Voyager program

• 1. Introduction

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  The Voyager program launched two spacecraft into the Solar System in 1977, and both are still transmitting from interstellar space today. What began as a mission to study Jupiter and Saturn became something far larger, a decades-long journey that no one at launch could have fully imagined. The pair of probes exploited a rare planetary alignment that occurs once every 175 years, one that allowed a craft to reach all of the outer planets using the gravity of each world to sling itself toward the next. The questions this documentary will answer are simple enough to ask, but staggering to answer: how did two spacecraft built in the early 1970s end up in interstellar space, what did they find along the way, and what exactly are they carrying with them into the dark between the stars?

• 2. The Grand Tour That Almost Wasn't

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  Gary Flandro, an aerospace engineer at the Jet Propulsion Laboratory, worked out in 1964 that a rare alignment of the outer planets could be exploited to visit all of them in a single mission using gravitational assists. The alignment came around once every 175 years, and the window to use it was not going to wait. NASA's Jet Propulsion Laboratory endorsed the plan in 1966, and the Planetary Grand Tour gained momentum. Then, in December 1971, the project was canceled. Funding was redirected to the Space Shuttle program, and the dream of a full outer-planet tour appeared finished.

  In 1972, a more modest proposal came forward. Instead of the ambitious multi-probe Grand Tour, a scaled-down version would send two identical spacecraft to four planets rather than the full set. These probes were derived from the Mariner series and were initially called Mariner 11 and Mariner 12, under a program called Mariner Jupiter-Saturn. As the design of the two spacecraft evolved beyond what the Mariner family had done before, engineers felt the name no longer fit. On the 4th of March 1977, NASA announced a public competition to rename the mission. The winner was Voyager, a name that referenced an earlier suggestion by William Pickering, who had proposed Navigator. Because the new name arrived so close to launch, the probes were occasionally still called by their old Mariner designations, and some sources even used Voyager 11 and Voyager 12.

• 3. Two Paths Through the Solar System

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  Voyager 2 lifted off first, despite carrying the higher number. Its trajectory, known as JSX, was designed for maximum flexibility, allowing flybys of Jupiter, Saturn, Uranus, and Neptune. Voyager 1 followed on a different path, one that was shorter and faster, optimized specifically for a close encounter with Saturn's moon Titan, which was known at the time to be unusually large and to hold a dense atmosphere.

  That encounter with Titan carried a real cost. Flying past Titan sent Voyager 1 out of the plane of the ecliptic, ending its planetary science work. Engineers had prepared for this outcome. Two trajectories had been built into the mission design from the start: the JST path focused on the Titan flyby, while JSX kept the option open for Uranus and Neptune. If Voyager 1 had failed at Titan, Voyager 2 could have been redirected to make a separate Titan pass, giving up its own chance at the ice giants entirely. Voyager 1 could have skipped Titan and gone on to Pluto, but that path was never chosen. Once Voyager 1 completed its Saturn objectives successfully, Voyager 2 received a mission extension to carry through to Uranus and Neptune. Voyager 1 then spent years pulling ahead of the slower Pioneer 10 and Pioneer 11 probes during the 1990s, eventually claiming the title of most distant human-made object from Earth, a record tied to its gravity boost from Saturn.

• 4. What the Spacecraft Carry

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  Each Voyager weighed 815 kg at launch. Of that total, 105 kg was devoted to scientific instruments. The probe's design centered on a 3.7 m high-gain antenna pointed permanently at Earth, mounted atop a hollow decagonal electronics container. Attached to one side of the bus is the Voyager Golden Record, a 12-inch golden phonograph disc containing pictures and sounds of Earth, symbolic instructions for playing it, and data pinpointing Earth's location. The record was intended as both a time capsule and a message to any civilization, alien or far-future human, that might one day recover either spacecraft. Carl Sagan chaired the committee that selected its contents, which also included Timothy Ferris.

