Voyager program

Mariner Jupiter-Saturn

Voyager did things no one predicted, found scenes no one expected, and promises to outlive its inventors. Like a great painting or an abiding institution, it has acquired an existence of its own, a destiny beyond the grasp of its handlers.

The two Voyager space probes were originally conceived as part of the Planetary Grand Tour planned during the late 1960s and early 70s that aimed to explore Jupiter, Saturn, Saturn's moon Titan, Uranus, Neptune, and Pluto. The mission originated from the Grand Tour program, conceptualized by Gary Flandro, an aerospace engineer at the Jet Propulsion Laboratory, in 1964, which leveraged a rare planetary alignment occurring once every 175 years.^[15][16] This alignment allowed a craft to reach all outer planets using gravitational assists. The mission was to send several pairs of probes and gained momentum in 1966 when it was endorsed by NASA's Jet Propulsion Laboratory. However, in December 1971, the Grand Tour mission was canceled when funding was redirected to the Space Shuttle program.^[17]

In 1972, a scaled-down (four planets, two identical spacecraft) mission was proposed, utilizing a spacecraft derived from the Mariner series, initially intended to be Mariner 11 and Mariner 12. The gravity-assist technique, successfully demonstrated by Mariner 10, would be used to achieve significant velocity changes by maneuvering through an intermediate planet's gravitational field to minimize time towards Saturn.^[18] The spacecraft were then moved into a separate program named Mariner Jupiter-Saturn (also Mariner Jupiter-Saturn-Uranus,^[19] MJS, or MJSU), part of the Mariner program, later renamed because it was thought that the design of the two space probes had progressed sufficiently beyond that of the Mariner family to merit a separate name.^[20]

Voyager probes

On March 4, 1977, NASA announced a competition to rename the mission, believing the existing name was not appropriate as the mission had differed significantly from previous Mariner missions. Voyager was chosen as the new name, referencing an earlier suggestion by William Pickering, who had proposed the name Navigator. Due to the name change occurring close to launch, the probes were still occasionally referred to as Mariner 11 and Mariner 12, or even Voyager 11 and Voyager 12.^[17]

Two mission trajectories were established: JST aimed at Jupiter, Saturn, and enhancing a Titan flyby, while JSX served as a contingency plan. JST focused on a Titan flyby, while JSX provided a flexible mission plan. If JST succeeded, JSX could proceed with the Grand Tour, but in case of failure, JSX could be redirected for a separate Titan flyby, forfeiting the Grand Tour opportunity.^[18] The second probe, now Voyager 2, followed the JSX trajectory, granting it the option to continue on to Uranus and Neptune. Upon Voyager 1 completing its main objectives at Saturn, Voyager 2 received a mission extension, enabling it to proceed to Uranus and Neptune. This allowed Voyager 2 to diverge from the originally planned JST trajectory.^[17]

The probes would be launched in August or September 1977, with their main objective being to compare the characteristics of Jupiter and Saturn, such as their atmospheres, magnetic fields, particle environments, ring systems, and moons. They would fly by planets and moons in either a JST or JSX trajectory. After completing their flybys, the probes would communicate with Earth, relaying vital data using their magnetometers, spectrometers, and other instruments to detect interstellar, solar, and cosmic radiation. Their radioisotope thermoelectric generators (RTGs) would limit the maximum communication time with the probes to roughly a decade. Following their primary missions, the probes would continue to drift into interstellar space.^[18]

Voyager 2 was the first to be launched. Its trajectory was designed to allow flybys of Jupiter, Saturn, Uranus, and Neptune. Voyager 1 was launched after Voyager 2, but along a shorter and faster trajectory that was designed to provide an optimal flyby of Saturn's moon Titan,^[21] which was known to be quite large and to possess a dense atmosphere. This encounter sent Voyager 1 out of the plane of the ecliptic, ending its planetary science mission.^[22] Had Voyager 1 been unable to perform the Titan flyby, the trajectory of Voyager 2 could have been altered to explore Titan, forgoing any visit to Uranus and Neptune.^[23] Voyager 1 was not launched on a trajectory that would have allowed it to continue to Uranus and Neptune, but could have continued from Saturn to Pluto without exploring Titan.^[24]

During the 1990s, Voyager 1 overtook the slower deep-space probes Pioneer 10 and Pioneer 11 to become the most distant human-made object from Earth, a record that it will keep for the foreseeable future. The New Horizons probe, which had a higher launch velocity than Voyager 1, is travelling more slowly due to the extra speed Voyager 1 gained from its flyby of Saturn. Voyager 1 and Pioneer 10 are the most widely separated human-made objects anywhere since they are travelling in roughly opposite directions from the Solar System.

