Written by Neuan H. (KIS'27)
━━ Nov 13, 2023━━
Diagram of the location of the Voyager spacecraft
The Voyager Missions
In 1977, the Kennedy Space Center in Cape Canaveral, Florida launched the Voyager satellites with the intention of investigating the ever-distant and enormous gas giants Jupiter and Saturn. 35 years later, they would achieve the unfathomable and become the first human-made objects to reach interstellar space. Both Voyager spacecraft remain operational as of this writing in 2023, long past the 5 years of their initial mission. To this day, the Voyager spacecraft continues the mission it started in 1977 and ventures through interstellar space to fulfill its new initiative: to explore space beyond the influence of stars. With the guidance of technicians on Earth, the Voyager mission is expected to continue until at least 2026 when the Voyager spacecraft’s primary systems are predicted to shut down.
Programming of the Voyagers
The Voyager satellites were built with the intent of lasting about 5 years or long enough to reach Jupiter and Saturn with the possibility of extending their use to explore Uranus and Neptune. While NASA technicians remotely reprogrammed both Voyagers after obtaining authorization to extend the initial Voyager mission for planetary exploration and include Uranus and Neptune as mission objectives, the spacecraft themselves remained mostly unchanged from their launch in 1977.
The command computer subsystem (CCS) of the Voyager spacecraft is responsible for utilizing data from sensors and uplink commands from Earth to manage most of the operations. This includes command decoding, fault detection, corrective routes, and using the information given by sensors to point its antenna toward Earth.
The Voyagers use a 3-axis stabilized attitude control subsystem. This is essential to keep the spacecraft’s antenna pointed towards Earth and therefore allow technicians to send and receive data. Attitude and Articulation Control Subsystems (AACS) are the computer systems on a spacecraft that control its path and orientation in space. In order to manipulate and stabilize its attitude, the Voyagers utilize small propulsion thrusters called mass-expulsion control systems (MEC) to nudge the spacecraft back and forth within a deadband of allowed attitude error. From their launch in 1977 to 2012 when Voyager 1 reached interstellar space, the spacecraft used about 75 kg of propellant out of the initial 100 kg that they were launched with. Each Voyager’s AACS is also responsible for the celestial reference of the spacecraft. A Sun sensor is used for the yaw and pitch references while a single bright star, Canopus, is used as the roll reference.
Image of a Voyager spacecraft
Scientific Equipment
The Voyager satellites both carry a number of scientific instruments as shown in the photo above. These instruments were launched in accordance with the Voyager Planetary Mission’s objectives and intended for usage in collecting data on the outer planets of our solar system. However, some instruments are currently in operation to this day, observing the surroundings of the Voyagers and returning crucial findings to facilities on Earth.
The Magnetometer, an instrument responsible for measuring the magnetic field of celestial objects remains in operation today and still returns information to Earth on the magnetic fields beyond our solar system. During the Voyager Planetary Mission, its role was to observe the magnetic fields of the outer planets and how the moons and rings of these planets interacted.
Power is generated on the Voyagers from the 3 Radioisotope Generators (RTG). This system converts the heat emitted from the decay of plutonium 238 into electricity. While this is a constant and reliable power source, the power decreases with time as the plutonium’s decay slows and eventually stops. In order to prevent the shutdown of the Voyagers, many of the non-essential instruments and systems aboard both probes have been shut down. This solution has extended the lifespan of the Voyager mission to at least 2026.
The Planetary Radio Astronomy (PRA) and Plasma Wave Subsystem (PWS) are separate systems that share 2 long antennas which are arranged at right angles. The PWS covers the frequencies between 10 Hz and 56 Hz, while the PRA covers 2 frequency ranges from 20.4 kHz to 1300 kHz and 2.3 MHz to 40.5 MHz. These systems also remain functioning and continue to return information to Earth.
