Voyager program

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Montage of planets and moons with the Voyagers

The Voyager program is an American scientific program that launched two unmanned space missions, the probes Voyager 1 and Voyager 2. These were launched in 1977 to take advantage of a favorable alignment of the planets during the late 1970s. Although they were designated officially to study just the planetary systems of Jupiter and Saturn, the space probes were able to continue their mission.

On August 25, 2012 Voyager 1 became the first human-made object to enter the previously unexplored region of space known as interstellar space, traveling "further than anyone, or anything, in history".[1][2][3][4][5][6][7] Voyager 2 is expected to enter interstellar space within a few years of 2016, and its plasma spectrometer should provide the first direct measurements of the density and temperature of the interstellar plasma.[8]

As of 2013, Voyager 1 was moving with a relative velocity to the Sun of 17 kilometres per second (11 mi/s).[9] The amount of power available to the probe has decreased over time, and will be no longer able to power any single instrument by 2025.

Both of the Voyager missions into outer space have gathered large amounts of data about the gas giants of the solar system, and their orbiting satellites, about which little had been previously known. In addition, the trajectories of the two spacecraft have been used to place limits on the existence of any hypothetical trans-Neptunian planets.

Data and photographs collected by the Voyagers’ cameras, magnetometers, and other instruments revealed previously unknown details about each of the giant planets and their moons. Close-up images from the spacecraft charted Jupiter’s complex cloud forms, winds, and storm systems and discovered volcanic activity on its moon Io. Saturn’s rings were found to have enigmatic braids, kinks, and spokes and to be accompanied by myriad of “ringlets.” At Uranus Voyager 2 discovered a substantial magnetic field around the planet and 10 additional moons. Its flyby of Neptune uncovered three complete rings and six hitherto unknown moons as well as a planetary magnetic field and complex, widely distributed auroras.

These two space probes were built at the Jet Propulsion Laboratory in Southern California, and they were paid for by the National Aeronautics and Space Administration (NASA), which also paid for their launchings from Cape Canaveral, Florida, their tracking, and everything else concerning the space probes.

History

Trajectories and expected location of Pioneer and Voyager spacecraft in April 2007
The trajectories that enabled Voyager spacecraft to visit the outer planets and achieve velocity to escape our solar system

The Voyager space probes were originally conceived as part of the Mariner program, and they were thus named Mariner 11 and Mariner 12, respectively. They were then moved into a separate program named Mariner Jupiter-Saturn, later renamed the Voyager Program because it was thought that the design of the two space probes had progressed sufficiently above those of the Mariner family that they merited a separate name.[10]

The Voyager Program is essentially a scaled-back version of the program "Grand Tour" of the Outer Planets planned during the late 1960s and early 70s. Gary Flandro, an aerospace engineer at the Jet Propulsion Laboratory on the study team, discovered that the alignment of the outer planets would make it possible to use gravitational assists from Jupiter to go to Saturn, and thence and on to Uranus and Neptune. The plan of the "Grand Tour" was to send several pairs of probes to fly by all the outer planets, including Pluto, along various trajectories, including Jupiter-Saturn-Pluto and Jupiter-Uranus-Neptune.

The major plans for the "Grand Tour" were dramatically scaled back because of lack of money appropriated by Congress. In the end, the Voyager Program fulfilled many of the flyby objectives of the "Grand Tour" excepting any mission to Pluto, and dual missions to Uranus and Neptune.

Of the two space probes of the Voyager Program, Voyager 2 was launched first. Its trajectory was designed to take advantage of an unusual alignment of the planets (that occurs once every 175 years[11]) that allowed one space probe to fly by Jupiter, Saturn, Uranus, and Neptune, if everything went well. Of course, in case of a serious malfunction, such as in all of the space probe's radio transmitters or receivers, then that would have been the end of the long mission (to four planets), since there was not a second space probe to fill the gap. That was the gamble that NASA and the JPL were forced to take.

Voyager 1 was launched after its sister probe, but along a shorter and faster trajectory that sent it to Jupiter and Saturn sooner—but at the cost of not visiting any more of the outer planets. Voyager 1 also had the high-priority mission of making a close fly-by of the Saturnian moon Titan, which was known to be quite large and to possess a dense atmosphere very much worth studying.[12]

During the 1990s, Voyager 1 overtook the slower deep-space probes Pioneer 10 and Pioneer 11 to become the most distant manmade object from Earth, a record that it will keep for the foreseeable future. Even the faster (at its launch) New Horizons space probe will not pass it, since the final speed of New Horizons (after maneuvering within the solar system) will be less than the current speed of Voyager 1.

