PUBLIC INFORMATION OFFICE
JET PROPULSION LABORATORY
CALIFORNIA INSTITUTE OF TECHNOLOGY
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION
PASADENA, CALIF. 91109. TELEPHONE (818) 354-5011
http://www.jpl.nasa.gov
Contact: Mary A. Hardin (818) 354-0344
FOR IMMEDIATE RELEASE
September 2, 1997
TWO VOYAGER SPACECRAFT STILL GOING STRONG AFTER 20 YEARS
Twenty years after their launch and long after their
planetary reconnaissance flybys were completed, both Voyager
spacecraft are now gaining on another milestone -- crossing that
invisible boundary that separates our solar system from
interstellar space, the heliopause.
Since 1989, when Voyager 2 encountered Neptune, both
spacecraft have been studying the environment of space in the
outer solar system. Science instruments on both spacecraft are
sensing signals that scientists believe are coming from the
heliopause -- the outer most edge of the Sun's magnetic field
that the spacecraft must pass through before they reach
interstellar space.
"During their first two decades the Voyager spacecraft have
had an unequaled journey of discovery. Today, even though
Voyager 1 is now more than twice as far from the Sun as Neptune,
their journey is only half over and more unique opportunities for
discovery await the spacecraft as they head toward interstellar
space," said Dr. Edward Stone, Voyager project scientist and
director of NASA's Jet Propulsion Laboratory, Pasadena, CA.
"The Voyagers owe their ability to operate at such great
distances from the Sun to their nuclear electric power sources,
which provide the electrical power they need to function."
The Sun emits a steady flow of electrically charged
particles called the solar wind. As the solar wind expands
supersonically into space, it creates a magnetized bubble around
the Sun, called the heliosphere. Eventually, the solar wind
encounters the electrically charged particles and magnetic field
in the interstellar gas. The boundary created between the solar
wind and interstellar gas is the heliopause. Before the
spacecraft reach the heliopause, they will pass through the
termination shock -- the zone in which the solar wind abruptly
slows down from supersonic to subsonic speed.
Reaching the termination shock and heliopause will be major
milestones for the spacecraft because no one has been there
before and the Voyagers will gather the first direct evidence of
their structure. Encountering the termination shock and
heliopause has been a long-sought goal for many space physicists,
and exactly where these two boundaries are located and what they
are like still remains a mystery.
"Based on current data from the Voyager cosmic ray
subsystem, we are predicting the termination shock to be in the
range of 62 to 90 astronomical units (AU) from the Sun. Most
'consensus' estimates are currently converging on about 85 AU.
Voyager 1 is currently at about 67 AU and moving outwards at 3.5
AU per year, so I would expect crossing the termination shock
sometime before the end of 2003," said Dr. Alan Cummings, a co-
investigator on the cosmic ray subsystem at the California
Institute of Technology.
"Based on a radio emission event detected by the Voyager 1
and 2 plasma wave instruments in 1992, we estimate that the
heliopause is located at110 to 160 AU from the Sun," said Dr.
Donald A. Gurnett, principal investigator on the plasma wave
subsystem at the University of Iowa. (One AU is equal to 150
million kilometers, or 93 million miles, or the distance from the
Earth to the Sun.)
"The low-energy charged particle instruments on the two
spacecraft continue to detect ions and electrons accelerated at
the Sun and at huge shock waves, tens of AU in radius, that are
driven outward through the solar wind. During the past five
years, we have observed marked variations in this ion population,
but have yet to see clear evidence of the termination shock. We
should always keep in mind that our theories may be incomplete
and the shock may be a lot farther out than we think," said Dr.
Stamatios M. Krimigis, principal investigator for the low energy
charged particle subsystem at The Johns Hopkins University
Applied Physics Laboratory.
Voyager 2 was launched first on Aug. 20, 1977 and Voyager 1
was launched a few weeks later on a faster trajectory on Sept. 5.
Initially both spacecraft were only supposed to explore two
planets -- Jupiter and Saturn. But the incredible success of
those two first encounters and the good health of the spacecraft
prompted NASA to extend Voyager 2's mission on to Uranus and
Neptune. As the spacecraft flew across the solar system,
remote-control reprogramming has given the Voyagers greater
capabilities than they possessed when they left the Earth.