The communications system on the Voyager satellites includes uplink and downlink from and to Earth. To account for the Voyager Planetary Mission’s initiative, the Voyager probes were designed with high-gain antennas (HGA). These are dish-shaped antennas capable of long-range communications with Earth from the distant interstellar Voyagers. Due to their amount of gain or the amount of radio power that the antenna is able to collect and transmit, the Voyagers are able to receive and send data from incredible distances with accuracy down to a fraction of a degree. In order to maintain this accuracy in pointing the antenna, the Voyagers rely on their AACS to correctly identify Earth’s location. Transmissions from Earth to the Voyagers, the uplink of information, are done on S-band radio frequencies. Data from both spacecrafts is sent to Earth on X-band frequencies.
The Low-Energy Charged Particle (LECP) subsystem of the Voyagers observes space for particles of higher energy than those detected by the Plasma Science subsystem (PLS). While some of its range overlaps with that of the Cosmic Ray Subsystem (CRS), the LECP’s range is the broadest among the particle sensors. This system functions by measuring the number of penetrations of the instrument by particles. The angle and the speed of the particles can be revealed by the depth and the direction of the penetrations. These measurements were taken in the solar wind and outer planets but this instrument is no longer in operation. This subsystem will remain in operation on the Voyagers for the rest of their journeys through space.
The Cosmic Ray Subsystem (CRS) observes energetic particles in the plasma of space. This instrument is the most sensitive of the particle detectors aboard the Voyagers and is used in the intense radiation fields of some planets and even some stars that have the highest-known particle energy. Due to the nature of these particles, the CRS does not slow or capture them but allows them to pass through the instrument, leaving traces and signs. The CRS is still powered on and will remain operational until the Voyagers shut down.
The Ultraviolet Spectrometer (UVS) is an instrument specifically designed to be sensitive to ultraviolet light with the ability to determine when certain atoms or ions are present by searching specific colors of ultraviolet light. These wavelengths are emitted by certain elements and compounds, the most noteworthy being the Sun. However, as sunlight passes through an atmosphere, some light is absorbed by elements and molecules in the atmosphere. Thus, this filtered sunlight lacks specific wavelengths and can be used to detect specific elements and compounds in an atmosphere.
The Imaging Science Subsystem (ISS) aboard the Voyagers is a version of a slow scan camera, similar to the instruments used on the Mariner missions. Among the two cameras, both have 8 filters in a Filter Wheel. The Imaging Narrow Angle is a narrow-angle but high-resolution camera while the Imaging Wide Angle has a wider angle but lower resolution. Contrary to many of the satellite, the ISS is not controlled autonomously but by a parameter table in the Flight Dynamics Service (FDS)
The Plasma Science (PLS) subsystem measures low-energy ions and electrons which make up most of the plasma in space. 3 plasma detectors in this subsystem point in the direction of Earth in order to observe the solar wind flow. The 4th observed the magnetospheres of the outer planets and the heliosphere. This instrument is used to determine many aspects of plasma in space such as flow speed, direction, density, and temperature. This subsystem will be in operation for the remainder of the Voyagers’ time.
The Photopolarimeter Subsystem (PPS) is a 0.2 m telescope fitted with filters and polarization analyzers. This instrument can scan 8 wavelengths between 235 nm and 750 nm. The purpose of this subsystem is to determine the physical properties of the outer planets’ particulate matter including the intensity and linear polarization of scattered sunlight. The PPS provided vital information on the texture and composition of the surfaces of the planets Jupiter and Saturn as well as the sodium cloud on one of Jupiter’s moons, Io. In addition, this system has proved vital in searching for evidence of lightning and auroral activity on the planets that the Voyagers have visited.
The IRIS or Infrared Interferometer Spectrometer and Radiometer is a subsystem that consists of 3 separate instruments. This subsystem acts as a thermometer to determine the distribution of heat energy and temperature of a celestial body or substance. In addition, the IRIS can determine which elements or compounds are in the atmosphere or surface of a planet. The radiometer measures the total amount of sunlight reflected off of an object and can scan ultraviolet, visible, and infrared frequencies. This subsystem can measure radiation in 2 regions of the spectrum.
