Voyager 1

In the 1960s, a Grand Tour to study the outer planets was proposed which prompted NASA to begin work on a mission in the early 1970s.[14] Information gathered by the Pioneer 10 spacecraft helped Voyager's engineers design Voyager to cope more effectively with the intense radiation environment around Jupiter.[15] However, shortly before launch, strips of kitchen-grade aluminum foil were applied to certain cabling to further enhance radiation shielding.[16]

Initially, Voyager 1 was planned as "Mariner 11" of the Mariner program. Due to budget cuts, the mission was scaled back to be a flyby of Jupiter and Saturn and renamed the Mariner Jupiter-Saturn probes. As the program progressed, the name was later changed to Voyager, since the probe designs began to differ greatly from previous Mariner missions.[17]

The 3.7 m (12 ft) diameter high gain dish antenna used on the Voyager craft

Voyager 1 was constructed by the Jet Propulsion Laboratory.[18][19][20] It has 16 hydrazine thrusters, three-axis stabilization gyroscopes, and referencing instruments to keep the probe's radio antenna pointed toward Earth. Collectively, these instruments are part of the Attitude and Articulation Control Subsystem (AACS), along with redundant units of most instruments and 8 backup thrusters. The spacecraft also included 11 scientific instruments to study celestial objects such as planets as it travels through space.[21]

The radio communication system of Voyager 1 was designed to be used up to and beyond the limits of the Solar System. The communication system includes a 3.7-meter (12 ft) diameter high gain Cassegrain antenna to send and receive radio waves via the three Deep Space Network stations on the Earth.[22] The craft normally transmits data to Earth over Deep Space Network Channel 18, using a frequency of either 2.3 GHz or 8.4 GHz, while signals from Earth to Voyager are transmitted at 2.1 GHz.[23]

When Voyager 1 is unable to communicate directly with the Earth, its digital tape recorder (DTR) can record about 67 megabytes of data for transmission at another time.[24] Signals from Voyager 1 take over 20 hours to reach Earth.[4]

Voyager 1 has three radioisotope thermoelectric generators (RTGs) mounted on a boom. Each MHW-RTG contains 24 pressed plutonium-238 oxide spheres.[25] The RTGs generated about 470 W of electric power at the time of launch, with the remainder being dissipated as waste heat.[26] The power output of the RTGs declines over time due to the 87.7-year half-life of the fuel and degradation of the thermocouples, but the craft's RTGs will continue to support some of its operations until 2025.[21][25]

• 
  Diagram of RTG fuel container, showing the plutonium-238 oxide spheres
• 
  Diagram of RTG shell, showing the power-producing silicon-germanium thermocouples
• 
  Model of an RTG unit

As of February 6, 2021, Voyager 1 has 70.95% of the plutonium-238 that it had at launch. By 2050, it will have 56.5% left. By 2080, it will have 42.04% left. By 2110, it will have 27.58% left. By 2140, it will have 13.12% left. By 2167, it will be empty.

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). Since the 1990s, most space probes have been equipped with completely autonomous cameras.[27]

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 1970s Viking orbiters.[28]

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 Voyagers are the same.[29][30]

