At 5:16am EST on November 18, 2026, a spacecraft that left Earth nearly a half-century ago will pass a rather nondescript point in deep space, defined not by what is there or what its instruments can detect, but by a combination of a physical property of the home world it left behind, and the most fundamental constant in physics.
Voyager 1, launched in 1977 from Cape Canaveral, will become the first human-made object to attain the great distance of one light-day from Earth.
A signal transmitted to the distant explorer from one of NASA’s three Deep Space Network listening stations, traveling at 299,976 km/s (671 million mph), which could circumnavigate Earth seven and one half times in a single second, will take a full 24 hours to reach Voyager 1, and another 24 hours for an acknowledgement signal to be received from the spacecraft – Its ground-based listening station will have rotated beneath its distant messenger two full times in the intervening time.
The Grand Tour
In the mid 1960s, as much of the nation’s attention was on the manned space program and building up the Apollo program to send astronauts to the Moon, engineers at Jet Propulsion Laboratory noted that a periodic planetary alignment that occurs only once in 175 years allows a spacecraft traveling from Earth through the outer solar system to visit all of the outer planets in a single trip with minimal energy, by using a maneuver known as a gravity assist. Such an alignment was to occur in the late 1970s.
NASA formed the Outer Planets Working Group in 1969 to develop a long-range plan which resulted in the proposal to develop two pairs of sophisticated spacecraft that would each visit three planets over a period of seven years.
As is often the case with large NASA projects, wrangling over funding and which projects should take priority (Grand Tour was competing for funds with the Mariner Mars program and the Space Shuttle) led to the project eventually getting cancelled.
Mission planners didn’t give up, however, noting that the rare planetary alignment offered the only opportunity to conduct such a mission quickly and efficiently, produced a revised plan under the name Mariner Jupiter-Saturn 1977. The name suggested a mission more limited in scope, and therefore lower budget, but built into it was the option for a mission extension to visit Uranus and Neptune. The mission was approved in 1972, the last year that men walked on the Moon.
Just a few months before launch, the mission was renamed Voyager.
Each spacecraft had a mass of 815 kg (1,797 lb) at launch, and was powered by three radioisotope thermoelectric generators, each producing 158 W of electrical power at launch.
They carried a suite of eleven science instruments: Ultraviolet Spectrometer, Infrared Interferometer Spectrometer, Planetary Radio Astronomy Experiment, Photopolarimeter, Triaxial Fluxgate Magnetometer, Plasma Spectrometer, Low-Energy Charged Particles Experiment, Plasma Waves Experiment, Cosmic Ray Telescope, Radio Science System, and Imaging Science System.
Cameras & Optics
Voyager’s Imaging Science System consists of a pair of cameras.
The wide-angle camera is a six-element Petzval optical system with a 200mm focal length operating at f/3.5, quite similar to many of the wide-field astrophotography lenses currently available to amateur astronomers.
Its narrow-angle camera is a catadioptric Cassegrain system consisting of five elements, with a diameter of 176mm with a focal length of 1503mm. The f/8.5 optics are similar in size and configuration to the common Celestron C8.
The sensors are 11.14mm square vacuum tube-based vidicons with an 800×800 pixel active area. Images were encoded with an 8-bit ADC, and the cameras had internal 8-position filter wheels. Each uncompressed image was just under 5.2 megabits of data.
Departing Earth
Voyager 1 was launched at 9:16:01am Eastern Daylight Time on Monday, September 5, 1977 aboard a Titan IIIE-Centaur rocket from Space Launch Complex 41 at Cape Canaveral, Florida. The rocket, powered by a 2,300 kN core stage and augmented by a pair of 5,300 kN solid rocket motors, lifted Voyager and its 68 kN Centaur upper stage into Earth orbit, where the Centaur would boost the spacecraft on an Earth escape trajectory. Finally, a 68 kN Thiokol Star 37 solid rocket motor kick stage sent Voyager towards its interplanetary planetary destination.