  The onboard computers are custom-built machines constructed from CMOS and TTL integrated circuits, many from the 7400 series of Texas Instruments. Together, the six computers across three types carry about 32,000 words of memory in total. The Computer Command System, which controls the spacecraft, has been running continuously since the 20th of August 1977, a record logged by the Guinness Book of Records as the longest period of continual operation for a computer. Power comes from three radioisotope thermoelectric generators fueled by plutonium-238, which has a half-life of 87.74 years. By the 7th of October 2011, power output on Voyager 1 had dropped to 267.9 W and on Voyager 2 to 269.2 W, roughly 57% of what each generated at launch, though those figures were still better than pre-launch predictions.

• 5. Crossing the Heliopause

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  The Solar System does not end at a sharp edge, and the Voyager probes spent decades mapping the transition. In December 2004, Voyager 1 crossed the termination shock at a distance of 94 AU from the Sun, where the solar wind drops to subsonic speed. Voyager 2 reached the same boundary in August 2007, but at only 84 AU, roughly a billion miles closer to the Sun. The asymmetry meant the Solar System itself was not the perfect sphere theorists had imagined.

  In 2011, data from both probes revealed that the heliosheath, the turbulent outer layer of the Sun's influence, was not smooth but filled with giant magnetic bubbles, apparently formed when the Sun's magnetic field became warped at the boundary region. On the 25th of August 2012, Voyager 1 crossed the heliopause and entered interstellar space. Voyager 2 followed on the 5th of November 2018. Scientists officially confirmed Voyager 2's crossing on the 4th of November 2019, reporting it had entered the interstellar medium, the region beyond the reach of the solar wind. In August 2018, NASA confirmed, based on data from the New Horizons spacecraft, the existence of a hydrogen wall at the outer edges of the Solar System, a feature that the two Voyagers had first detected back in 1992.

• 6. Keeping the Signal Alive

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  As the probes traveled farther from Earth, every link in the communication chain had to be strengthened. From Jupiter, the data rate was about 115,000 bits per second. By Saturn that rate had halved, and it kept falling as distance grew, following the inverse-square law of radio communications. Between 1982 and 1985, the three main parabolic dish antennas of the Deep Space Network were widened from 64 to 70 m, substantially increasing their ability to catch the weakening microwave signal. Between 1986 and 1989, engineers developed a new technique of combining signals from multiple antennas into a single more powerful reception. This was done at Goldstone in California, Canberra in Australia, and Madrid in Spain. For the Neptune flyby in 1989, Australia's Parkes Radio Telescope joined the array, and in the United States, the Very Large Array in New Mexico was brought into temporary service alongside the Goldstone dishes.

  On the 14th of November 2023, Voyager 1 stopped sending all telemetry. The signal itself was still present, but no readable data came through. Diagnosing the fault was complicated by a 45-hour round-trip time between Earth and the probe. After months of investigation, engineers traced the problem to a bad memory chip in the Flight Data Subsystem. New software was written to route around the damaged memory block, and engineering data resumed on the 20th of April 2024. Science data from two instruments came back in May 2024, and full recovery of all still-powered science instruments was achieved in June 2024.

• 7. The Pale Blue Dot and the Long Goodbye

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  On the 14th of February 1990, Voyager 1 turned its camera back toward the inner Solar System from a distance of approximately 6 billion kilometers, or about 40.5 AU, as part of a Family Portrait series of images. Earth appeared in that photograph as a tiny point of light, a fraction of a pixel. Carl Sagan, who had advocated for taking the image, wrote of it: "Consider again that dot. That's here. That's home. That's us.... The Earth is a very small stage in a vast cosmic arena.... To my mind, there is perhaps no better demonstration of the folly of human conceits than this distant image of our tiny world."

  Today, Voyager 1 moves at 61,000 km/h relative to the Sun and sits 24.5 billion kilometers away. Voyager 2 travels at 55,000 km/h and is 20.4 billion kilometers from the Sun. Both spacecraft are expected to retain adequate power and attitude control propellant to keep operating through at least 2026, and to communicate until 2036, barring further failures. In about 40,000 years, Voyager 1 will pass within 1.6 light years of the star AC+79 3888, also known as Gliese 445. Voyager 2 will come within 1.7 light years of the star Ross 248 on the same timescale, and in 296,000 years it will pass within 4.6 light years of Sirius, the brightest star in the night sky. The spacecraft are not expected to collide with any star for 1 sextillion years.