Instrument name	Abr.	Description
Imaging Science System (disabled)	(ISS)	Utilized a two-camera system (narrow-angle/wide-angle) to provide images of Jupiter, Saturn and other objects along the trajectory. More Filters Narrow-angle camera[31] 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[32] 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 Principal investigator: Bradford Smith / University of Arizona (PDS/PRN website) Data: PDS/PDI data catalog, PDS/PRN data catalog
Radio Science System (disabled)	(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 Saturn's rings and the ring dimensions. More Principal investigator: G. Tyler / Stanford University PDS/PRN overview Data: PDS/PPI data catalog, PDS/PRN data catalog (VG_2803), NSSDC data archive
Infrared Interferometer Spectrometer (disabled)	(IRIS)	Investigates both global and local energy balance and atmospheric composition. Vertical temperature profiles are also obtained from the planets and satellites as well as the composition, thermal properties, and size of particles in Saturn's rings. More Principal investigator: Rudolf Hanel / NASA Goddard Space Flight Center (PDS/PRN website) Data: PDS/PRN data catalog, PDS/PRN expanded data catalog (VGIRIS_0001, VGIRIS_002), NSSDC Jupiter data archive
Ultraviolet Spectrometer (disabled)	(UVS)	Designed to measure atmospheric properties, and to measure radiation. More Principal investigator: A. Broadfoot / University of Southern California (PDS/PRN website) Data: PDS/PRN data catalog
Triaxial Fluxgate Magnetometer (active)	(MAG)	Designed to investigate the magnetic fields of Jupiter and Saturn, the interaction of the solar wind with the magnetospheres of these planets, and the magnetic field of interplanetary space out to the boundary between the solar wind and the magnetic field of interstellar space. More Principal investigator: Norman F. Ness / NASA Goddard Space Flight Center (website) Data: PDS/PPI data catalog, NSSDC data archive
Plasma Spectrometer (defective)	(PLS)	Investigates the microscopic properties of the plasma ions and measures electrons in the energy range from 5 eV to 1 keV. More Principal investigator: John Richardson / MIT (website) Data: PDS/PPI data catalog, NSSDC data archive
Low Energy Charged Particle Instrument (active)	(LECP)	Measures the differential in energy fluxes and angular distributions of ions, electrons and the differential in energy ion composition. More Principal investigator: Stamatios Krimigis / JHU / APL / University of Maryland (JHU/APL website / UMD website / KU website) Data: UMD data plotting, PDS/PPI data catalog, NSSDC data archive
Cosmic Ray System (active)	(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 Principal investigator: Edward Stone / Caltech / NASA Goddard Space Flight Center (website) Data: PDS/PPI data catalog, NSSDC data archive
Planetary Radio Astronomy Investigation (disabled)	(PRA)	Utilizes a sweep-frequency radio receiver to study the radio-emission signals from Jupiter and Saturn. More Principal investigator: James Warwick / University of Colorado Data: PDS/PPI data catalog, NSSDC data archive
Photopolarimeter System (defective)	(PPS)	Utilized a telescope with a polarizer to gather information on surface texture and composition of Jupiter and Saturn and information on atmospheric scattering properties and density for both planets. More Principal investigator: Arthur Lane / JPL (PDS/PRN website) Data: PDS/PRN data catalog
Plasma Wave Subsystem (active)	(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 Principal investigator: Donald Gurnett / University of Iowa (website) Data: PDS/PPI data catalog

For more details on the Voyager space probes' identical instrument packages, see the separate article on the overall Voyager Program.

Voyager 1 lifted off atop a Titan IIIE.

Animation of Voyager 1's trajectory from September 1977 to December 31, 1981
   Voyager 1  ·   Earth ·   Jupiter ·   Saturn ·   Sun

Animation of Voyager 1's trajectory around Jupiter
  Voyager 1 ·   Jupiter ·   Io ·   Europa ·   Ganymede ·   Callisto

The trajectory of Voyager 1 through the Jupiter system

The Voyager 1 probe was launched on September 5, 1977, from Launch Complex 41 at the Cape Canaveral Air Force Station, aboard a Titan IIIE launch vehicle. The Voyager 2 probe had been launched two weeks earlier, on August 20, 1977. Despite being launched later, Voyager 1 reached both Jupiter[34] and Saturn sooner, following a shorter trajectory.[35]

Voyager 1's initial orbit had an aphelion of 8.9 AU, just a little short of Saturn's orbit of 9.5 AU. Voyager 2's initial orbit had an aphelion of 6.2 AU, well short of Saturn's orbit.[36]

Voyager 1 began photographing Jupiter in January 1979. Its closest approach to Jupiter was on March 5, 1979, at a distance of about 349,000 kilometers (217,000 miles) from the planet's center.[34] Because of the greater photographic resolution allowed by a closer approach, most observations of the moons, rings, magnetic fields, and the radiation belt environment of the Jovian system were made during the 48-hour period that bracketed the closest approach. Voyager 1 finished photographing the Jovian system in April 1979.[37]

Discovery of ongoing volcanic activity on the moon Io was probably the greatest surprise. It was the first time active volcanoes had been seen on another body in the Solar System. It appears that activity on Io affects the entire Jovian system. Io appears to be the primary source of matter that pervades the Jovian magnetosphere – the region of space that surrounds the planet influenced by the planet's strong magnetic field. Sulfur, oxygen, and sodium, apparently erupted by Io's volcanoes and sputtered off the surface by impact of high-energy particles, were detected at the outer edge of the magnetosphere of Jupiter.[34]

The two Voyager space probes made a number of important discoveries about Jupiter, its satellites, its radiation belts, and its never-before-seen planetary rings.