In December 2004, Voyager 1 crossed the termination shock, where the solar wind is slowed to subsonic speed, and entered the heliosheath, where the solar wind is compressed and made turbulent due to interactions with the interstellar medium. On 10 December 2007, Voyager 2 also reached the termination shock, about 1.6 billion kilometres (1 billion miles) closer to the Sun than from where Voyager 1 first crossed it, indicating that the Solar System is asymmetrical.^[25]

In 2010 Voyager 1 reported that the outward velocity of the solar wind had dropped to zero, and scientists predicted it was nearing interstellar space.^[26] In 2011, data from the Voyagers determined that the heliosheath was not smooth, but filled with giant magnetic bubbles. It was theorized that they formed when the magnetic field of the Sun became warped at the edge of the Solar System.^[27]

In June 2012, scientists at NASA reported that Voyager 1 was very close to entering interstellar space, which was indicated by a sharp rise in high-energy particles from outside the Solar System.^[28][29] In September 2013, NASA announced that Voyager 1 had crossed the heliopause on 25 August 2012, making it the first spacecraft to enter interstellar space.^[30][31][32]

In December 2018, NASA announced that Voyager 2 had crossed the heliopause on 5 November 2018, making it the second spacecraft to enter interstellar space.^[3]

As of 2017^[update] Voyager 1 and Voyager 2 continued to monitor conditions in the outer expanses of the Solar System.^[33] The Voyager spacecraft were expected to be able to operate science instruments through 2020, when limited power would require instruments to be deactivated one by one. It was expected that circa 2025 there would no longer be sufficient power to operate any scientific instruments.

In July 2019, a revised power management plan was implemented for the two probes' dwindling power supplies.^[34]

Scientific instruments

List of scientific instruments

Instrument name	Abbreviation	Description
Imaging Science System	ISS	Used a two-camera system (narrow-angle/wide-angle) to provide imagery of Jupiter, Saturn and other objects along the trajectory. Filters Narrow-angle camera[39] Name Wavelength Spectrum Sensitivity 0 – Clear 280–640 nm 4 – Clear 280–640 nm 7 – UV 280–370 nm 1 – Violet 350–450 nm 2 – Blue 430–530 nm 5 – Green 530–640 nm 6 – Green 530–640 nm 3 – Orange 590–640 nm Wide-angle camera[40] Name Wavelength Spectrum Sensitivity 2 – Clear 280–640 nm 3 – Violet 350–450 nm 1 – Blue 430–530 nm 6 – CH4-U 536–546 nm 5 – Green 530–640 nm 4 – Na-D 588–590 nm 7 – Orange 590–640 nm 0 – CH4-JST 614–624 nm Principal investigator: Bradford A. Smith / University of Arizona Data: PDS/PDI data catalog, PDS/PRN data catalog
Radio Science System	RSS	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. Principal investigator: Von R. Eshleman Data: PDS/PPI data catalog, PDS/PRN data catalog (VG_2803), NSSDC data archive
Infrared interferometer spectrometer and radiometer	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. Principal investigator: Rudolf Hanel / NASA Goddard Space Flight Center Data: PDS/PRN data catalog, PDS/PRN expanded data catalog (VGIRIS_0001, VGIRIS_002), NSSDC Jupiter data archive
Ultraviolet Spectrometer	UVS	Designed to measure atmospheric properties, and to measure radiation. Principal investigator: A. Lyle Broadfoot / University of Southern California Data: PDS/PRN data catalog
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. Principal investigator: Norman F. Ness / NASA Goddard Space Flight Center Data: PDS/PPI data catalog, NSSDC data archive
Plasma Spectrometer	PLS	Investigated the macroscopic properties of the plasma ions and measures electrons in the energy range from 5 eV to 1 keV. Principal investigator: Herbert S. Bridge / MIT Data: PDS/PPI data catalog, NSSDC data archive
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. Principal investigator: Stamatios Krimigis / JHU/APL / University of Maryland Data: UMD data plotting, PDS/PPI data catalog, NSSDC data archive
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. Principal investigator: Rochus Eugen Vogt, Edward C. Stone, Alan C. Cummings / Caltech / NASA Goddard Space Flight Center Data: PDS/PPI data catalog, NSSDC data archive
Planetary Radio Astronomy Investigation	PRA	Used a sweep-frequency radio receiver to study the radio-emission signals from Jupiter and Saturn. Principal investigator: James W. Warwick / University of Colorado Data: PDS/PPI data catalog, NSSDC data archive
Photopolarimeter System	PPS	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. Principal investigator: Charles F. Lillie, Charles W. Hord Data: PDS/PRN data catalog and PDS Atmospheric Node
Plasma Wave Subsystem	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. Principal investigator: Frederick L. Scarf / TRW, Donald Gurnett / University of Iowa Data: PDS/PPI data catalog

