to send signals back to Earth. The team estimates the probe
still has enough power from its plutonium power plant to operate all
its instruments through 2020, when it will begin shutting them off
one by one, until it goes dark in 2025. That still gives Voyager 1
more than a decade to study the realm of the universe it has entered.
"It marks the beginning of a new era of exploration for Voyager: the
exploration of the space between the stars," said Ed Stone, Voyager's
longtime project scientist.

The new findings represent a turnaround for the Voyager team, which
as recently as June predicted the spacecraft may have years to go
before it reached interstellar space. But the latest data remove most
of the team members' doubts, Stone said.

The new report, published online today in Science, also agrees with
the conclusions of a separate paper claiming Voyager 1 had left the
solar system, based on magnetic field data, which was published
August 14 in The Astrophysical Journal Letters. The lead author of
that paper, University of Maryland, College Park, physicist Marc
Swisdak, says the two projects are complementary. "I thought the
authors make an excellent case for their measurements and their
interpretation," Swisdak says.

Over the course of 36 years, between the two of them, the Voyager
probes have visited all the outer planets of the solar system and
discovered 23 moons around these worlds. They also each carry time
capsules in the form of phonograph records loaded with recordings of
music, natural sounds, voice greetings in 56 languages, and
photographs and diagrams of life on Earth. Voyager 1 is on a course
that should eventually take it within 1.6 light-years of a star
called AC+79 3888, which lies in the constellation of Camelopardalis.
It's due to arrive in about 40,000 years, long after the spacecraft
loses its ability to tell whomever is on Earth what comes of that
SIDEBAR: A depiction of Voyager 1 crossing into interstellar space.
Image: NASA/JPL-Caltech

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