• 
  Jupiter's Great Red Spot, an anti-cyclonic storm larger than Earth, as seen from Voyager 1
• 
  View of sulfur-rich lava flows radiating from the volcano Ra Patera on Io
• 
  The eruption plume of the volcano Loki rises 160 km (100 mi) over the limb of Io
• 
  Europa's lineated but un-cratered face, evidence of currently active geology, at a distance of 2.8 million km.
• 
  Ganymede's tectonically disrupted surface, marked with bright impact sites, from 253,000 km.

Media related to the Voyager 1 Jupiter encounter at Wikimedia Commons

The gravitational assist trajectories at Jupiter were successfully carried out by both Voyagers, and the two spacecraft went on to visit Saturn and its system of moons and rings. Voyager 1 encountered Saturn in November 1980, with the closest approach on November 12, 1980, when the space probe came within 124,000 kilometers (77,000 mi) of Saturn's cloud-tops. The space probe's cameras detected complex structures in the rings of Saturn, and its remote sensing instruments studied the atmospheres of Saturn and its giant moon Titan.[38]

Voyager 1 found that about seven percent of the volume of Saturn's upper atmosphere is helium (compared with 11 percent of Jupiter's atmosphere), while almost all the rest is hydrogen. Since Saturn's internal helium abundance was expected to be the same as Jupiter's and the Sun's, the lower abundance of helium in the upper atmosphere may imply that the heavier helium may be slowly sinking through Saturn's hydrogen; that might explain the excess heat that Saturn radiates over energy it receives from the Sun. Winds blow at high speeds in Saturn. Near the equator, the Voyagers measured winds about 500 m/s (1,100 mph). The wind blows mostly in an easterly direction.[35]

The Voyagers found aurora-like ultraviolet emissions of hydrogen at mid-latitudes in the atmosphere, and auroras at polar latitudes (above 65 degrees). The high-level auroral activity may lead to the formation of complex hydrocarbon molecules that are carried toward the equator. The mid-latitude auroras, which occur only in sunlit regions, remain a puzzle, since bombardment by electrons and ions, known to cause auroras on Earth, occurs primarily at high latitudes. Both Voyagers measured the rotation of Saturn (the length of a day) at 10 hours, 39 minutes, 24 seconds.[38]

Voyager 1's mission included a flyby of Titan, Saturn's largest moon, which had long been known to have an atmosphere. Images taken by Pioneer 11 in 1979 had indicated the atmosphere was substantial and complex, further increasing interest. The Titan flyby occurred as the spacecraft entered the system to avoid any possibility of damage closer to Saturn compromising observations, and approached to within 6,400 km (4,000 mi), passing behind Titan as seen from Earth and the Sun. Voyager's measurement of the atmosphere's effect on sunlight and Earth-based measurement of its effect on the probe's radio signal were used to determine the atmosphere's composition, density, and pressure. Titan's mass was also measured by observing its effect on the probe's trajectory. The thick haze prevented any visual observation of the surface, but the measurement of the atmosphere's composition, temperature, and pressure led to speculation that lakes of liquid hydrocarbons could exist on the surface.[39]

Because observations of Titan were considered vital, the trajectory chosen for Voyager 1 was designed around the optimum Titan flyby, which took it below the south pole of Saturn and out of the plane of the ecliptic, ending its planetary science mission.[40] Had Voyager 1 failed or been unable to observe Titan, Voyager 2's trajectory would have been altered to incorporate the Titan flyby,[39]^:94 precluding any visit to Uranus and Neptune.[6] The trajectory Voyager 1 was launched into would not have allowed it to continue on to Uranus and Neptune,[40]^:155 but could have been altered to avoid a Titan flyby and travel from Saturn to Pluto, arriving in 1986.[8]

