Space Voyager 1 and 2: Grand Tour of Gas Giants and Headed for the Stars

	Jupiter seen by Voyager 2. picture courtesy NASA/JPL

Managed for NASA by the Jet Propulsion Laboratory (JPL), Voyager 1 and 2 were on tracks of Pioneer missions. They explored gas giants worlds: Jupiter, Saturn, Uranus, and Neptune. In the late 1970s and the 1980s this four planets were to be in a configuration occurring each 175 years only, allowing a four-planet tour with a minimum of propellant and time, using gravity assist flybys. Craft cost for such a mission was deemed too expensive however. As a consequence, engineers transformed instead two last Mariner series missions (Mariner 11 and 12) into Voyager 1 and 2. Voyager 1 and 2 mission was to perform a closeup study of Jupiter and Saturn and of their main moons, Io and Titan, only. Voyager 2 trajectory was preserving the option to continue to Uranus and Neptune however. Missions were to last five years. Conditions made that this two-planet, five-year missions eventually turned into a 12-year, four gas giants, 48 of their moons, and their environment, tour. Voyager 1 and 2 twelve-year journeys revolutionarized planetology as they produced a never seen wealth of data. It is these missions, e.g. which brought the until then never seen views of Io surface or which confirmed, discovered, or first saw rings at Jupiter, Uranus, and Neptune. Voyager 2’s 'Grand Tour' of the solar system was to require that the spacecraft travel through Saturn’s outer A ring in order to get close enough to use gravitational assist to propel it to Uranus as a journey through the Asteroid Belt had been risk-free by the previous Pioneer 10 and 11 missions. Scientists at the time were unsure of the makeup of the rings and were worried that the spacecraft might be impacted and destroyed by objects in the ring. The material in the rings (now known to be mainly comprised of water ice) needed, on average, to be smaller than 1 mm, in which case they would be unlikely to damage the spacecraft, or larger than 1 cm so that they would be spaced far enough apart to allow a spacecraft to pass through them. If they fell between these two sizes, then a lethal impact was nearly inevitable. The Pioneer 11 mission was used to pass into the outer ring of Saturn like a precursor to the Voyagers. The suite of 11 instruments for the Voyager 1 included: an imaging science system consisting of narrow-angle and wide-angle cameras to photograph the planet and its satellites; a radio science system to determine the planet’s physical properties; an infrared interferometer spectrometer to investigate local and global energy balance and atmospheric composition; an ultraviolet spectrometer to measure atmospheric properties; a magnetometer to analyze the planet’s magnetic field and interaction with the solar wind; a plasma spectrometer to investigate microscopic properties of plasma ions; a low energy charged particle device to measure fluxes and distributions of ions; a cosmic ray detection system to determine the origin and behavior of cosmic radiation; a planetary radio astronomy investigation to study radio emissions from Jupiter; a photopolarimeter to measure the planet’s surface composition; and a plasma wave system to study the planet’s magnetosphere