While the Titan family of rockets has long been retired, Space Launch Complex 41, which was deactivated after the Voyager probes were launched, was reconfigured in the 1980s to service the Atlas V family of rockets, and has seen a bevy of notable mission launches, including Mars Reconnaissance Orbiter (2005), New Horizons to Pluto (2006), Lunar Reconnaissance Orbiter (2009), Solar Dynamics Observatory (2010), Mars Science Laboratory Curiosity (2011), Juno mission to Jupiter (2011), Perseverance Mars rover (2020), the Lucy mission to Jupiter’s trojan asteroids (2021), and the Starliner Crew Flight Test that launched Barry Willore and Sunita Williams to the International Space Station for a nine-month stay. SLC-41 now supports Vulcan-Centaur launches, which, as the name suggests, still flies derivatives of the venerable Centaur upper stage that sent the Voyagers on their Grand Tour and interstellar missions.
Less than two weeks after leaving Earth, from a distance of 39 light seconds (11.66 million kilometers, (7.25 million miles, or 31.8 times greater than the Earth-Moon distance), Voyager 1 turned its narrow-angle camera back towards its home world to capture the first-ever image of Earth and Moon together in a single frame, and notably, not the last time that Voyager 1’s camera would turn homeward.
Later missions to the outer planets would take advantage of their passage through the asteroid belt to visit at least one minor planet along the way, but Voyager’s trajectory was an express to its first planetary encounter at Jupiter, and no suitable targets would be close enough to explore. At a distance of 6 light minutes (0.754 au), Voyager 1 entered the asteroid belt on December 10, 1977, and exited on September 8, 1978, 40 light minutes (4.827 au) from Earth.
Jupiter
On Saturday, January 6, 1979, when Voyager 1 was 33 light minutes from Earth (598.2 million kilometers, 371.7 million miles), and 57.6 million kilometers (35.8 million miles) from Jupiter, it captured the first of many images of the giant planet.
Voyager 1 wasn’t the first spacecraft to visit Jupiter: NASA’s Pioneer 10 spacecraft flew by the giant planet in December 1973, and its twin, Pioneer 11, flew by one year later, on its way to Saturn.
On March 4, 1979, a few hours before its closest approach, Voyager 1 looked back at Jupiter during its eclipse and revealed a thin, gossamer ring of ultrafine particles encircling the giant planet, backlit by the Sun. Jupiter was the third planet found to possess a ring system, just two years after rings around Uranus had been detected by a stellar occultation observed by Kuiper Airborne Observatory.
One of the most remarkable discoveries, not just by Voyager 1, but in the age of planetary exploration altogether, occurred two days past closest approach to Jupiter, on March 8, 1979, when Voyager 1 captured an image of crescent Io, not as part of the science program, but to get a navigational reading for a precise positioning of the spacecraft. When the image was enhanced to reveal background stars, JPL astronomer Linda Morabito saw a 300 km (190 mi) plume on the limb of Io. Later image analysis revealed several more volcanic plumes, and follow-up observations by Voyager 2, which arrived at Jupiter four months later confirmed active volcanism, the first observed on another world in our solar system.
Prior to Voyager 1’s arrival at Jupiter, the king of the planets had 13 known moons, the most recent discovery being that of Leda by Charles T. Kowal at Palomar Observatory three years prior to the arrival of the interplanetary space probe. All but five of Jupiter’s known moons at the time were discovered photographically, the standard method by which all discoveries of solar system bodies other than comets had been made since Amalthea was spotted by E. E. Barnard in 1892.
At Jupiter, Voyager 1 introduced us to an entirely new method of discovery: remote imaging by spacecraft. The two inner moons that would later be named Metis and Thebe were discovered by Stephen P. Synnott on images returned from Voyager 1. These moons are small (45km and 100km), faint, and located inside the orbit of Io at periods of seven hours and 16 hours, respectively.
Voyager 1’s encounter with Jupiter provided the spacecraft with a velocity boost of about 10 km/s, which put it on a path to reach Saturn 13 months later.
Saturn
Voyager 1 turned its instruments to Saturn on August 22, 1980, when the spacecraft was 81 light minutes (9.684 au) from Earth.
Only the second spacecraft to explore the ringed world, Voyager 1, with its suite of instruments, made some incredible discoveries and returned spectacular images of the planet, its rings and moons before, during, and after its November 12 flyby.
