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Voyages Beyond the Solar System: The Voyager Interstellar Mission
Editor's Note: This is the 25th in a series of essays
on exploration by Steven J. Dick.
The epic
journeys of Voyagers 1 and 2 are among the most astonishing tales of the Space
Age. Originally planned to explore the gas giant planets and their satellites,
which they did in brilliant fashion, they have continued their journeys and are
now the most distant human objects in the cosmos, tossed out into the sea of
space from our home planet.
Launched in the summer of 1977, both Voyager
spacecraft were designed to last five years, and both encountered Jupiter and
Saturn between 1979 and 1981. After the flyby of Saturn's moon Titan, Voyager 1
took a trajectory north of Saturn's orbital plane out of the solar system, while
Voyager 2 headed onward to Uranus and Neptune, courtesy of a gravity assist and
a rare planetary alignment. After encountering Uranus in 1986 and Neptune in
1989, Voyager 2 took a southward trajectory out of the solar system. The last
Voyager images were taken Valentine's Day, 1990, when Voyager 1 looked back from
3.7 billion miles to take a portrait of seven of the nine planets in our own
solar system, including the "pale blue dot" that is Earth. The data the Voyagers
returned revolutionized our knowledge of the outer planets and their intriguing
panoply of satellites.
It would have been an extraordinary voyage had it
ended there. But it did not. In 1989 the missions were jointly renamed the
Voyager Interstellar Mission. Both Voyagers continued their journeys, Voyager 1
heading northward at about 320 million miles per year, and Voyager 2 heading
southward at the slower rate of 290 million miles per year. As we enter 2007
Voyager 1 is more than 100 astronomical units distant (one AU is the distance
from the Earth to the Sun), about 9.5 billion miles, with Voyager 2 trailing on
its southward journey. Both are way beyond the dwarf planet Pluto, which
averages 40 astronomical units from the Sun and was formerly considered the edge
of the solar system.
Both Voyagers are also well beyond their
trailblazing predecessor spacecraft, Pioneer 10 and 11. The Pioneer 10
spacecraft, which returned the first close-up images of Jupiter in 1973 and was
for many years the most distant spacecraft from Earth, was passed in 1998 by
Voyager 1 at a distance of 69 astronomical units. Communications were lost in
2003, but Pioneer 10, with its famous engraved plaque message, continues onward
behind the Voyagers. Communication with its sister ship, Pioneer 11, was lost in
1995, but it too continues its outward journey at a slower rate than the
Voyagers.
The Voyager Interstellar Mission therefore provides an
opportunity to explore for the first time uncharted territory - interstellar
space. These journeys now raise an interesting practical question - where does
the solar system end and interstellar space begin? The answer requires a sojourn
into the celestial cartography of our solar system's edge. That edge, it turns
out, is defined not by objects or barriers, but by the phenomenon of the wind
from the Sun. Early spacecraft discovered a steady stream of electrically
charged gas flowing from the Sun; we now know that it creates a bubble called
the heliosphere (see image below). The solar wind travels outward at speeds of
700,000 to 1.5 million miles per hour, until it begins to feel the effects of
the interstellar wind composed of gas from other stars. At this point, called
the "termination shock," the speed of the solar wind slows abruptly. Continuing
outward, the solar wind comes to a boundary where the pressure of the two winds
is in balance, a point called the "heliopause". Here a bow shock is created like
that of a boat moving through water. Beyond the heliopause is interstellar
space.
 Locations of Voyager 1 & 2: Voyager 1
is traveling faster, and reached the termination shock sooner. The dramatic
orange border to the left represents the bow shock, a theoretical area created
as interstellar gas runs into the solar atmosphere. The location of the
termination shock, or the boundary into the area where interstellar gas and
solar wind start to mix, has been a mystery to scientists because it moves with
regard to the power of the solar wind. Image credit: NASA.