Voyager 1 and Pioneer 10 are the most widely separated manmade objects anywhere, since they are traveling in roughly opposite directions from the Solar System.

Periodic contact has been maintained with Voyager 1 and Voyager 2 to monitor conditions in the outer expanses of the Solar System. The radioactive power sources of both spacecraft were still producing significant amounts of electric power as of 2012, keeping them operational, and it is hoped that this will allow the heliopause of the Solar System to be located and investigated.

In late 2003 Voyager 1 began sending data that seemed to indicate it had crossed the termination shock, but interpretations of these data are in dispute, and it was later believed that the termination shock was crossed in December 2004. The heliopause remains an unknown distance ahead.

On December 10, 2007, instruments on board Voyager 2 sent data back to Earth indicating that the solar system is asymmetrical. It has also reached the termination shock, about 10 billion miles from where Voyager 1 first crossed it, and is traveling outward at roughly 3.3 AU per year.

In August 2009 Voyager 1 was over 16.5 terameters (16.5×1012 meters, or 16.5×109 km, 110.7 AU, or 10.2 billion miles) from the Sun, and thus had entered the heliosheath region between the solar wind's termination shock and the heliopause (the limit of the solar wind). Beyond the heliopause is the bow shock of the interstellar medium, beyond which the probes enter interstellar space and the Sun's gravitational influence on them is exceeded by that of the Milky Way galaxy in general. At the heliopause, light from the Sun takes over 16 hours to reach the probe.

By December 2010 Voyager 1 had reached a region of space where there was no net velocity of the solar wind. At this point, the wind from the Sun may be canceled out by the interstellar wind. It does not appear that the spacecraft has yet crossed the heliosheath into interstellar space.[13]

On June 10, 2011, scientists studying the Voyager data noticed what may be giant magnetic bubbles located in the heliosphere, the region of our solar system that separates us from the violent solar winds of interstellar space. The bubbles, scientists theorize, form when the magnetic field of the Sun becomes warped at the edge of our Solar System.[14]

On 15 June 2012, scientists at NASA reported that Voyager 1 might be very close to entering interstellar space and becoming the first manmade object to leave the inner Solar System.[15][16]

On September 12, 2013, NASA announced that Voyager 1 had crossed the heliopause and entered interstellar space on August 25, 2012, making it the first manmade object to do so.[2][3][5][7][17][18]

Spacecraft design

A space probe with squat cylindrical body topped by a large parabolic radio antenna dish pointing left, a three-element radioisotope thermoelectric generator on a boom extending down, and scientific instruments on a boom extending up. A disk is fixed to the body facing front left. A long triaxial boom extends down left and two radio antennas extend down left and down right.
Voyager spacecraft structure

The Voyager spacecraft weighs 773 kilograms. Of this, 105 kilograms are scientific instruments.[19] The identical Voyager spacecraft use three-axis-stabilized guidance systems that use gyroscopic and accelerometer inputs to their attitude control computers to point their high-gain antennas towards the Earth and their scientific instruments pointed towards their targets, sometimes with the help of a movable instrument platform for the smaller instruments and the electronic photography system.

The diagram at the right shows the high-gain antenna (HGA) with a 3.66 meter diameter dish attached to the hollow decagonal electronics container. There is also a spherical tank that contains the hydrazine monopropellant fuel.

The Voyager Golden Record is attached to one of the bus sides. The angled square panel to the right is the optical calibration target and excess heat radiator. The three radioisotope thermoelectric generators (RTGs) are mounted end-to-end on the lower boom.

The scan platform comprises: the Infrared Interferometer Spectrometer (IRIS) (largest camera at top right); the Ultraviolet Spectrometer (UVS) just above the UVS; the two Imaging Science Subsystem (ISS) vidicon cameras to the left of the UVS; and the Photopolarimeter System (PPS) under the ISS.

Only five investigation teams are still supported, though data is collected for two additional instruments.[20] The Flight Data Subsystem (FDS) and a single eight-track digital tape recorder (DTR) provide the data handling functions.

The FDS configures each instrument and controls instrument operations. It also collects engineering and science data and formats the data for transmission. The DTR is used to record high-rate Plasma Wave Subsystem (PWS) data. The data is played back every six months.

