Editing Voyager program (section)

==Spacecraft design==
[[File:Voyager Program - spacecraft diagram.png|thumb|upright=1.3|alt=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 diagram]]

The Voyager spacecraft each weighed {{convert|815|kg|lb|abbr=off}} at launch, but after fuel usage are now about {{convert|733|kg|lb|abbr=off}}.<ref name="FAQ"/>  Of this weight, each spacecraft carries {{convert|105|kg|lb|abbr=off}} of scientific instruments.<ref>{{cite journal|last=Haynes|first=Robert|title=How We Get Pictures from Space, Revised Edition|url=https://ntrs.nasa.gov/search.jsp?R=19880001821|journal=NASA Facts|date=January 1987|publisher=NTRS|access-date=7 July 2017|archive-date=30 July 2023|archive-url=https://web.archive.org/web/20230730174044/https://ntrs.nasa.gov/search.jsp?R=19880001821|url-status=live}}</ref> The identical Voyager spacecraft use three-axis-stabilized [[guidance system]]s that use [[gyroscopic]] and [[accelerometer]] inputs to their [[Spacecraft attitude control|attitude control]] computers to point their [[high-gain antenna]]s towards [[the Earth]] and their scientific instruments towards their targets, sometimes with the help of a movable instrument platform for the smaller instruments and the [[video camera tube|electronic photography]] system.

The diagram shows the high-gain antenna (HGA) with a {{convert|3.7|m|ft|abbr=on}} diameter dish attached to the hollow [[decagon]]al [[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 IRIS; the two Imaging Science Subsystem (ISS) [[Vidicon#Vidicon|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.<ref>[http://voyager.jpl.nasa.gov/spacecraft/index.html ''Voyager - Spacecraft''] {{Webarchive|url=https://web.archive.org/web/20070324035810/http://voyager.jpl.nasa.gov/spacecraft/index.html |date=24 March 2007 }} Nasa website</ref>
The Flight Data Subsystem (FDS) and a single eight-track [[Magnetic tape|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 [[Data transmission|transmission]]. The DTR is used to record high-rate [[Plasma (physics)|Plasma]] Wave Subsystem (PWS) data, which 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 {{convert|200|mm|in|abbr=on}} [[focal length]] wide-angle lens with an [[aperture]] of f/3 (the wide-angle camera), while the other uses a higher resolution {{cvt|1500|mm}} narrow-angle f/8.5 lens (the narrow-angle camera).

Three spacecraft were built, ''Voyager 1'' (VGR 77-1), ''Voyager 2'' (VGR 77-3), and test spare model (VGR 77-2).{{sfn|Pyne|2010|p=39}}<ref name=":0">{{cite journal | last = Folger | first = Tim | url = https://www.scientificamerican.com/article/record-breaking-voyager-spacecraft-begin-to-power-down/ | title = Record-Breaking Voyager Spacecraft Begin to Power Down | journal = Scientific American | date = July 2022 | access-date = 12 April 2024 | archive-date = 23 June 2022 | archive-url = https://web.archive.org/web/20220623222423/https://www.scientificamerican.com/article/record-breaking-voyager-spacecraft-begin-to-power-down/ | url-status = live }}</ref>