Voyager 1 revealed the braided nature of Saturn’s tenuous F ring, an effect of the gravity of the previously unknown shepherd moon Prometheus, also discovered by Voyager 1.
Looking at Titan, one of the primary scientific targets of the entire mission, Voyager revealed the moon’s atmosphere to be 60% denser than Earth’s, and composed of 90% nitrogen, with the rest being methane and trace hydrocarbons. Despite Voyager 1’s close 6,490 km (4,033 mi) flyby of the giant moon, its thick haze, which extended up to 300 km above the moon, obscured its surface features.
An image of Saturn’s moon Mimas returned by Voyager 1 revealed a giant impact crater ⅓ of the moon’s diameter, almost large enough to have shattered the moon. The crater was given the name Herschel, after astronomer William Herschel, who discovered the moon in 1789.
Saturn had eleven known moons prior to Voyager 1’s arrival in late 1980. In addition to Prometheus, Voyager also returned images that led to the discoveries of two other small moons: Atlas and Pandora.
Voyager 1 looked at Saturn’s atmosphere, determined it to be mostly hydrogen, with some helium and trace elements, and measured wind speeds of up to 1,800 kph (over 1,100 mph), at the time the fastest known in the solar system.
Voyager 1 gave us the now famous “departing Saturn” image on November 15, 1980, when the spacecraft was 5,300,000 km (3,300,000 miles) from the planet, and 1 hour 25 minutes (10.180 au) from Earth. Although this would not be the last image Voyager 1 returned, it marked the end of its planetary encounters due to the trajectory chosen to give us the close flyby of Titan, Voyager 1 exited the ecliptic plane at an angle of about 34° north. Unlike its sister ship, it would not be able to continue on to Uranus and Neptune.
On to Pluto?
A lesser-known fact of the Voyager mission is that the Grand Tour originally considered Pluto as the final planetary destination of Voyager 1. The flyby of Pluto would have occurred in March 1986, around the same time as public fascination with Halley’s Comet was at its height, and just a few weeks after Voyager 2’s encounter with Uranus (Voyager 1 was on a faster trajectory than Voyager 2, and Pluto was closer to the Sun than Neptune in the late 1980s). However, because Voyager 2 was destined to fly by Neptune’s large moon Triton, which was seen as a close analogue of Pluto, it was decided that a close flyby of Saturn’s moon Triton would net a greater scientific return, and therefore the spacecraft would not be on the correct trajectory to visit Pluto.
Pluto eventually got its own dedicated mission when the New Horizons spacecraft visited in 2015.
Pale Blue Dot
With the Grand Tour completed, and there being no more planetary encounters for both of the Voyager spacecraft, and therefore no longer a need for the Imaging Science System to remain powered, the instruments were to be deactivated in early 1990.
Before the Imaging Science System was powered down, however, Voyager 1 had one more task to complete, and one that would produce one of the most memorable images from the entire Voyager program. Carl Sagan, a member of the Voyager Imaging Team, had long planned for one of the spacecraft to capture an image of Earth from the edge of the solar system.
On February 14, 1990, Voyager 1’s cameras were commanded to point at Neptune to begin a 60-image mosaic encompassing the entire solar system from its vantage point 5 hours and 36 minutes (40.477 au) away. It then slewed to Uranus, Saturn, Mars, the Sun, Jupiter, Earth, and Venus.
The frame containing Earth, which illuminated just a single pixel, would become famous as the Pale Blue Dot, of which Sagan described in a magnificent piece of prose as “a mote of dust suspended in a sunbeam.”
Following the family portrait, both Voyagers would enter an extended coast period. With their spectacular tour of the solar system behind them, and their cameras having gone dark, the Voyager program faded from memory as a new fleet of explorers spread out across the solar system, including the Galileo mission to Jupiter and the Cassini mission to Saturn, among others.
The presence of the Kuiper Belt was not known until after the primary mission of the twin probes was completed, and because neither of the Voyager spacecraft was close to the plane of the solar system where the denser portions of the Kuiper belt resided, there were no lost opportunities to make any meaningful observations there.