The Imaging Science Subsystem, made up of a wide angle and a narrow angle camera, is a modified version of the slow scan vidicon camera designs that were used in the earlier Mariner flights. The Imaging Science Subsystem consists of two television-type cameras, each with eight filters in a commandable Filter Wheel mounted in front of the vidicons. One has a low resolution 200 millimeter wide-angle lens with an aperture of f/3 (the wide angle camera), while the other uses a higher resolution 1.500 meter narrow-angle f/8.5 lens (the narrow angle camera).

Scientific instruments

Expand
Instrument Name Abr. Description
Imaging Science System
(ISS)
 Utilized a two-camera system (narrow-angle/wide-angle) to provide imagery of Jupiter, Saturn and other objects along the trajectory. More
Filters
Narrow Angle Camera Filters[21]
Name Wavelength Spectrum Sensitivity
Clear
280–640 nm
UV
280–370 nm
Violet
350–450 nm
Blue
430–530 nm
'
'
'
Green
530–640 nm
'
'
'
Orange
590–640 nm
'
'
'
Wide Angle Camera Filters[22]
Name Wavelength Spectrum Sensitivity
Clear
280–640 nm
'
'
'
Violet
350–450 nm
Blue
430–530 nm
CH4-U
536–546 nm
Green
530–640 nm
Na-D
588–590 nm
Orange
590–640 nm
CH4-JST
614–624 nm
Radio Science System
(RSS)
 Utilized the telecommunications system of the Voyager spacecraft to determine the physical properties of planets and satellites (ionospheres, atmospheres, masses, gravity fields, densities) and the amount and size distribution of material in the Saturn rings and the ring dimensions. More
Infrared Interferometer Spectrometer
(IRIS)
 Investigated both global and local energy balance and atmospheric composition. Vertical temperature profiles were also obtained from the planets and satellites, as well as the composition, thermal properties, and size of particles in Saturn's rings. More
Ultraviolet Spectrometer
(UVS)
 Designed to measure atmospheric properties, and to measure radiation. More
Triaxial Fluxgate Magnetometer
(MAG)
 Designed to investigate the magnetic fields of Jupiter and Saturn, the solar-wind interaction with the magnetospheres of these planets, and the interplanetary magnetic field out to the solar wind boundary with the interstellar magnetic field and beyond, if crossed. More
Plasma Spectrometer
(PLS)
 Investigated the macroscopic properties of the plasma ions and measures electrons in the energy range from 5 eV to 1 keV. More
Low Energy Charged Particle Instrument
(LECP)
 Measures the differential in energy fluxes and angular distributions of ions, electrons and the differential in energy ion composition. More
Cosmic Ray System
(CRS)
 Determines the origin and acceleration process, life history, and dynamic contribution of interstellar cosmic rays, the nucleosynthesis of elements in cosmic-ray sources, the behavior of cosmic rays in the interplanetary medium, and the trapped planetary energetic-particle environment. More
Planetary Radio Astronomy Investigation
(PRA)
 Utilized a sweep-frequency radio receiver to study the radio-emission signals from Jupiter and Saturn. More
Photopolarimeter System
(PPS)
 Utilized a 6-inch f/1.4 Dahl-Kirkham-type Cassegrain telescope with an analyzer wheel containing five analyzers of 0,60,120,45 and 135 degrees and filter wheel with eight spectral bands covering 2350 to 7500A to gather information on surface texture and composition of Jupiter, Saturn, Uranus and Neptune and information on atmospheric scattering properties and density for these planets. More
Plasma Wave System
(PWS)
 Provides continuous, sheath-independent measurements of the electron-density profiles at Jupiter and Saturn as well as basic information on local wave-particle interaction, useful in studying the magnetospheres. More

Computers

Unlike the other onboard instruments, the operation of the cameras for visible light is not autonomous, but rather it is controlled by an imaging parameter table contained in one of the on-board digital computers, the Flight Data Subsystem (FDS). More recent space probes, since about 1990, usually have completely autonomous cameras.

The computer command subsystem (CCS) controls the cameras. The CCS contains fixed computer programs such as command decoding, fault detection, and correction routines, antenna pointing routines, and spacecraft sequencing routines. This computer is an improved version of the one that was used in the Viking orbiter.[23] The hardware in both custom-built CCS subsystems in the Voyagers is identical. There is only a minor software modification for one of them that has a scientific subsystem that the other lacks.

