The Story of Voyager Probes in the Solar System: This year marks the 45th anniversary of the Voyager launch. Of course, anniversaries like these have no meaning for spacecraft that have left Earth’s orbit; but they do for us, who have remained on this planet waiting and counting the days.
Incredibly, it’s possible that these spacecraft, which have literally changed the face of the solar system, could last until the next big anniversary, the 50th, in 2027. We’re really on the edge here, although some say that turning off instruments and saving onboard power will even get us to 2030.
In the meantime, it will certainly be a pleasure for us to retrace the steps of the mission, summarizing all the extraordinary discoveries given to us in these 45 years. We are all agreed? In the 18th and 19th centuries, the Grand Tour was defined as the journey of education through Europe made by the scions of the wealthy class, almost always with the final destination Italy.
The concept of the Space Grand Tour was borrowed in 1964 when Gary Flandro of the Jet Propulsion Laboratory noted that an alignment of Jupiter, Saturn, Uranus, and Neptune that would occur in the late 1970s would allow a single probe to visit all the outer planets using the then-experimental “gravitational assist” technique.
The particular alignment envisaged by Flandro happens very rarely, once every 175 years… so the JPL immediately accepted the idea, considering the great savings in time and economic resources, giving the start to the Grand Tour program.
After years of changes and rethinking, finally, it was decided to make the mission to two completely identical spacecraft, which on different trajectories and with different objectives would eventually explore all the systems of the outer planets, including moons.
It was followed on September 5, 1977, by Voyager 1, which was put on a faster, shorter trajectory to Jupiter. Voyager 1’s course was optimized for the Titan flyby and Voyager 2 for the Grand Tour. Both launches took place from Cape Canaveral.
At that time, the gas giant planets were mysterious, barely resolved by the largest ground-based telescopes. Their moons were assumed to be inert and uninteresting rocks like Mercury or the Moon. Yet the outer Solar System seems inhuman and inhospitable for life.
So, the Voyagers took a part of the Solar System that had been studied briefly and in little detail and fleshed it out into a family portrait of four giant planets, their ring systems, and magnetic fields, plus forty-eight of their moons.
The additions and revisions have been enough to cause textbooks on astronomy to be rewritten. First up was Jupiter. This mighty gas giant is three times more massive than any other planet and 318 times more massive than the Earth.
The probes reached Jupiter in 1979, with a separation of four months: Voyager 1 on March 5 and Voyager 2 on July 9. Even after centuries of telescopic observations, there were surprises. The Great Red Spot was revealed to be a huge anticyclone, large enough to swallow the Earth, with eddies and smaller storms around its periphery.
It had changed color from orange to dark brown in the six years since the Pioneer flybys. As they passed behind the planet the Voyagers saw lightning illuminating the darkness of the night-side atmosphere.
Voyager 1’s closest encounter with Jupiter was at a range of about 280,000 km, following which it encountered several of Jupiter’s moons, including Amalthea at the 420,000-kilometer range, Io (21,000 km), Europa (733,000 km), Ganymede (115,000 km), and Callisto (126,000 km), in that order, returning spectacular photos of their terrains and opening up completely new worlds for planetary scientists.
Voyager 1 also discovered a very faint ring system around Jupiter made of dust ejected from the inner moons after high-velocity impacts. The rings are far less dramatic than Saturn’s but are equally interesting scientifically. The main ring circles from 125,000 km away from the center of the planet.
Voyager 1 discovered two new moons of Jupiter: Thebe and Metis. Both are irregular in shape; Thebe is 116 km in its largest dimension and Metis is only 60 km long. However, the real excitement came from Io, Jupiter’s closest moon and the fourth-largest moon in the Solar System.
This strange-looking rock, with its mottled yellow-brown surface, looking like a moldy orange, is the most geologically active world in the Solar System. Images captured by the two high-resolution cameras revealed to scientists a relatively young surface world, dotted with oddly shaped pits, mountains taller than Mount Everest, and volcanic lava flows.
In addition, a NASA engineer noticed in one of the images transmitted by the probe a plume emerging from the curvature of the horizon. The discovery generated great excitement throughout the Voyager 1 team, and the hunt for more evidence began immediately.
Analysis of the other photos of the Jovian moon led to the discovery of a total of nine plumes that dispersed in Io’s atmosphere up to 300 km altitude, proving conclusively that the moon was volcanically active.