• 
  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.
• 
  Voyager's fully extended 13-meter-long magnetometer boom

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 TTL medium-scale CMOS integrated circuits and discrete components, mostly from the 7400 series of Texas Instruments.^[41] The total number of words among the six computers is about 32K. Voyager 1 and Voyager 2 have identical computer systems.^[35][42]

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.^[43]

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 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.^[43] 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.^[44]

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^[45] 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 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 3.7-meter (12 ft) high-gain antennas. The high-gain antenna has a beamwidth of 0.5° for X-band, and 2.3° for S-band.^[46]: 17 (The low-gain antenna has a 7 dB gain and 60° beamwidth.)^[46]: 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.^[46] 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 were increased from 64 to 70 m (210 to 230 ft)^[46]: 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-correcting encoding.^[46]: 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.^[46]: 34 This was done at Goldstone, California, Canberra (Australia), and 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.^[46]: 34 Using this new technology of antenna arrays helped to compensate for the immense radio distance from Neptune to the Earth.

Power

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,^[47] 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, the thermal power generated by such an RTG would be reduced to (1/2)^(34/87.74) ≈ 76% of its initial power. The RTG thermocouples, 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.^[48]

Mission details

By the start of VIM, Voyager 1 was at a distance of 40 AU from the Earth, while Voyager 2 was at 31 AU. VIM is in three phases: termination shock, heliosheath exploration, and interstellar exploration phase. The spacecraft began VIM in an environment controlled by the Sun's magnetic field, with the plasma particles being dominated by those contained in the expanding supersonic solar wind. This is the characteristic environment of the termination shock phase. At some distance from the Sun, the supersonic solar wind will be held back from further expansion by the interstellar wind. The first feature encountered by a spacecraft as a result of this interaction – between interstellar wind and solar wind – was the termination shock, where the solar wind slows to subsonic speed, and large changes in plasma flow direction and magnetic field orientation occur. Voyager 1 completed the phase of termination shock in December 2004 at a distance of 94 AU, while Voyager 2 completed it in August 2007 at a distance of 84 AU. After entering into the heliosheath, the spacecraft were in an area that is dominated by the Sun's magnetic field and solar wind particles. After passing through the heliosheath, the two Voyagers began the phase of interstellar exploration. The outer boundary of the heliosheath is called the heliopause. This is the region where the Sun's influence begins to decrease and interstellar space can be detected.^[62]

Voyager 1 is escaping the Solar System at the speed of 3.6 AU per year 35° north of the ecliptic in the general direction of the solar apex in Hercules, while Voyager 2's speed is about 3.3 AU per year, heading 48° south of the ecliptic. The Voyager spacecraft will eventually go on to the stars. In about 40,000 years, Voyager 1 will be within 1.6 light years (ly) of AC+79 3888, also known as Gliese 445, which is approaching the Sun. In 40,000 years Voyager 2 will be within 1.7 ly of Ross 248 (another star which is approaching the Sun), and in 296,000 years it will pass within 4.6 ly of Sirius, which is the brightest star in the night-sky.^[2] The spacecraft are not expected to collide with a star for 1 sextillion (10²⁰) years.^[63]

In October 2020, astronomers reported a significant unexpected increase in density in the space beyond the Solar System, as detected by the Voyager space probes. According to the researchers, this implies that "the density gradient is a large-scale feature of the VLISM (very local interstellar medium) in the general direction of the heliospheric nose".^[64][65]