The Attitude and Articulation Control Subsystem (AACS) controls the spacecraft orientation (its attitude). It keeps the high-gain antenna pointing towards the Earth, controls attitude changes, and points the scan platform. The custom-built AACS systems on both craft are identical.

It has been erroneously reported[citation needed] on the Internet that the Voyager space probes were controlled by a version of the RCA 1802 (RCA CDP1802 "COSMAC" microprocessor), but such claims are not supported by the primary design documents. The CDP1802 microprocessor was used later in the Galileo space probe, which was designed and built years later. The digital control electronics of the Voyagers were based on RCA CD4000 radiation-hardened, silicon-on-sapphire (SOS) custom-made integrated circuit chips, combined with standard transistor-transistor logic (TTL) integrated circuits.

Communications

The uplink communications are executed via S-band microwave communications. The downlink communications are carried out by an X-band microwave transmitter on board the spacecraft, with an S-band transmitter as a back-up. All long-range communications to and from the two Voyagers have been carried out using their 3.67-meter high-gain antennas.

Because of the inverse-square law in radio communications, the digital data rates used in the downlinks from the Voyagers have been continually decreasing the farther that they get from the Earth. For example, the data rate used from Jupiter was about 115,000 bits per second. That was halved at the distance of Saturn, and it has gone down continually since then. Some measures were taken on the ground along the way to reduce the effects of the inverse-square law. In between 1982 and 1985, the diameters of the three main parabolic dish antennas of the Deep Space Network was increased from 240 feet to 270 feet, dramatically increasing their areas for gathering weak microwave signals.

Then between 1986 and 1989, new techniques were brought into play to combine the signals from multiple antennas on the ground into one, more powerful signal, in a kind of an antenna array. This was done at Goldstone, California, Canberra, and Madrid using the additional dish antennas available there. Also, in Australia, the Parkes Radio Telescope was brought into the array in time for the fly-by of Neptune in 1989. In the United States, the Very Large Array in New Mexico was brought into temporary use along with the antennas of the Deep Space Network at Goldstone. Using this new technology of antenna arrays helped to compensate for the immense radio distance from Neptune to the Earth.

Power

Radioisotope thermoelectric generators for the Voyager program.

Electrical power is supplied by three MHW-RTG radioisotope thermoelectric generators (RTGs). They are powered by plutonium-238 (distinct from the Pu-239 isotope used in nuclear weapons) and provided approximately 470 W at 30 volts DC when the spacecraft was launched. Plutonium-238 decays with a half-life of 87.74 years,[24] so RTGs using Pu-238 will lose a factor of 1−0.5{1/87.74} = 0.79% of their power output per year.

In 2011, 34 years after launch, such an RTG would inherently produce 470 W × 2−(34/87.74) ≈ 359 W, about 76% of its initial power. Additionally, the thermocouples that convert heat into electricity also degrade, reducing available power below this calculated level.

By 7 October 2011 the power generated by Voyager 1 and Voyager 2 had dropped to 267.9 W and 269.2 W respectively, about 57% of the power at launch. The level of power output was better than pre-launch predictions based on a conservative thermocouple degradation model. As the electrical power decreases, spacecraft loads must be turned off, eliminating some capabilities.

It is notable that the electrical utility of the two Voyager craft would seem to average out ((470 W+268.5 W)÷2) at about 369 W or about (369 W ÷ 773 kg) 0.4773 W/kg over a period of about 34 years for a work record of about (0.4773 × 34 X 31,556,925.22) 512 megajoules per kilogram. By comparison gasoline contains about 42 MJ per kg and compared to a typical gasoline engine's efficiency of 14%[25] Voyager has produced (512 MJ/kg ÷ (42 MJ/kg × 0.14)) 87 times as much energy per kilogram. If a spacecraft with such an efficient radioisotope powerplant were also equipped with a high efficiency ion motor and a convenient mass ratio of e (ca 2.71828) then it could calculably escape from the solar system by electric propulsion from low earth orbit though it would still not match the high velocities of the Voyagers.