=== Scientific instruments ===
{| class="wikitable mw-collapsible mw" <!-- or mw-collapsed-->
|+ {{nowrap|List of scientific instruments}}
|-
! scope="col" style="width:135px;"| Instrument name
! scope="col" style="width:50px;"| Abbreviation
<!--! scope="col" width="50" | Image-->
! Description
|-
| {{center|Imaging Science System <br />}}
| {{center|ISS}}
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| Used a two-camera system (narrow-angle/wide-angle) to provide imagery of Jupiter, Saturn and other objects along the trajectory.
{| class="wikitable collapsible"
|-
! colspan="2" | Filters
|-
|
{| style="text-align:center; width:320px;"
! colspan="4" scope="col" |Narrow-angle camera<ref name="NACam3">{{cite web |title=Voyager 1 Narrow Angle Camera Description |url=https://pds-rings.seti.org/voyager/iss/inst_cat_na1.html#filters |access-date=January 17, 2011 |publisher=NASA |archive-date=11 August 2011 |archive-url=https://web.archive.org/web/20110811232250/http://pds-rings.seti.org/voyager/iss/inst_cat_na1.html#filters |url-status=live }}</ref>
|-
! scope="col" style="background:#e5e5e5; width:60px;" |Name
! scope="col" style="background:#e5e5e5;" |Wavelength
! scope="col" style="background:#e5e5e5;" |Spectrum
! scope="col" style="background:#e5e5e5;" |Sensitivity
|-
|<small>0 – Clear</small>
|<small>280–640&nbsp;nm</small>
|[[File:Voyager - Filters - Clear.png|center|50x50px]]
| style="background:#fff;" |
|-
| style="text-align:center; height:25px;" |<small>4 – Clear</small>
|<small>280–640&nbsp;nm</small>
|[[File:Voyager - Filters - Clear.png|center|50x50px]]
| style="background:#fff;" |
|-
|<small>7 – [[Ultraviolet|UV]]</small>
|<small>280–370&nbsp;nm</small>
|[[File:Voyager - Filters - UV.png|center|50x50px]]
| style="background:#1d0036;" |
|-
|<small>1 – Violet</small>
|<small>350–450&nbsp;nm</small>
|[[File:Voyager - Filters - Violet.png|center|50x50px]]
| style="background:#8300b5;" |
|-
|<small>2 – Blue</small>
|<small>430–530&nbsp;nm</small>
|[[File:Voyager - Filters - Blue.png|center|50x50px]]
| style="background:#00d5ff;" |
|-
|<small>5 – Green</small>
|<small>530–640&nbsp;nm</small>
|[[File:Voyager - Filters - Green.png|center|50x50px]]
| style="background:#ffef00;" |
|-
| style="text-align:center; height:25px;" |<small>6 – Green</small>
|<small>530–640&nbsp;nm</small>
|[[File:Voyager - Filters - Green.png|center|50x50px]]
| style="background:#ffef00;" |
|-
|<small>3 – Orange</small>
|<small>590–640&nbsp;nm</small>
|[[File:Voyager - Filters - Orange.png|center|50x50px]]
| style="background:#ff8900;" |
|}
|
{| style="text-align:center; width:320px;"
! colspan="4" scope="col" |Wide-angle camera<ref name="WACam3">{{cite web |title=Voyager 1 Wide Angle Camera Description |url=https://pds-rings.seti.org/voyager/iss/inst_cat_wa1.html#filters |access-date=January 17, 2011 |publisher=NASA |archive-date=7 November 2021 |archive-url=https://web.archive.org/web/20211107025433/https://pds-rings.seti.org/voyager/iss/inst_cat_wa1.html#filters |url-status=live }}</ref>
|-
! scope="col" style="background:#e5e5e5; width:60px;" |Name
! scope="col" style="background:#e5e5e5;" |Wavelength
! scope="col" style="background:#e5e5e5;" |Spectrum
! scope="col" style="background:#e5e5e5;" |Sensitivity
|-
|<small>2 – Clear</small>
|<small>280–640&nbsp;nm</small>
|[[File:Voyager - Filters - Clear.png|center|50x50px]]
| style="background:#fff;" |
|-
|<small>3 – Violet</small>
|<small>350–450&nbsp;nm</small>
|[[File:Voyager - Filters - Violet.png|center|50x50px]]
| style="background:#8300b5;" |
|-
|<small>1 – Blue</small>
|<small>430–530&nbsp;nm</small>
|[[File:Voyager - Filters - Blue.png|center|50x50px]]
| style="background:#00d5ff;" |
|-
|<small>6 – [[Methane|CH<sub>4</sub>]]-U</small>
|<small>536–546&nbsp;nm</small>
|[[File:Voyager - Filters - CH4U.png|center|50x50px]]
| style="background:#81ff00;" |
|-
|<small>5 – Green</small>
|<small>530–640&nbsp;nm</small>
|[[File:Voyager - Filters - Green.png|center|50x50px]]
| style="background:#ffef00;" |
|-
|<small>4 – [[Sodium|Na]]-D</small>
|<small>588–590&nbsp;nm</small>
|[[File:Voyager - Filters - NaD.png|center|50x50px]]
| style="background:#ffe200;" |
|-
|<small>7 – Orange</small>
|<small>590–640&nbsp;nm</small>
|[[File:Voyager - Filters - Orange.png|center|50x50px]]
| style="background:#ff8900;" |
|-
|<small>0 – [[Methane|CH<sub>4</sub>]]-JST</small>
|<small>614–624&nbsp;nm</small>
|[[File:Voyager - Filters - CH4JST.png|center|50x50px]]
