...Where No Man (Or Machine) Has Gone Before...
NASA's Voyager 1 spacecraft has entered the solar system's final
frontier. It is entering a vast, turbulent expanse where the Sun's
influence ends and the solar wind crashes into the thin gas between
stars.
"Voyager 1 has entered the final lap on its race to the edge of
interstellar space," said Dr. Edward Stone, Voyager project
scientist at the California Institute of Technology in Pasadena.
Caltech manages NASA's Jet Propulsion Laboratory in Pasadena, which
built and operates Voyager 1 and its twin, Voyager 2.
In November 2003, the Voyager team announced it was seeing
events unlike any in the mission's then 26-year history. The team
believed the unusual events indicated Voyager 1 was approaching a
strange region of space, likely the beginning of this new frontier
called the termination shock region. There was considerable
controversy over whether Voyager 1 had indeed encountered the
termination shock or was just getting close.
The termination shock is where the solar wind, a thin stream of
electrically charged gas blowing continuously outward from the Sun,
is slowed by pressure from gas between the stars. At the
termination shock, the solar wind slows abruptly from a speed that
ranges from 700,000 to 1.5 million miles per hour and becomes
denser and hotter. The consensus of the team is that Voyager 1, at
approximately 8.7 billion miles from the Sun, has at last entered
the heliosheath, the region beyond the termination shock.
Predicting the location of the termination shock was hard,
because the precise conditions in interstellar space are unknown.
Also, changes in the speed and pressure of the solar wind cause the
termination shock to expand, contract and ripple.
The most persuasive evidence that Voyager 1 crossed the
termination shock is its measurement of a sudden increase in the
strength of the magnetic field carried by the solar wind, combined
with an inferred decrease in its speed. This happens whenever the
solar wind slows down.
In December 2004, the Voyager 1 dual magnetometers observed the
magnetic field strength suddenly increasing by a factor of
approximately 2-1/2, as expected when the solar wind slows down.
The magnetic field has remained at these high levels since
December. NASA's Goddard Space Flight Center, Greenbelt, Md., built
the magnetometers.
Voyager 1 also observed an increase in the number of high-speed
electrically charged electrons and ions and a burst of plasma wave
noise before the shock. This would be expected if Voyager 1 passed
the termination shock. The shock naturally accelerates electrically
charged particles that bounce back and forth between the fast and
slow winds on opposite sides of the shock, and these particles can
generate plasma waves.
"Voyager's observations over the past few years show the
termination shock is far more complicated than anyone thought,"
said Dr. Eric Christian, Discipline Scientist for the Sun-Solar
System Connection research program at NASA Headquarters,
Washington.
The result is being presented today
at a press conference in the Morial Convention Center, New Orleans,
during the 2005 Joint Assembly meeting of Earth and space science
organizations.
For their original missions to Jupiter and Saturn, Voyager 1 and
sister spacecraft Voyager 2 were destined for regions of space far
from the Sun where solar panels would not be feasible, so each was
equipped with three radioisotope thermoelectric generators to
produce electrical power for the spacecraft systems and
instruments. Still operating in remote, cold and dark conditions 27
years later, the Voyagers owe their longevity to these Department
of Energy-provided generators, which produce electricity from the
heat generated by the natural decay of plutonium dioxide.