To further confirm the new discovery there were also data from the study of the thin atmosphere and its surface, which were both composed mainly of sulfur and sulfur dioxide, chemical compounds associated with volcanic phenomena.
The observations of Voyager 1 were confirmed four months later, when Voyager 2 made a close pass over Io, although keeping a greater distance than the first probe. Being able to count on two overflights at a short distance from each other, it was also possible to study the evolution of volcanic activity over time.
The surface of Io changed considerably in the four months that elapsed between the two meetings. It was noted during the second passage, that only seven of the nine volcanoes discovered by the first probe were still erupting.
Subsequent studies suggested that the extreme geological activity of Io was attributable to tidal heating due to the friction caused in its interior by the gravitational stresses of Jupiter and the other Galilean moons: Europa, Callisto, and Ganymede.
The discovery became even more sensational when scientists realized that volcanic activity on Io has repercussions on the entire Jovian system. The products of volcanism such as sulfur, oxygen, and sodium, are in fact lifted for kilometers by the power of the eruptions and remain suspended in the highest layers of the atmosphere.
From there, these particles are torn apart by interaction with Jupiter’s intense magnetic field. They then arrange themselves around the Jupiter system, in a series of belts that have important consequences from the point of view of the magnetic fields and plasma surrounding the gas giant’s system. Other surprises came from the overflight of Europa.
Scientists were confronted with a crust of water ice characterized by a complex web of linear striations. Initially, they thought of deep cracks generated by crustal rifting or tectonic processes. This new discovery suggested that the crust was young and warm, probably due to the same tidal phenomena that underlie Io’s volcanic activity. Voyager’s observations of Europa provided significant indications of a liquid water ocean beneath the icy crust.
Following the Jupiter encounter, Voyager 1 completed an initial course correction on April 9, 1979, in preparation for its meeting with Saturn in November 1980, During its flyby, Voyager 1 found five new moons, a ring system consisting of thousands of bands, wedge-shaped transient clouds of tiny particles in the B-ring that scientists called “spokes,” a new ring (the G-ring), and “shepherding” satellites on either side of the F-ring; satellites that keep the rings well-defined.
Also, the spacecraft photographed Saturn’s moons Mimas, Enceladus, Tethys, Dione, and Rhea. Based on incoming data, all the moons appeared to be composed largely of water ice. The main goal, however, was to explore Titan, Saturn’s largest moon. The reason for so much interest was its atmosphere.
Titan is in fact the only moon in the solar system characterized by a dense atmosphere, and images taken a few years earlier by the Pioneer 1 probe had revealed a complex composition, worthy of investigation.
The overflight of the large moon took place without problems on November. 12, at a range of about 4,000 km, but at first sight, it was quite disappointing. No instrument was able to penetrate the thick atmosphere to observe the surface, which remained a big question mark.
Nevertheless, important data were obtained on the composition of the atmosphere itself, which was found to be composed mainly of nitrogen, like the Earth’s, and dotted with clouds of methane and ethane. During the overflight were also studied the atmospheric density, temperature, and pressure, while the mass was estimated evaluating the effects on the trajectory of the probe.
The data collected during the overflight were of fundamental importance because they led to the formulation hypotheses about the possible presence of hydrocarbon lakes on the surface.
Following the encounter with Saturn, Voyager 1 headed on a trajectory to escape the solar system at a speed of about 500 million km per year. So long Voyager 1, thank you for all! Okay, but what happened to Voyager 2 in the meantime?
Voyager 2’s closest encounter to Jupiter was on July 9, 1979, at a range of about 650,000 km. It transmitted new data on the planet’s clouds, its newly discovered four moons, and ring system as well as 17,000 new pictures.
During its encounter, it relayed back spectacular photos of the entire Jovian system, including its moons Callisto (at a range of about 215,000 km), Ganymede (at 62,000 km), Europa (at 200,000 km), Io, and Amalthea, all of which had already been surveyed by Voyager 1.
With the combined cameras of the two Voyagers, at least 80% of the surfaces of Ganymede and Callisto were mapped out to a resolution of only 5 km. And then, after a course correction that occurred two hours after its maximum approach to Jupiter, Voyager 2 headed toward Saturn.