• 
  Crescent Saturn from 5.3 million km, four days after closest approach
• 
  Voyager 1 image of Saturn's narrow, twisted and braided F Ring.
• 
  Mimas at a range of 425,000 km; the crater Herschel is at upper right
• 
  Tethys, with its giant rift valley Ithaca Chasma, from 1.2 million km.
• 
  Fractured 'wispy terrain' on Dione's trailing hemisphere.
• 
  The icy surface of Rhea is nearly saturated with impact craters.
• 
  Titan's thick haze layer is shown in this enhanced Voyager 1 image.
• 
  Layers of haze, composed of complex organic compounds, covering Saturn's satellite Titan.

Media related to the Voyager 1 Saturn encounter at Wikimedia Commons

Close flybys of gas giants gave gravity assists to both Voyagers

Scientists at the Johns Hopkins University Applied Physics Laboratory believe that Voyager 1 entered the termination shock in February 2003.[50] This marks the point where the solar wind slows to subsonic speeds. Some other scientists expressed doubt, discussed in the journal Nature of November 6, 2003.[51] The issue would not be resolved until other data became available, since Voyager 1's solar-wind detector ceased functioning in 1990. This failure meant that termination shock detection would have to be inferred from the data from the other instruments on board.[52][53][54]

In May 2005, a NASA press release said that the consensus was that Voyager 1 was then in the heliosheath.[55] In a scientific session at the American Geophysical Union meeting in New Orleans on May 25, 2005, Dr. Ed Stone presented evidence that the craft crossed the termination shock in late 2004.[56] This event is estimated to have occurred on December 15, 2004 at a distance of 94 AU from the Sun.[56][57]

On March 31, 2006, amateur radio operators from AMSAT in Germany tracked and received radio waves from Voyager 1 using the 20-meter (66 ft) dish at Bochum with a long integration technique. Retrieved data was checked and verified against data from the Deep Space Network station at Madrid, Spain.[58] This seems to be the first such amateur tracking of Voyager 1.[58]

It was confirmed on December 13, 2010 that Voyager 1 had passed the reach of the radial outward flow of the solar wind, as measured by the Low Energy Charged Particle device. It is suspected that solar wind at this distance turns sideways because of interstellar wind pushing against the heliosphere. Since June 2010, detection of solar wind had been consistently at zero, providing conclusive evidence of the event.[59][60] On this date, the spacecraft was approximately 116 AU or 17.3 billion kilometers (10.8 billion miles) from the Sun.[61]

Voyager 1 was commanded to change its orientation to measure the sideways motion of the solar wind at that location in space in March 2011 (~33yr 6mo from launch). A test roll done in February had confirmed the spacecraft's ability to maneuver and reorient itself. The course of the spacecraft was not changed. It rotated 70 degrees counterclockwise with respect to Earth to detect the solar wind. This was the first time the spacecraft had done any major maneuvering since the Family Portrait photograph of the planets was taken in 1990. After the first roll the spacecraft had no problem in reorienting itself with Alpha Centauri, Voyager 1's guide star, and it resumed sending transmissions back to Earth. Voyager 1 was expected to enter interstellar space "at any time". Voyager 2 was still detecting outward flow of solar wind at that point but it was estimated that in the following months or years it would experience the same conditions as Voyager 1.[62][63]

The spacecraft was reported at 12.44° declination and 17.163 hours right ascension, and at an ecliptic latitude of 34.9° (the ecliptic latitude changes very slowly), placing it in the constellation Ophiuchus as observed from the Earth on May 21, 2011.[6]

On December 1, 2011, it was announced that Voyager 1 had detected the first Lyman-alpha radiation originating from the Milky Way galaxy. Lyman-alpha radiation had previously been detected from other galaxies, but because of interference from the Sun, the radiation from the Milky Way was not detectable.[64]