Voyager Interstellar Mission

The Voyager primary mission was completed in 1989, with the close flyby of Neptune by Voyager 2. The Voyager Interstellar Mission (VIM) is a mission extension, which began when the two spacecraft had already been in flight for over 12 years.[26] The Heliophysics Division of the NASA Science Mission Directorate conducted a Heliophysics Senior Review in 2008. The panel found that the VIM "is a mission that is absolutely imperative to continue" and that VIM "funding near the optimal level and increased DSN (Deep Space Network) support is warranted."[27]

As of the present date, the Voyager 2 and Voyager 1 scan platforms, including all of the platform instruments, have been powered down. The ultraviolet spectrometer (UVS)[28] on Voyager 1 was active until 2003, when it too was deactivated. Gyro operations will end in 2015 for Voyager 2 and 2016 for Voyager 1. Gyro operations are used to rotate the probe 360 degrees six times per year to measure the magnetic field of the spacecraft, which is then subtracted from the magnetometer science data.

This diagram about the heliosphere was released on June 28, 2013 and incorporates results from the Voyager spacecraft.[1]
Humanity's Farthest Journey

The two Voyager spacecraft continue to operate, with some loss in subsystem redundancy, but retain the capability of returning scientific data from a full complement of Voyager Interstellar Mission (VIM) science instruments.

Both spacecraft also have adequate electrical power and attitude control propellant to continue operating until around 2025, after which there may not be available electrical power to support science instrument operation. At that time, science data return and spacecraft operations will cease.[29]

Telemetry

The telemetry comes to the telemetry modulation unit (TMU) separately as a "low-rate" 40-bit-per-second (bit/s) channel and a "high-rate" channel.

Low rate telemetry is routed through the TMU such that it can only be downlinked as uncoded bits (in other words there is no error correction). At high rate, one of a set of rates between 10 bit/s and 115.2 kbit/s is downlinked as coded symbols.

The TMU encodes the high rate data stream with a convolutional code having constraint length of 7 with a symbol rate equal to twice the bit rate (k=7, r=1/2)

Voyager telemetry operates at these transmission rates:

  • 7200, 1400 bit/s tape recorder playbacks
  • 600 bit/s real-time fields, particles, and waves; full UVS; engineering
  • 160 bit/s real-time fields, particles, and waves; UVS subset; engineering
  • 40 bit/s real-time engineering data, no science data.

Note: At 160 and 600 bit/s different data types are interleaved.

The Voyager craft have three different telemetry formats

High rate

  • CR-5T (ISA 35395) Science , note that this can contain some engineering data.
  • FD-12 higher accuracy (and time resolution) Engineering data, note that some science data may also be encoded.

Low rate

  • EL-40 Engineering , note that this format can contain some science data, but not all systems represented.
  • This is an abbreviated format, with data truncation for some subsystems.

It is understood that there is substantial overlap of EL-40 and CR-5T (ISA 35395) telemetry, but the simpler EL-40 data does not have the resolution of the CR-5T telemetry. At least when it comes to representing available electricity to subsystems, EL-40 only transmits in integer increments—so similar behaviours are expected elsewhere.

Memory dumps are available in both engineering formats. These routine diagnostic procedures have detected and corrected intermittent memory bit flip problems, as well as detecting the permanent bit flip problem that caused a two-week data loss event mid-2010.

Voyager Golden Record

Main article: Voyager Golden Record

Voyager 1 and 2 both carry with them a golden record that contains pictures and sounds of Earth, along with symbolic directions for playing the record and data detailing the location of Earth.[16] The record is intended as a combination time capsule and interstellar message to any civilization, alien or far-future human, that may recover either of the Voyager craft. The contents of this record were selected by a committee that included Timothy Ferris[16] and was chaired by Carl Sagan.

Pale blue dot

Seen from 6 billion kilometers (3.7 billion miles), Earth appears as a "pale blue dot" (the blueish-white speck approximately halfway down the brown band to the right).
Main article: Pale Blue Dot

The Voyager program's discoveries during the primary phase of its mission, including never-before-seen close-up color photos of the major planets, were regularly documented by both print and electronic media outlets. Among the best-known of these is an image of the Earth as a pale blue dot, taken in 1990 by Voyager 1, and popularised by Carl Sagan.