Its encounter with the sixth planet began two years after leaving the Jovian system, with imaging of the moon Iapetus. Once again, Voyager 2 repeated the photographic mission of its predecessor. The closest encounter to Saturn was on August 26, at a range of about 100,000 km.
The spacecraft provided more detailed images of the ring “spokes” and kinks, and also the F-ring and its shepherding moons, all found by Voyager 1. Voyager 2’s data suggested that Saturn’s A-ring was perhaps only about 300 meters thick.
As it flew behind and up past Saturn, the probe passed through the plane of Saturn’s rings at a speed of 13 Km per second. For several minutes during this phase, the spacecraft was hit by thousands of micron-sized dust grains that created “puff” plasma as they were vaporized.
Because the vehicle’s attitude was repeatedly shifted by the particles, attitude control jets automatically fired many times to stabilize the vehicle. During the encounter, Voyager 2 also photographed the Saturn moons Hyperion (the “hamburger moon”), Enceladus, Tethys, and Phoebe as well as the more recently discovered Helene, Telesto, and Calypso.
Although Voyager 2 had fulfilled its primary mission goals with the two planetary encounters, mission planners directed the veteran spacecraft to Uranus – a journey that would take about 4.5 years. Long-range observations of the planet began on November 4, 1985, when signals took approximately 2.5 hours to reach Earth. Light conditions were 400 times less than terrestrial conditions.
The closest approach to Uranus took place on January 24, 1986, at a range of about 80,000 km. During its flyby, Voyager 2 discovered ten little moons, two new rings in addition to the “older” nine rings, and a magnetic field tilted at 55 degrees off-axis and off-center.
The spacecraft found wind speeds in Uranus’ atmosphere as high as 700 km per hour and found evidence of a boiling ocean of water some 800 km below the top cloud surface. Its rings were found to be extremely variable in thickness and opacity.
Voyager 2 also returned spectacular photos of Miranda, Oberon, Ariel, Umbriel, and Titania, five of Uranus’ larger moons. In flying by Miranda at a range of only 28,000 km, the spacecraft came closest to any object so far in its nearly decade-long travels.
Following the Uranus encounter, the spacecraft performed a single midcourse correction on February 14, 1986, to set it on a precise course to Neptune. Voyager 2’s encounter with Neptune capped a 7 billion-kilometer journey when, on August 25, 1989, it flew about 4,700 km over the cloud tops of the giant planet, the closest of its four flybys.
During the encounter, the spacecraft discovered six little moons and four new rings. Because Neptune receives so little sunlight, many scientists had expected to see a placid, featureless planet. Instead, Voyager showed a dynamic atmosphere with 1,100-kilometer per hour winds blowing westward, opposite the direction of rotation, at speeds faster than the winds of any other planet.
Neptune revealed its Great Dark Spot, a storm system that resembled Jupiter’s Great Red Spot, and a smaller, eastwardly moving cloud, called ‘scooter’, which went around the planet about every 16 hours. The planet was circled by diffuse, dusty rings; six new moons were discovered.
Hydrogen was found to be the most common atmospheric element, although the abundant methane gave the planet its blue appearance. Voyager 2 passed over the north polar region, using the planet’s gravity to redirect the trajectory for a final encounter with Neptune’s largest moon Triton, which turned out to be one of the most interesting moons in the Solar System.
It is in fact the coldest body among those in orbit around the sun, with a surface temperature of -235 degrees centigrade. The images were taken by the probe also revealed the presence of geysers that raised frozen nitrogen and dark dust up to several kilometers high above the polar cap.
The observations made also denied any assumption about its atmosphere, which turned out to be extremely rarefied. To date, Voyager 2 remains the only probe to have ever visited the Neptune system and this makes its discoveries absolutely unique.
The flyby of Neptune concluded Voyager 2’s planetary encounters, which spanned an amazing 12 years in deep space, virtually accomplishing the originally planned “Grand Tour” of the solar system, at least in terms of targets reached if not in science accomplished.
Once past the Neptune system, Voyager 2 followed a course below the ecliptic plane and out of the solar system. So, years after their launch, the data collected by Voyagers continue to be studied and processed, new discoveries continue to be made, and surely this engineering masterpiece will continue to inspire future missions to the Solar System. Way to go Voyagers, we look forward to seeing you for the 50th!
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