NASA announced on December 5, 2011, that Voyager 1 had entered a new region referred to as a "cosmic purgatory". Within this stagnation region, charged particles streaming from the Sun slow and turn inward, and the Solar System's magnetic field is doubled in strength as interstellar space appears to be applying pressure. Energetic particles originating in the Solar System decline by nearly half, while the detection of high-energy electrons from outside increases 100-fold. The inner edge of the stagnation region is located approximately 113 AU from the Sun.[65]

NASA announced in June 2012 that the probe was detecting changes in the environment that were suspected to correlate with arrival at the heliopause.[66] Voyager 1 had reported a marked increase in its detection of charged particles from interstellar space, which are normally deflected by the solar winds within the heliosphere from the Sun. The craft thus began to enter the interstellar medium at the edge of the Solar System.[67]

Voyager 1 became the first spacecraft to cross the heliopause in August 2012, then at a distance of 121 AU from the Sun, although this was not confirmed for another year.[68][69][70][71][72]

As of September 2012, sunlight took 16.89 hours to get to Voyager 1 which was at a distance of 121 AU. The apparent magnitude of the Sun from the spacecraft was -16.3 (30 times less than the brightness of the full Moon).[73] The spacecraft was traveling at 17.043 km/s (10.590 mi/s) relative to the Sun. It would need about 17,565 years at this speed to travel a light-year.[73] To compare, Proxima Centauri, the closest star to the Sun, is about 4.2 light-years (2.65×10⁵ AU) distant. Were the spacecraft traveling in the direction of that star, 73,775 years would pass before Voyager 1 reaches it. (Voyager 1 is heading in the direction of the constellation Ophiuchus.[73])

In late 2012, researchers reported that particle data from the spacecraft suggested that the probe had passed through the heliopause. Measurements from the spacecraft revealed a steady rise since May in collisions with high energy particles (above 70 MeV), which are thought to be cosmic rays emanating from supernova explosions far beyond the Solar System, with a sharp increase in these collisions in late August. At the same time, in late August, there was a dramatic drop in collisions with low-energy particles, which are thought to originate from the Sun.[74] Ed Roelof, space scientist at Johns Hopkins University and principal investigator for the Low-Energy Charged Particle instrument on the spacecraft declared that "Most scientists involved with Voyager 1 would agree that [these two criteria] have been sufficiently satisfied."[74] However, the last criterion for officially declaring that Voyager 1 had crossed the boundary, the expected change in magnetic field direction (from that of the Sun to that of the interstellar field beyond), had not been observed (the field had changed direction by only 2 degrees[69]), which suggested to some that the nature of the edge of the heliosphere had been misjudged. On December 3, 2012, Voyager project scientist Ed Stone of the California Institute of Technology said, "Voyager has discovered a new region of the heliosphere that we had not realized was there. We're still inside, apparently. But the magnetic field now is connected to the outside. So it's like a highway letting particles in and out."[75] The magnetic field in this region was 10 times more intense than Voyager 1 encountered before the termination shock. It was expected to be the last barrier before the spacecraft exited the Solar System completely and entered interstellar space.[76][77][78]

In March 2013, it was announced that Voyager 1 might have become the first spacecraft to enter interstellar space, having detected a marked change in the plasma environment on August 25, 2012. However, until September 12, 2013, it was still an open question as to whether the new region was interstellar space or an unknown region of the Solar System. At that time, the former alternative was officially confirmed.[79] [80]

In 2013 Voyager 1 was exiting the Solar System at a speed of about 3.6 AU per year, while Voyager 2 is going slower, leaving the Solar System at 3.3 AU per year.[81] Each year Voyager 1 increases its lead over Voyager 2.

Voyager 1 reached a distance of 135 AU from the Sun on May 18, 2016.[4] By September 5, 2017, that had increased to about 139.64 AU from the Sun, or just over 19 light-hours, and at that time Voyager 2 was 115.32 AU from the Sun.[4]

Its progress can be monitored at NASA's website (see § External links).[4]

• 
  Plot showing a dramatic increase in the rate of cosmic ray particle detection by the Voyager 1 spacecraft (October 2011 through October 2012)
• 
  Plot showing a dramatic decrease in the rate of solar wind particle detection by Voyager 1 (October 2011 through October 2012)