The Optical Calibration Target is a flat rectangle that includes known color and brightness. This instrument is fixed to the spacecraft and is used by other instruments including cameras and infrared instruments onboard the scan platforms of the Voyagers to point and calibrate.
Finally, the ‘BUS’ Housing Electronics section of the spacecraft is essentially a ten-sided box that carries the satellite’s subsystems and scientific instruments. The centerline or z-axis is aligned with the HGA which in turn must be pointed at Earth in order to send and receive signals. The spacecraft rolls around this axis using thrusters. Each side of the ‘BUS’ is a bay or compartment that holds electronic assemblies. These bays are numbered from 1 to 10 in a clockwise direction from the perspective of Earth.
The Voyager Planetary Mission
The Voyagers were initially launched with the objective of completing flybys of Jupiter and Saturn, returning data to Earth on Saturn’s rings and the moons of both gas giants. Voyager 2 was launched aboard a Titan-Centaur rocket from NASA’s Kennedy Space Center at Cape Canaveral, Florida on September 5, 1977, days before the launch of Voyager 1 at the same site on September 5, 1977. On December 15, 1977, Voyager 1 passed Voyager 2 due to its being launched on a faster and shorter trajectory. The information returned by the Voyager mission revolutionized planetary astronomy and our understanding of the origin and evolution of our solar system and its planets, answering and raising new questions.
More than 10,000 trajectories were studied as possibilities for the Voyager Planetary Mission, utilizing an alignment of the outer planets that would allow for a series of gravity assists to swing the spacecraft from planet to planet while minimizing usage of the onboard propulsion systems. This rare alignment of the outer planets, an event that occurs approximately once every 175 years, could be used to curve the flight path of the Voyagers while increasing their velocity as they passed these planets. This planetary alignment allowed for the flight times of the Voyagers to be decreased from 30 years to 12. Voyager 1 reached Jupiter on March 5, 1979, about three months ahead of Voyager 2 on July 8, 1979. Voyager 1 ventured on to reach Saturn on November 12, 1980. About 9 months later, on August 25, 1981, Voyager 2 completed the objective of the Voyager Planetary Missions and reached Saturn.
Image of Jupiter (left) and Uranus (right) taken during Voyager mission
While the Voyager Planetary Mission was initially designed only to reach Jupiter and Saturn, its success led to the authorization for a flyby of Uranus and eventually, Neptune, the outermost planet in our solar system.
Image of Saturn (left) and Neptune (right) taken during the Voyager mission
Pale Blue Dot
On February 14, 1990, before leaving the solar system forever, Voyager 1 was instructed to turn its camera around and take a family photo of our solar system. After taking 3 hours to warm up its onboard camera, at 4:48 GMT, Voyager 1 pointed its camera toward Earth for a final glance at our home planet before permanently powering it off to conserve power. This photograph was taken from a distance of 3.7 billion miles or 6 billion kilometers from the Sun.
The most famous of these photographs, Pale Blue Dot, shows Earth in a scattered ray of sunlight. Due to the distance of the Voyagers from the planets, our planet, Earth, is depicted as a single pixel of light, emphasizing vulnerability and insignificance to the wider universe.
Voyager 1 was turned around for this photo at the request of Carl Sagan, a leader in the U.S. space program who later wrote a book inspired by this photo: “Pale Blue Dot: A Vision of the Human Future in Space” which was released in 1994. Since the 1950s, Sagan has been a planetary scientist consultant and advisor for NASA. He had helped design and manage the Venus probe Mariner 2, Mars probes Mariner 9, Viking 1, and Viking 2, and Jupiter probe Galileo.
Thus, on May 1, 1990, 4 communications arrived from Voyager 1, containing images of 6 out of 7 target planets and the Sun, making Voyager 1 the first and only attempt of a satellite to photograph the solar system.
Pale Blue Dot as photographed by Voyager 1
The Voyager Interstellar Mission
After completing their initial objective, the Voyagers were given a new mission called the Voyager Interstellar Mission. The next phase of the Voyagers’ decades-long odyssey through space was to reach interstellar space to observe interstellar fields, particles, and waves. This mission included 3 phases: termination, heliosheath exploration, and interstellar exploration.