Going Interstellar
After over two decades in space, and at a distance of 9 hours 38 minutes (69.581 au), Voyager 1 overtook Pioneer 10 to become the most distant human-made object on February 17, 1998.
On October 7, 2002, Voyager 1 reached a distance of 86.572 au, which equates to one day round trip light time.
On December 16, 2004, at a distance of 13 hours 9 minutes (94.814 au), Voyager 1 entered the solar termination shock, the boundary where the velocity of the solar wind becomes subsonic as the plasma particles begin to press against the interstellar medium.
Outside of the termination shock, Voyager 1 sailed through the heliosheath for nearly eight years. This is the outer zone of the heliosphere where the Sun’s magnetic field has diminishing influence and solar wind particles begin to move in directions other than radially out from the direction of the Sun as they interact with the greater influence of the interstellar medium.
Voyager 1 crossed the heliopause, the outer boundary of the heliosheath, and into interstellar space at a distance of 16h 49m (121.421 au), on August 25, 2012.
Deep Space Tech Support
Just as extraordinary as the spacecraft’s incredible distance from Earth of 173.145 astronomical units is that it and its twin Voyager 2 not only had illustrious careers exploring the outer solar system, but that JPL engineers have been able to keep both spacecraft functional across the depths of space for nearly a half-century.
Among the challenges engineers have faced in keeping Voyager 1 alive include diagnosing and fixing a garbled data issue, switching to a backup thruster system when the primary thrusters became clogged, and working around issues causing disruptions in communications with the spacecraft. Many of these issues were severe enough to potentially end the mission, but JPL’s engineers, working with 1970s hardware and computer code that must run on less than 70 kB of memory, have done amazing work in resolving every problem.
The ongoing management of diminishing available power is a constant consideration during Voyager’s interstellar mission. Prioritizing which instruments will yield the most scientific value with a tight power budget has kept engineers busy, and they recently switched off some equipment and activated lower-power alternatives in order to extend the life of Voyager’s remaining instruments.
Voyagers 1 and 2 continue to operate with their Magnetometer and Plasma Wave Subsystem, while Voyager 2’s Cosmic Ray Subsystem remains active. Voyager 1’s Cosmic Ray Subsystem was deactivated two years ago to conserve power.
Location & Scale
Where in our sky is Voyager 1? Although far too dim to be seen with any optical telescope on Earth, Voyager 1 is in northern Ophiuchus, just over 2° south of Rasalgethi (α Herculis).
Although Voyager 1 is not in heliocentric orbit, and is moving away from the Sun, it still exhibits the effect of parallax by drawing an ellipse in the sky that is presently about the width of the full Moon. This elliptical path diminishes in size each year as the space probe gains 3.5 au of distance, and it is also migrating eastward by 16 arcminutes annually, slowly moving towards Rasalhague (alpha Ophiuchi).
Our Sun, as seen from Voyager 1, shines at a relatively dim magnitude -15.5, only 13 times brighter than the full Moon, and its 1.16 million kilometer-wide photosphere appears at a mere 9.2 arcseconds, about the same size as Mercury when it is near inferior conjunction. Earth’s maximum elongation from the Sun is a mere ⅓ °, smaller than the apparent size of the Moon in our sky.
This maximum separation of Earth and Moon is a mere 3.2 arcseconds.
A classroom globe in Buffalo, New York (600 km away) would appear the same size as Earth does from Voyager 1.
Voyager 1 is traveling at 16.88 kilometers per second relative to the Sun. At that velocity, it covers the distance from Seagrave Memorial Observatory to Ladd Observatory in just 0.936 seconds. It would reach the Jet Propulsion Laboratory in California in just four minutes, and traverse the distance from Earth to the Moon in 6.5 hours.
By comparison, the Artemis II astronauts reached a maximum velocity of 24,664 mph, 11.03 km/s. Voyager 1 is moving 53% faster.
During science downlinks, Voyager is capable of transmitting at 7200 bits per second. Although this is a theoretical maximum rate, it is interesting to consider that at one light-day distance, the intervening space between Voyager 1 and Earth could “store” 622 megabits of data, enough to contain twelve full resolution images.
