Voyager 2

• 1. Introduction

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  Voyager 2 left Earth on the 20th of August 1977, riding a Titan IIIE/Centaur rocket out of Space Launch Complex 41 at Cape Canaveral, Florida. It was aimed at the outer Solar System, carrying a mandate no single spacecraft had attempted before: visit all four of the giant planets in a single journey. What made that possible was a rare alignment of Jupiter, Saturn, Uranus, and Neptune that occurs only once every few centuries, and NASA was determined not to miss it. How did engineers design a probe that would survive four decades in the cold dark beyond the Sun? What would those planets actually look like up close? And once the planetary tour was done, where would Voyager 2 go next?

• 2. Born from the Grand Tour

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  Planetary scientists in the early space age noticed that the outer planets would line up in the late 1970s in a way that allowed a single probe to use each planet's gravity to slingshot toward the next. NASA turned that insight into an ambitious concept called the Grand Tour, initially envisioning two pairs of spacecraft visiting Jupiter, Saturn, Pluto, Uranus, and Neptune in sequence. By 1972, budget pressures forced a drastic simplification. The mission was folded into the Mariner program and redesigned as a pair of Mariner Jupiter-Saturn probes, with flyby trajectories that preserved the possibility of continuing onward but made no formal commitment to it. When the program was eventually renamed Voyager, the two spacecraft were given complementary roles. Voyager 1 was tasked with the highest-priority scientific target: a close pass by Titan, Saturn's largest moon, whose dense and complex atmosphere had been hinted at by Pioneer 11 images taken in 1979. Voyager 2 was placed on a longer, more circular trajectory that left the Uranus and Neptune option open, with a Titan flyby held in reserve only if Voyager 1 failed. When Voyager 1 completed its Titan encounter successfully, Voyager 2 was cleared to continue toward the ice giants.

• 3. A Machine Built to Last

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  JPL engineers shaped Voyager 2 around a decagonal-prism bus fitted with 16 hydrazine thrusters, a three-axis stabilization system, and a 3.7-meter parabolic high-gain antenna pointed perpetually back at Earth. Power came from three radioisotope thermoelectric generators, each loaded with 24 pressed plutonium oxide spheres. At launch the three RTGs together produced 470 watts of electricity, with output halving every 87.7 years. That slow decay was the mission's long clock: engineers projected the RTGs could support operations until at least 2020, and by April 2023 JPL was drawing on a reservoir of backup power originally reserved for a safety mechanism just to keep the remaining science instruments running. The communications link relied on the S-band and X-band radio systems, capable of sending data at up to 115.2 kilobits per second at Jupiter's distance; the rate fell off with the inverse-square law as the probe traveled farther. A digital tape recorder could capture roughly 64 megabytes of data when the spacecraft passed out of line-of-sight. In April 1978, before Voyager 2 had even reached Jupiter, its primary radio receiver failed entirely. From that point forward the mission depended on a backup receiver with a failed capacitor, which forced controllers to transmit at a precise frequency that had to be constantly adjusted for the Doppler effect caused by Earth's rotation and the receiver's own temperature.

• 4. Jupiter and Saturn

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  At 22:29 UT on the 9th of July 1979, Voyager 2 reached its closest point to Jupiter, passing within 570,000 km of the planet's cloud tops. Scientists had been puzzled by Voyager 1 images of Jupiter's moon Europa, which showed a large network of intersecting linear features that initially looked like deep crustal rifts. High-resolution photos from Voyager 2 complicated the picture: the lines had no topographic relief whatsoever, leading one scientist to say they looked as though they had been painted on with a felt marker. Europa was later understood to have a thin crust of water ice, probably less than 30 km thick, possibly floating above a 50-km-deep ocean. During the Jupiter encounter the two Voyager spacecraft together observed nine active volcanoes erupting on Io, confirming and extending the volcanic discoveries Voyager 1 had made four months earlier. Two new small satellites, Adrastea and Metis, were found orbiting just outside Jupiter's ring, and a third, Thebe, was discovered between the orbits of Amalthea and Io. Saturn came next, with closest approach at 03:24:05 UT on the 26th of August 1981. Voyager 2 probed Saturn's upper atmosphere by watching radio signals pass through it: at a pressure of 70 millibars the temperature measured 82 K, while deeper at 1200 millibars it climbed to 143 K. The spacecraft also detected that Saturn's north pole ran about 10 degrees Celsius cooler at 100 millibars than mid-latitudes. Shortly after the Saturn flyby, the probe's scan platform seized when its azimuth actuator jammed, traced to a combination of bearing-design flaws, gear lubrication failure, corrosion, debris, dissimilar-metal reactions, and a lack of relief ports. Engineers on the ground issued a sequence of commands that partially restored the platform, allowing the mission to continue.