Although the Hubble Space Telescope has photographed examples of
this bow shock around other stars, the precise locations of the termination
shock and heliopause for our solar system have been matters of conjecture. Until
now, that is. On December 16, 2004, the magnetometers aboard Voyager 1 showed a
sudden increase in the surrounding magnetic field, the result of a solar wind
slowdown. At approximately 8.7 billion miles from the Sun (94 astronomical
units) Voyager 1 passed the termination shock and entered the heliosheath.
"Voyager 1 has entered the final lap on its race to the edge of interstellar
space," Voyager project scientist Ed Stone remarked at the time. Voyager 2, now
at about 82 astronomical units, will cross the termination shock in the next few
years, certainly by 2010. The exact timing in uncertain because the boundary
changes with changing conditions on the Sun, such as the sunspot
cycle.
What Voyager 1 did not find at the termination shock is as
important as what it did find. As expected, it found a steady source of
low-energy protons. It was also expected that the termination shock was the
source of something called "anomalous cosmic ray helium." But the intensity of
these cosmic rays did not peak at the shock, indicating that the termination
shock (at least in the vicinity of Voyager 1) was not the source of this type of
cosmic ray. Scientists consider it a "shocking" result. As Len Fisk (a former
head of space science at NASA) said in the Science magazine cover story where
the Voyager 1 results were announced, "Once again the mantra of space
exploration is fulfilled: when we go somewhere that is new, we find the
unexpected, and that's what makes it so exciting."
It will take another
10 or 20 years for Voyager 1 to reach the edge of the heliopause and enter true
interstellar space. It will indeed be a race, not between Voyagers 1 and 2, but
with the electrical power supply that provides communication with the
spacecraft. At these immense distances from the Sun, solar arrays are useless
and nuclear power is used. But even the radioisotope thermoelectric generators
(RTGs) aboard Voyager have a limited lifetime, and it is entirely possible the
Voyagers will pass silently into interstellar space.
Even if
communications are lost, the silence will not be total. Long after last contact
with Earth, the Voyagers will continue their interstellar journeys. Aside from
the technological message represented by the spacecraft themselves, both
spacecraft carry a more explicit message from Earth. The Voyager interstellar
records attached to each spacecraft carry images, sounds, music, and greetings
in 55 languages. In some ways these time capsules from Earth already seem quaint
even to us in the age of CDs and DvDs. How much more quaint will they seem to
any extraterrestrial civilizations that may intercept them? But even if the
messages cannot be deciphered, the spacecraft themselves will be mute testimony
that our species, perhaps one among many, has entered the new ocean of space.
 Voyager 1 and 2
carry a golden record, intended to communicate a story of our world to
extraterrestrials. The Voyager message is carried by a phonograph record - a
12-inch gold-plated copper disk containing sounds and images selected to portray
the diversity of life and culture on Earth. The contents of the record were
selected for NASA by a committee chaired by Carl Sagan of Cornell University.
More information on the record is available at http://voyager.jpl.nasa.gov/spacecraft/goldenrec.html,
and in Sagan's book cited below. Image credit: NASA.
Further
Reading
Dethloff, Henry C. and Ronald A. Schorn. To the Outer
Planets and Beyond: Voyager's Grand Tour (Smithsonian Institution Press,
2003).
Kraemer, Robert S. Beyond the Moon: A Golden Age of Planetary
Exploration, 1971-1978 (Smithsonian Institution Press: Washington and
London, 2000).
NASA, Voyager website, http://www.nasa.gov/mission_pages/voyager/index.html
Sagan, Carl, Frank Drake, Ann Druyan, Timothy Feerris, Jon Lomerg and
Linda Salzman Sagan. Murmers of Earth: The Voyager Interstellar Record
(Random House: New York, 1978).
Science magazine cover story and special
section, "Voyager 1 Passes the Termination Shock," Science, vol. 309 (23
September, 2005), pp. 2015-2029.
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