| style="background:#ff7b00;" |
|}
|}
* <small>'''Principal investigator:''' [[Bradford A. Smith]] / University of Arizona</small>
* <small>'''Data:''' PDS/PDI data catalog, PDS/PRN data catalog</small>
|-
| {{center|Radio Science System <br />}}
| {{center|RSS}}
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| Used 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.
* <small>'''Principal investigator:''' G. Tyler / Stanford University</small>
* <small>'''Data:''' PDS/PPI data catalog , PDS/PRN data catalog '''('''VG_2803''')''', NSSDC data archive</small>
|-
| {{center|[[Infrared interferometer spectrometer and radiometer]] <br />}}
| {{center|IRIS}}
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| 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]].
* <small>'''Principal investigator:''' Rudolf Hanel / NASA Goddard Space Flight Center</small>
* <small>'''Data:''' PDS/PRN data catalog, PDS/PRN expanded data catalog '''('''VGIRIS_0001, VGIRIS_002''')''', NSSDC Jupiter data archive</small>
|-
| {{center|Ultraviolet [[Spectrometer]] <br />}}
| {{center|UVS}}
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| Designed to measure atmospheric properties, and to measure radiation.
* <small>'''Principal investigator:''' A. Broadfoot / University of Southern California</small>
* <small>'''Data:''' PDS/PRN data catalog</small>
|-
| {{center|Triaxial Fluxgate [[Magnetometer]] <br />}} [[File:Deployed magnetometer boom of one of NASA's Voyager PIA21738.jpg|center|140px]]
| {{center|MAG}}
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| 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.
* <small>'''Principal investigator:''' [[Norman F. Ness]] / NASA Goddard Space Flight Center</small>
* <small>'''Data:''' PDS/PPI data catalog , NSSDC data archive</small>
|-
| {{center|[[Plasma (physics)|Plasma]] [[Spectrometer]] <br />}} [[File:PIA22922~orig.jpg|center|140px]]
| {{center|PLS}}
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| Investigated the macroscopic properties of the [[Plasma (physics)|plasma]] ions and measures electrons in the energy range from 5 eV to 1 keV.
* <small>'''Principal investigator:''' John Richardson / MIT</small>
* <small>'''Data:''' PDS/PPI data catalog , NSSDC data archive</small>
|-
| {{center|Low Energy [[Charged Particle]] Instrument <br />}} [[File:Voyager Low Energy Charged Particle Instrument.jpg|center|140px]]
| {{center|LECP}}
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| Measures the differential in energy fluxes and angular distributions of ions, electrons and the differential in energy ion composition.
* <small>'''Principal investigator:''' [[Stamatios Krimigis]] / JHU/APL / University of Maryland</small>
* <small>'''Data:''' UMD data plotting, PDS/PPI data catalog , NSSDC data archive</small>
|-
| {{center|[[Cosmic Ray System]] <br />}}
| {{center|CRS}}
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| 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.
* <small>'''Principal investigator:''' [[Edward C. Stone|Edward Stone]] / Caltech / NASA Goddard Space Flight Center</small>
* <small>'''Data:''' PDS/PPI data catalog , NSSDC data archive</small>
|-
| {{center|Planetary [[Radio Astronomy]] Investigation <br />}}
| {{center|PRA}}
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| Used a sweep-frequency radio receiver to study the radio-emission signals from Jupiter and Saturn.
* <small>'''Principal investigator:''' James Warwick / University of Colorado</small>
* <small>'''Data:''' PDS/PPI data catalog, NSSDC data archive</small>
|-
| {{center|[[Polarimeter|Photopolarimeter]] System <br />}}
| {{center|PPS}}
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<!--| Used a 6-inch f/1.4 Dahl-Kirkham-type Cassegrain telescope with an analyzer wheel containing five analyzers of {{formatnum:060120}},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. [https://nssdc.gsfc.nasa.gov/nmc/experiment/display.action?id=1977-084A-11 '''More''']-->
| Used 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.
* <small>'''Data:''' PDS/PRN data catalog and PDS Atmospheric Node</small>
|-
| {{center|[[Plasma Wave Subsystem]] <br />}}
| {{center|PWS}}
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| 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.
* <small>'''Principal investigator:''' [[Donald Gurnett]] / University of Iowa</small>
* <small>'''Data:''' PDS/PPI data catalog</small>
|}