There are four other science instruments that are still
functioning and collecting data as part of the Voyager
Interstellar Mission. The plasma subsystem measures the protons
in the solar wind. "Our instrument has recently observed a slow,
year-long increase in the speed of the solar wind which peaked in
late 1996, and we are now observing a slow decrease in solar wind
velocity," said Dr. John Richardson, of the Massachusetts
Institute of Technology, principal investigator on the plasma
subsystem. "We think the velocity peak coincided with the recent
solar minimum. As we approach the solar maximum in 2000 the
solar wind pressure should decrease, which will result in the
termination shock and heliopause moving inward towards the
Voyager spacecraft."
The magnetometer on board the Voyagers measures the magnetic
fields that are carried out into interplanetary space by the
solar wind. The Voyagers are currently measuring the weakest
interplanetary magnetic fields ever detected and those magnetic
fields being measured are responsive to charged particles that
cannot be detected directly by any other instruments on the
spacecraft, according to Dr. Norman Ness, principal investigator
on the magnetometer subsystem at the Bartol Research Institute,
University of Delaware.
Other science instruments still collecting data include the
planetary radio astronomy subsystem and the ultraviolet
spectrometer subsystem.
Voyager 1 encountered Jupiter on March 5, 1979, and Saturn
on November 12, 1980 and then, because its trajectory was
designed to fly close to Saturn's large moon Titan, Voyager 1's
path was bent northward by Saturn's gravity, sending the
spacecraft out of the ecliptic plane, the plane in which all the
planets except Pluto orbit the Sun. Voyager 2 arrived at
Jupiter on July 9, 1979, and Saturn on August 25, 1981, and was
then sent on to Uranus on January 25, 1986 and Neptune on August
25, 1989. Neptune's gravity bent Voyager 2's path southward,
sending it out of the ecliptic plane as well and on toward
interstellar space.
Both spacecraft have enough electrical power and attitude
control propellant to continue operating until about 2020, when
the available electrical power will no longer support science
instrument operation. Spacecraft electrical power is supplied by
Radioisotope Thermoelectric Generators (RTGs) that provided
approximately 470 watts power at launch. Due to the natural
radioactive decay of the plutonium fuel source, the electrical
energy provided by the RTGs is continually declining. At the
beginning of 1997, the power generated by Voyager 1 had dropped
to 334 watts and to 336 watts for Voyager 2. Both of these power
levels represent better performance than had been predicted
before launch.
The Voyagers are now so far from home that it takes nine
hours for a radio signal traveling at the speed of light to reach
the spacecraft. Science data are returned to Earth in real-time
to the 34-meter Deep Space Network (DSN) antennas located in
California, Australia and Spain. Voyager 1 will pass the Pioneer
10 spacecraft in January 1998 to become the most distant human-
made object in our solar system.
Voyager 1 is currently 10.1 billion kilometers (6.3 billion
miles) from Earth, having traveled 11.9 billion kilometers (7.4
billion miles) since its launch. The Voyager 1 spacecraft is
departing the solar system at a speed of 17.4 kilometers per
second (39,000 miles per hour).
Voyager 2 is currently 7.9 billion kilometers (4.9 billion
miles) from Earth, having traveled 11.3 billion kilometers (6.9
billion miles) since its launch. The Voyager 2 spacecraft is
departing the solar system at a speed of 15.9 kilometers per
second (35,000 miles per hour).
JPL, a division of the California Institute of Technology,
manages the Voyager Interstellar Mission for NASA's Office of
Space Science, Washington, D. C.
#####
NOTE TO EDITORS: A NASA TV video file featuring animation of the
Voyager spacecraft will air today, Sept. 2 at 9 a.m., noon, 3
p.m., 6 p.m. and 9 p.m. Pacific time. NASA TV is located at GE-2,
transponder 9C at 85 degrees West longitude, with vertical
polarization. Frequency is on 3880.0 megahertz, with audio on
6.8 megahertz.
8/26/97 MAH
#9774