• 5. Uranus and Neptune

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  On the 24th of January 1986, Voyager 2 closed to within 81,500 km of Uranus's cloud tops, becoming the first and still only spacecraft to visit the planet. It discovered 11 previously unknown moons, named Cordelia, Ophelia, Bianca, Cressida, Desdemona, Juliet, Portia, Rosalind, Belinda, Puck, and Perdita, and revealed that Uranus carries a magnetic field tilted far off its rotational axis, trailing a helix-shaped magnetic tail extending 10 million km away from the Sun. Voyager 2 also found two previously unknown Uranian rings, whose particles may be the remnants of a shattered moon. The Planetary Radio Astronomy experiment recorded 140 lightning discharges, each lasting an average of 120 milliseconds and emitting between 10 million and 20 million joules, detected from as far as 600,000 km away. The moon Miranda, seen in detail for the first time, showed enormous canyons carved by geological faults, sparking a hypothesis that Miranda might have been shattered by a violent impact and then reaggregated from its own debris. Following a course correction in 1987, Voyager 2 reached Neptune on the 25th of August 1989, sweeping within 4,950 km of the planet's north pole and then passing within about 40,000 km of Triton five hours later. At Neptune, Voyager 2 discovered the Great Dark Spot, a feature that later disappeared according to Hubble Space Telescope observations, and confirmed six previously unknown moons: Despina, Galatea, Larissa, Proteus, Naiad, and Thalassa. In March 2020, astronomers revisiting Voyager 2's old Uranus data reported the detection of a large atmospheric magnetic bubble, called a plasmoid, that had been quietly hidden in the archived readings for more than three decades.

• 6. Into Interstellar Space

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  After the Neptune encounter ended on the 2nd of October 1989, Voyager 2 began what NASA formally called the Voyager Interstellar Mission. On the 30th of August 2007, it crossed the termination shock into the heliosheath, doing so at a point roughly a billion miles closer to the Sun than Voyager 1 had, a difference attributed to the interstellar magnetic field compressing the southern hemisphere of the heliosphere inward. On the 5th of November 2018, at a distance of 119.7 AU from the Sun and traveling at 15.341 km/s, Voyager 2 crossed the heliopause and entered the interstellar medium. In December 2018, Voyager project scientist Edward C. Stone announced the crossing publicly, noting that scientists had for years been waiting for a shift in the direction of the magnetic field as the definitive signature. In October 2020, measurements from both Voyager probes indicated an unexpected jump in plasma density beyond the Solar System, suggesting the density gradient is a large-scale feature of the very local interstellar medium in the direction of the heliospheric nose. On the 21st of July 2023, a programming error tilted Voyager 2's high-gain antenna 2 degrees off Earth, severing communications. By August 1 the spacecraft's carrier signal was recovered using multiple antennas of the Deep Space Network, and a high-power command sent from the Canberra station on August 4 successfully reoriented the probe. The mission is now wholly dependent on the DSS 43 antenna near Canberra, Australia, and NASA projects that by around 2030 the RTGs will no longer be able to power any instrument, with the Deep Space Network losing contact entirely around 2036.

• 7. The Golden Record

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  Voyager 2 carries a gold-plated audio-visual disc assembled under the direction of a team that included Carl Sagan and Timothy Ferris. The record holds photographs of Earth and its living things, a range of scientific information, and spoken greetings from people including the Secretary-General of the United Nations, recorded in 55 different languages. Its musical medley, titled Sounds of Earth, opens with the sounds of whales, a baby crying, and waves breaking on a shore, then moves through music spanning cultures and eras, with works by Wolfgang Amadeus Mozart, Blind Willie Johnson, Chuck Berry, and Valya Balkanska among the selections. The disc was intended as a message to any extraterrestrial finder, a gesture toward contact across distances that dwarf any human frame of reference. Voyager 2 is not aimed at any particular star. In roughly 42,000 years it will pass the star Ross 248, which lies 10.30 light-years from Earth, at a distance of 1.7 light-years. If undisturbed for 296,000 years, it will pass Sirius, 8.6 light-years from Earth, at a distance of 4.3 light-years, carrying the record with it.