<gallery mode=packed heights=200>
File:Voyager instruments boom.jpg|A view of some of Voyager's instruments from below. Left: the cameras, ultraviolet and infrared spectrometers (far left), plasma detector (black box lower right), particle and radiation detectors (far right). On the boom, center and right, are plasma, particle, and cosmic ray detectors.
File:Voyager’s 13-meter-long magnetometer boom.jpg|Voyager's fully extended 13-meter-long magnetometer boom
</gallery>

=== Computers and data processing ===
There are three different computer types on the Voyager spacecraft, two of each kind, sometimes used for redundancy. They are proprietary, custom-built computers built from [[CMOS]] and [[Transistor–transistor logic|TTL]] medium-scale CMOS integrated circuits and discrete components, mostly from the [[7400-series integrated circuits|7400 series]] of [[Texas Instruments]].<ref>{{cite web | url=https://www.eejournal.com/article/voyagers-1-and-2-take-embedded-computers-into-interstellar-space/ | title=Voyagers 1 and 2 Take Embedded Computers into Interstellar Space | date=25 July 2022 | access-date=5 August 2023 | archive-date=5 August 2023 | archive-url=https://web.archive.org/web/20230805033258/https://www.eejournal.com/article/voyagers-1-and-2-take-embedded-computers-into-interstellar-space/ | url-status=live }}</ref> Total number of words among the six computers is about 32K. Voyager&nbsp;1 and Voyager&nbsp;2 have identical computer systems.<ref name="FAQ">{{cite web |url=http://voyager.jpl.nasa.gov/faq.html |title=Voyager Frequently Asked Questions |url-status=dead |archive-url=https://web.archive.org/web/20110721050617/http://voyager.jpl.nasa.gov/faq.html |archive-date=21 July 2011}}</ref><ref>{{cite web |url=https://pds-rings.seti.org/voyager/spacecraft/vg1host.html |title=Voyager 1 Instrument Host Information |publisher=seti.org |access-date=10 August 2019 |archive-date=24 July 2024 |archive-url=https://web.archive.org/web/20240724092319/https://pds-rings.seti.org/voyager/spacecraft/vg1host.html |url-status=live }}</ref>

The Computer Command System (CCS), the central controller of the spacecraft, has two 18-bit word, interrupt-type processors with 4096 words each of non-volatile [[plated-wire memory]]. During most of the Voyager mission the two CCS computers on each spacecraft were used non-redundantly to increase the command and processing capability of the spacecraft. The CCS is nearly identical to the system flown on the Viking spacecraft.<ref name="Tomayko">{{cite book
 |first         = James E.
 |last          = Tomayko
 |editor-last1  = Kent
 |editor-first1 = Allen
 |editor-last2  = Williams
 |editor-first2 = James G.
 |chapter       = Distributed Computing On Board Voyager and Galileo (chapter 6)
 |url           = https://ntrs.nasa.gov/citations/19880069935
 |title         = Computers in Spaceflight: The NASA Experience
 |series        = Encyclopedia of Computer Science and Technology
 |chapter-url   = https://history.nasa.gov/computers/Ch6-2.html
 |publisher     = NASA
 |date          = 1987-08-03
 |isbn          = 978-0-8247-2268-5
 |volume        = 18. Supplement&nbsp;3
 |via           = NASA History
 |access-date   = 26 July 2022
 |archive-date  = 18 October 2023
 |archive-url   = https://web.archive.org/web/20231018062947/https://ntrs.nasa.gov/citations/19880069935
 |url-status    = live
}}</ref>

The Flight Data System (FDS) is two 16-bit word machines with modular memories and 8198 words each.

The Attitude and Articulation Control System (AACS) is two 18-bit word machines with 4096 words each.

Unlike the other on-board 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 computer]]s, the Flight Data Subsystem (FDS). More recent space probes, since about 1990, usually have completely [[automaton|autonomous]] cameras.

The computer command subsystem (CCS) controls the cameras. The CCS contains fixed [[computer program]]s 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 program|''Viking'' orbiter]].<ref name="Tomayko"/> 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.

According to Guinness Book of Records, CCS holds record of "longest period of continual operation for a computer". It has been running continuously since 20 August 1977.<ref>{{Cite web |title=Longest period of continual operation for a computer |url=https://www.guinnessworldrecords.com/world-records/635980-longest-period-of-continual-operation-for-a-computer |access-date=2023-04-28 |website=Guinness World Records |date=20 August 1977 |language=en-gb |archive-date=28 April 2023 |archive-url=https://web.archive.org/web/20230428123751/https://www.guinnessworldrecords.com/world-records/635980-longest-period-of-continual-operation-for-a-computer |url-status=live }}</ref>

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<ref>{{cite web |last=Johnson |first=Herb |date=November 2014 |url=http://www.retrotechnology.com/memship/1802_spacecraft.html |access-date=27 July 2015 |title=COSMAC 1802 History in Space |archive-date=15 July 2015 |archive-url=https://web.archive.org/web/20150715200158/http://www.retrotechnology.com/memship/1802_spacecraft.html |url-status=live }}</ref> on the [[Internet]] that the Voyager space probes were controlled by a version of the [[RCA&nbsp;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 (spacecraft)|''Galileo'' space probe]], which was designed and built years later. The digital control electronics of the Voyagers were not based on a microprocessor integrated-circuit chip.