The Voyager Interstellar Mission began in 1989, with both satellites hurtling towards the unknown and reaching unprecedented distances of the most far-off reaches of our solar system. At this time, Voyager 1 was 40 AU from the Sun and leaving the solar system at a rate of 3.6 AU per year at an angle of 35 degrees north of the ecliptic plane. Voyager 2 was at a distance of 31 AU and moving at a rate of 3.3 AU per year at an angle of 48 degrees south of the ecliptic plane.
The first phase of the mission was at the region of our solar system where the solar wind slows, the termination shock zone. Within the termination shock, the Sun’s influence creates a bubble of plasma particles and the magnetic field of the Sun. However, at the termination shock, the particles within the supersonic solar wind are slowed by the interstellar wind. This results in a change in plasma flow and magnetic field orientation.
The second phase of the mission was the heliosheath exploration. This phase consisted of the studies on the outer layer of the Sun’s area of influence, the heliosphere. In this region, the Sun’s magnetic field and solar wind maintain influence on the space surrounding it. Voyager 1 reached the heliosheath at 94 AU in December 2004 with Voyager 2 three years behind, reaching the heliosheath at 84 AU in 2007.
The third phase of the mission was interstellar exploration. To reach this stage in the mission, both satellites passed the heliopause, the boundary of the bubble that the Sun has influenced. Voyager 1 became the first man-made object to leave our star’s domain, interstellar space, on August 25, 2012. Voyager 1 was at a distance of 122 AU, about 11 billion miles or 18 billion kilometers from the Sun. On November 5, 2018, Voyager 2’s trajectory brought the spacecraft beyond the heliopause, becoming the second man-made object to reach interstellar space.
The Voyagers are expected to maintain communication with Earth until at least 2026. By that time, Voyager 1 will be 13.8 billion miles or 22.1 billion kilometers from the Sun. Voyager 2 will be 11.4 billion miles or 18.4 billion kilometers from the Sun. Eventually, the Voyagers will even pass stars. In about 40,000 years, Voyager will come within 1.6 light-years or 9.3 trillion miles of the star AC+79 3999, a part of the constellation Camelopardalis as Voyager 2 will come within 1.7 light-years or 9.7 trillion miles of the star Ross 248. In 296,000 years, Voyager 2 will even come within 4.3 light-years or 25 trillion miles of Sirius, the brightest star in our night sky.
The Voyagers relative to the layers of the heliosphere
The Golden Record
Aboard the Voyagers, a committee led by Carl Sagan mounted a disk of gold-plated copper, hence the name, containing a narrative of the human existence on Earth in the hopes that life elsewhere in the distant future may know of the life and culture of our planet.
The content of the Golden Record includes 115 images and 35 sounds deemed representative of our world such as the ocean, wind, thunder, and animals. Select music was chosen from a variety of cultures and eras along with spoken greetings in 55 languages. There is a printed message from President Carter and the U.N. Secretary-General Waldheim.
The Golden Record is a disk with a 12-inch or 30-cm diameter. It is plated with gold with an aluminum cover. Along with the disk, the Voyagers carry cartridges and needles, providing all equipment required to play the information onboard. Etched by hand on the cover is the inscription “To the makers of music - all worlds, all times.”
Photos of the Golden Record
Conclusion
While the Voyagers were initially only designed for a 5-year mission to Jupiter and Saturn, they are expected to survive and remain operational until 2026, almost 50 years after their launch. This was made possible by the work of NASA technicians, systematically turning off many instruments of the spacecraft in order to save the dwindling energy aboard the satellite. This strategy is in accordance with SDG 12, Responsible Consumption and Production. By utilizing similar strategies to the Voyager mission in future spacecraft, manned and unmanned, it is possible that space missions can reduce their consumption of energy and produce less waste whether it be on our planet or beyond.
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