(Submitted August 30, 1997)
As it is 9 years and 1 week since Voyager left the vicinity of Neptune, I
I would ask some Voyager questions.
Voyager is without doubt the most fantastic probe ever to leave the earth,
but what of them now?
Are the probes still transmitting receivable data?
Where are they in relation to our Solar System? i.e. still within the
What is the life expectancy of the on board computers and internal power
Where (roughly) are the probes heading, and once there, will they still
have enough power to transmit data?
Are any more similar missions planned?
The best answer to your question is to direct you to the JPL web site
devoted to voyager:
It has the answers to at least some of your questions, and probably
more information which you will find interesting. There is also
a recent press release which we are appending which you might
Tim Kallman and Maggie Masetti
for the Ask an Astrophysicist team
Date: Tue, 2 Sep 1997 16:12:38 -0400 (EDT)
Subject: Two Voyager Spacecraft Still Going Strong After 20 Years
Headquarters, Washington, DC
Jet Propulsion Laboratory, Pasadena, CA
TWO VOYAGER SPACECRAFT STILL GOING STRONG AFTER 20 YEARS
Twenty years after their launch and long after their
planetary reconnaissance flybys have been 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 outermost 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, the 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 place where 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-after 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 from 110 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 93
million miles (150 million kilometers), 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 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
The magnetometer instrument 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
Voyager 1 encountered Jupiter on March 5, 1979, and Saturn
on Nov. 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 but Pluto orbit
the Sun. Voyager 2 arrived at Jupiter on July 9, 1979, and
Saturn on Aug. 25, 1981, and was then sent on to Uranus on Jan.
25, 1986, and Neptune on Aug. 25, 1989. Neptune's gravity bent
Voyager 2's path southward sending it also out of the ecliptic
plane 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 of 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
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 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 6.3 billion miles (10.1 billion
kilometers) from Earth, having traveled 7.4 billion miles (11.9
billion kilometers) since its launch. The Voyager 1 spacecraft is
departing the solar system at a speed of 39,000 miles per hour
(17.4 kilometers per second).
Voyager 2 is currently 4.9 billion miles (7.9 billion
kilometers) from Earth, having traveled 6.9 billion miles (11.3
billion kilometers) since its launch. The Voyager 2 spacecraft is
departing the solar system at a speed of 35,000 miles per hour
(15.9 kilometers per second).
JPL, a division of the California Institute of Technology,
manages the Voyager Interstellar Mission for NASA's Office of
Space Science, Washington, DC.
Questions on this topic are no longer responded to by the "Ask an Astrophysicist" service. See http://imagine.gsfc.nasa.gov/docs/ask_astro/ask_an_astronomer.html
for help on other astronomy Q&A services.