===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 {{convert|3.7|m|ft|adj=on|sp=us}} high-gain antennas. The high-gain antenna has a beamwidth of 0.5° for X-band, and 2.3° for S-band.<ref name=Ludwig2002/>{{rp|17}} (The low-gain antenna has a 7&nbsp;dB gain and 60° beamwidth.)<ref name=Ludwig2002/>{{rp|17}}

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.<ref name=Ludwig2002>{{cite web|last1=Ludwig|first1=Roger|last2=Taylor|first2=Jim|publisher=NASA|date=March 2002|url=http://descanso.jpl.nasa.gov/DPSummary/Descanso4--Voyager_new.pdf|access-date=26 March 2016|title=Voyager Telecommunications|archive-date=18 March 2021|archive-url=https://web.archive.org/web/20210318092548/http://descanso.jpl.nasa.gov/DPSummary/Descanso4--Voyager_new.pdf|url-status=live}}</ref> 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 antenna]]s of the [[Deep Space Network]] were increased from {{convert|64|to|70|m|abbr=on}}<ref name=Ludwig2002/>{{rp|34}} dramatically increasing their areas for gathering weak microwave signals.

Whilst the craft were between Saturn and Uranus the onboard software was upgraded to do a degree of image compression and to use a more efficient [[Reed–Solomon error correction#Space transmission|Reed-Solomon error-correcting encoding]].<ref name=Ludwig2002/>{{rp|33}}

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 (electromagnetic)|antenna array]].<ref name=Ludwig2002/>{{rp|34}} This was done at [[Goldstone, California]], [[Canberra Deep Space Communication Complex|Canberra (Australia)]], and [[Madrid Deep Space Communication Complex|Madrid (Spain)]] 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.<ref name=Ludwig2002/>{{rp|34}} Using this new technology of antenna arrays helped to compensate for the immense radio distance from Neptune to the Earth.

===Power===
[[File:MHW-RTGs.gif|right|thumb|[[Radioisotope thermoelectric generator|RTGs]] for the Voyager program|238x238px]]
[[electric power|Electrical power]] is supplied by three [[MHW-RTG]] [[radioisotope thermoelectric generator]]s (RTGs). They are powered by [[plutonium-238]] (distinct from the [[Plutonium-239|Pu-239]] isotope used in nuclear weapons) and provided approximately 470 [[Watt|W]] at 30 [[volt]]s [[direct current|DC]] when the spacecraft was launched. Plutonium-238 decays with a [[half-life]] of 87.74 years,<ref>{{Cite web|url=https://lanl.gov/source/orgs/nmt/nmtdo/AQarchive/97summer/Pu_238.html|title=The Actinide Research Quarterly: Summer 1997|website=lanl.gov|access-date=6 February 2020|archive-date=8 March 2022|archive-url=https://web.archive.org/web/20220308131514/https://lanl.gov/source/orgs/nmt/nmtdo/AQarchive/97summer/Pu_238.html|url-status=live}}</ref> so RTGs using Pu-238 will lose a factor of 1−0.5<sup>(1/87.74)</sup> = 0.79% of their power output per year.

In 2011, 34 years after launch, the thermal power generated by such an RTG would be reduced to (1/2)<sup>(34/87.74)</sup> ≈ 76% of its initial power. The RTG [[thermocouple]]s, which convert thermal power into electricity, also degrade over time reducing available electric 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. There may be insufficient power for communications by 2032.<ref>{{cite news|last1=Segal|first1=Michael|title=Beyond Voyager|url=http://nautil.us/issue/51/limits/beyond-voyager|access-date=2 September 2017|work=[[Nautilus (science magazine)|Nautilus]]|date=1 September 2017|archive-date=2 September 2017|archive-url=https://web.archive.org/web/20170902052932/http://nautil.us/issue/51/limits/beyond-voyager|url-status=dead}}</ref>
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