See also

References

  1. ↑ Jpl.Nasa.Gov. "Voyager Enters Interstellar Space - NASA Jet Propulsion Laboratory". Jpl.nasa.gov. Retrieved 2013-09-14. 
  2. ↑ 2.0 2.1 Cook, Jia-Rui C.; Agle, D. C.; Brown, Dwayne (12 September 2013). "NASA Spacecraft Embarks on Historic Journey Into Interstellar Space". NASA. Retrieved 12 September 2013. 
  3. ↑ 3.0 3.1 Barnes, Brooks (September 12, 2013). "In a Breathtaking First, NASA Craft Exits the Solar System". New York Times. Retrieved September 12, 2013. 
  4. ↑ Morin, Monte (September 12, 2013). "NASA confirms Voyager 1 has left the solar system". Los Angeles Times. 
  5. ↑ 5.0 5.1 "Voyager 1 has entered a new region of space, sudden changes in cosmic rays indicate". Retrieved 20 March 2013. 
  6. ↑ "Report: Humanity Leaves the Solar System — Or Maybe Not". Time News Magazine. Retrieved 20 March 2013. 
  7. ↑ 7.0 7.1 "Report: NASA Voyager Status Update on Voyager 1 Location". NASA. Retrieved 20 March 2013. 
  8. ↑ "At last, Voyager 1 slips into interstellar space – Atom & Cosmos". Science News. 2013-09-12. Retrieved 2013-09-17. 
  9. ↑ "Voyager Mission Operations Status Report # 2013-05-31, Week Ending May 31, 2013". JPL. Retrieved 19 August 2013. 
  10. ↑ Chapter 11 "Voyager: The Grand Tour of Big Science" (sec. 268.), by Andrew,J. Butrica, found in From Engineering Science To Big Science ISBN 978-0-16-049640-0 edited by Pamela E. Mack, NASA, 1998
  11. ↑ "Planetary Voyage". USA.gov. 2013-10-30. Archived from the original on 2013-12-09. Retrieved 2014-02-04. 
  12. ↑ Chapter 11 "Voyager: The Grand Tour of Big Science" (sec. 269.), by Andrew,J. Butrica, found in From Engineering Science To Big Science ISBN 978-0-16-049640-0 edited by Pamela E. Mack, NASA, 1998
  13. ↑ Brown, Dwayne; Cook, Jia-Rui; Buckley, M. (December 14, 2010). "Nearing Interstellar Space, NASA Probe Sees Solar Wind Decline". Applied Physics Lab, Johns Hopkins University. Retrieved 2011-01-28. 
  14. ↑ Smith, Catharine (2011-06-10). "WATCH: NASA Discovers 'Bubbles' At Solar System's Edge". Huffington Post. 
  15. ↑ Amos, Jonathan (15 June 2012). "Particles point way for Nasa's Voyager". BBC News. Retrieved 15 June 2012. 
  16. ↑ 16.0 16.1 16.2 Ferris, Timothy (May 2012). "Timothy Ferris on Voyagers' Never-Ending Journey". Smithsonian Magazine. Retrieved 15 June 2012. 
  17. ↑ Morin, Monte (September 12, 2013). "NASA confirms Voyager 1 has left the solar system". Los Angeles Times. 
  18. ↑ "Report: Humanity Leaves the Solar System — Or Maybe Not". Time News Magazine. Retrieved 20 March 2013. 
  19. ↑ Haynes, Robert. "How We Get Pictures from Space, Revised Edition". NASA facts. NTRS. 
  20. ↑ Voyager - Spacecraft Nasa website
  21. ↑ "Voyager 1 Narrow Angle Camera Description". NASA / PDS. 2003-08-26. Retrieved 2011-01-17. 
  22. ↑ "Voyager 1 Wide Angle Camera Description". NASA / PDS. 2003-08-26. Retrieved 2011-01-17. 
  23. ↑ Tomayko, James (April 1987). "Computers in Spaceflight: The NASA Experience". NASA. Retrieved February 6, 2010. 
  24. ↑ The Actinide Research Quarterly: Summer 1997
  25. ↑ Serway, Raymond A, Physics for Scientists and Engineers Philadelphia: Saunders College Publishing, 1983, page 131
  26. ↑ "Interstellar Mission". NASA. 
  27. ↑ "Senior Review 2008 of the Mission Operations and Data Analysis Program for the Heliophysics Operating Missions" (PDF). NASA. p. 7. 
  28. ↑ "Ultraviolet Spectrometer". Voyager: The Interstellar Mission. NASA JPL. Retrieved 2006-06-11. 
  29. ↑ "Voyager – Spacecraft Lifetime NASA website". Retrieved 2011-09-13. 

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