While Voyager 1’s crossing of one light-day of distance is largely symbolic for us on the surface of Earth with its 24-hour solar day, it is worthwhile to include that the distance covered by radio signals in one sidereal day, that is three minutes and 55.9 seconds shorter than our solar day, is crossed on October 22 at 22:41 Universal Time. This value is more meaningful at Voyager’s great distance, as it indicates when the space probe is at (very close to) the same position in the sky relative to the receiving antennas on successive rotations of our planet.
Next spacecraft heading to one light-day
Following Voyager 1 there are four other spacecraft on escape trajectories. Voyager 1’s twin ship Voyager 2 is 20 hours, 6 minutes away in the southern constellation Pavo, and will cross the one light-day distance on November 1, 2035. New Horizons, launched in 2006, still has a ways to catch up. Located in Sagittarius, its signals take 9 hours, 15 minutes to reach us, and traveling at a slower 13.5 km/s, the spacecraft will pass one light-day in 2065.
Pioneer 10 and 11, launched in 1972 and 1973, respectively, are also escaping the solar system. Although both probes were deactivated long ago and are no longer able to be tracked, we still know where they are based on their previously tracked positions and the fact that there are no significant gravitational forces altering their trajectories. While Voyager 1 crosses one light-day, Pioneer 10, in Taurus, will be 19 hours, 36 minutes away, and Pioneer 11, in Scutum, will be 16 hours, 29 minutes from Earth. They will each cross one light-day distance in 2039 and 2050, respectively.
Silently sailing through the Milky Way
Voyager 1 will remain the fastest and most distant spacecraft escaping the solar system well into the foreseeable future. Although there have been proposals for interstellar missions, a prominent example being Breakthrough Starshot, which would use a light sail to attain speeds at a substantial fraction of the speed of light, none have been developed beyond initial study phases. Voyager’s distance record is likely to be safe for at least a few more decades.
But Voyager won’t last that long. Its radioisotope thermoelectric generators, which produce all of the spacecraft’s electrical power using heat from the radioactive decay of plutonium-238, will not have enough power to keep the spacecraft running in about another decade.
Coasting silently across the vastness of space, but still not completely outside of our Sun’s gravitational influence, the spacecraft will next cross into the Oort cloud, the outermost region of our solar system where icy cometary debris resides. With its inner edge thought to be as close as 2,000 au from the Sun, Voyager will enter the area in about 500 years, and it may then take another 10,000 years to pass through its outer boundary.
Long after Voyager sends its last signal back to Earth, its final mission, which will truly be interstellar, will continue long into the future. Although the spacecraft will have fallen silent, it carries with it one last message from Earth. This message was sent, not by radio transmission, but on a gold-plated copper disk affixed to the spacecraft itself, traveling not at the speed of light, but at the brisk pace of the fastest object that humanity has hurled towards the stars.
Each of the twin Voyager probes carries a Golden Record along with instructions on how to play back its encoded messages, which consist of greetings in 55 languages, sounds of nature, and 115 images. Etched onto the record cover is a celestial map designed by Frank Drake depicting the location of our solar system relative to 14 nearby pulsars. The cover itself is plated with a highly pure source of uranium-238, an isotope with a half-life of 4.5 billion years. Together with the star map, this would allow an advanced civilization to determine the age and origin of Voyager.
Recorded in 1977, the Golden Record is simultaneously a time capsule and a cosmic message in a bottle. Carl Sagan, who was instrumental in creating the Golden Record, wrote “[T]he spacecraft will be encountered and the record played only if there are advanced spacefaring civilizations in interstellar space.”
Voyager’s path will not bring it close to any nearby stars – the closest encounter in the cosmic near future will be when it comes within 1.7 light years of Gliese 445 in about 40,000 years.
After it leaves our solar neighborhood, it will continue to revolve around the Milky Way for billions of years, carrying its message from Earth, and our hope that the message will be received.
While we’re not yet ready to bid farewell to our intrepid traveler, celebrating its one light-day milestone is an opportune time to look back and appreciate its voyage of discovery and the Earthlings that have kept it going for nearly a half-century.