	Saturn seen by Voyager 2. picture courtesy NASA/JPL

Voyager 2 launched from Cape Canaveral (now Kennedy Space Center, KSC) on August, 20th 1977 on top of a Titan-Centaur. Voyager 1 launched in second position only on September, 5th 1977. Voyager 1 was on a faster and shorter trajectory however. It thus arrived at Jupiter on March, 5th 1979 and at Saturn on November, 12th 1980 as Voyager 2 reached both planets on July, 9th 1979 and August, 25th 1981 only. As Voyager 1 trajectory had been devised to reach Titan, craft was drawn above ecliptic's plane to reach there as the moon was of interest. Hence its planetary mission stopped there. On its flyby of Saturn, Voyager 1 found a wealth of new data regarding the planet and its moons. At Saturn, the Voyagers discovered a menagerie of moons, each icy and with a past, or current geological activity. As American astronomer Gerard Kuiper, for whom the Kuiper Belt is named, had discovered in 1944 that Titan had an atmosphere containing methane. Observations from both Voyagers showed that Titan's thick atmosphere was primarily composed of nitrogen, with a few percent methane and smaller amounts of other complex hydrocarbons, such as ethane, propane and acetylene. No other moon in the solar system has a dense atmosphere. Three new moons were discoverd --Prometheus, Pandora (shepherding moons of the F-rings), and Atlas, shepherd of the A-rings. There were also surprising gaps in the rings, some caused by moons embedded within them. Also revealed a immense hexagonal feature in the clouds that surrounded Saturn's north pole and that, Saturn’s upper atmosphere to be composed of 7% helium and the rest mostly made up of hydrogen. Scientists inferred that because of Saturn’s atmospheric composition, Saturn radiates more heat than what it receives from the Sun. Voyager 1 also discovered the G-ring of Saturn. Since then and despite advances through the more recent Cassini mission, mysteries remain about Saturn. For example, the exact length of Saturn's day, or the origin of the rings. On another hand, since its approach to Saturn, Voyager 2 was directed to a fly-by point which was devised to send it to Uranus. Once Voyager 2 encountered Saturn it was officially extended as an Uranus and Neptune's probe as the Uranus encounter was deviced so that the spacecraft would receive a gravity assist to help it reach Neptune. Voyager 1 at the same time was officially recognized as an interplanetary medium mission. Comprehensive mission was renamed "Voyager Neptune Interstellar Mission". Voyager 2 arrived at Uranus on January, 24th 1986. It was the first probe ever to that planet. It then reached Neptune on August, 25th 1989 as its route had been finely programmed to reach the planet over the north pole and planetary weather specialists had been able to tell where to point the cameras at. There too Voyager 2 was first probe ever. Neptune got into view on August 25, 1989 in the middle of the night at NASA's Jet Propulsion Laboratory in Pasadena, California. Voyager 2 began to observe Neptune on June 5, 1989, at a distance of 73 million miles. Even at this range, Voyager’s images were already four times better than those obtained by Earth-based telescopes. It returned more than 9,000 images of the planet, its rings and its moons. Because of Neptune’s great distance from the Sun, engineers made changes to Voyager’s imaging techniques to accommodate light levels only 3 percent of what they were during the Jupiter encounter and image motion compensation techniques were programmed into Voyager’s computer to maintain clear photographs at those long exposures coupled with the spacecraft’s velocity. NASA also upgraded the tracking antennas of the Deep Space Network at the time, to increase their sensitivity to receive Voyager’s signals from Neptune’s distance. The DSN expanded the dishes to 230 ft (70 meters). The mission team, at that point, was going to places where no one had ever gone before! From there Voyager 2 trajectory was bent and craft was brought under the ecliptic, heading to the interplanetary medium, then stars bound as that a gravity assist also allowed a flyby to Neptune's moon Triton. Missions became the "Voyager Interstellar Mission". Both probes were successfully upgraded via remote-control reprogramming. As of February, 14th, 1990, the Valentine Day, Voyager 1 sent its famous solar system family portrait showing solar system planets as seen in a sole and afar point of view. It was to be its last picture as its cameras were turned off shortly after that portrait

Uranus seen by Voyager 2. picture courtesy NASA/JPL

Both craft continued their journey during the 1990s however and they are now reaching solar system outer reaches, like Pioneer 10 and 11. Pioneer 10 and 11 are no more able to make science nor may be contacted as Voyager 1 and 2 are still fit for science. They are scheduled to probe solar system boundaries. Their work is thus thought to be possible during two or three decades still. Communications will be maintained until their radioisotope power generators stop supplying them as engineers expect each spacecraft to continue operating at least one science instrument until around 2025. Voyager 1 is travelling at an angle of 35° to the ecliptic and is thought to have passed heliosphere "termination shock" in 2003. As of late 2011 it had reached 11 billion miles (17 billion km) from the Sun, and approaching the interstellar space as it had reached almost 13 billion miles (21 billion km) by 2017. Voyager 2 is journeying at minus 48° under the ecliptic and as of 2017 had reached 11 billion miles (17.7 billion km) away, and expected to enter interstellar space in the next few years. A Voyager clock is kept at the JPL to account for the journey of both craft. There is plenty of power for the rest of the journey. Both Voyagers are energized by the radioactive decay of a Plutonium 238 heat source. This should keep critical subsystems running through at least 2020. By November 2011 Voyager 2 spacecraft was ordered to switch to the backup set of thrusters that controls the roll of the spacecraft, at the effect of reducing the amount of power it requires to heat the fuel line, and use previously unused thrusters. Each Voyager featured six sets of thrusters including three pairs of primary thrusters and three backup pairs. Voyager 2 already was using its two pairs of backup thrusters to control pitch and yaw. Albeit the craft available power keeps declining slowly now, it is thus expected to continue to operate another decade. Voyager 1 changed to its three sets of backup thrusters in 2004. Team members estimate they will have to turn off the last science instrument by 2030. The technology, on a other hand, is many generations old, and it takes someone with 1970's design experience to understand how the spacecraft operate and what updates can be made to permit them to continue operating. Voyager 1 crossed the heliopause and entered interstellar space in August 2012. After that the Voyagers will become our silent ambassador to the stars. As they are stars bound the Voyager 1 and 2 are carrying a message to any intelligent life they might encounter: a disk containing sounds and images of life and cultures at Earth. Content had been devised by a committee chaired by Carl Sagan. Each so-called 'Golden Record,' literally is a gold-coated copper phonograph record. It contains 118 photographs of Earth, 90 minutes of the world's greatest music, an audio essay entitled Sounds of Earth (featuring everything from burbling mud pots to barking dogs to a roaring Saturn 5 liftoff), greetings in 55 human languages and one whale language, the brain waves of a young woman in love, and salutations from the Secretary General of the United Nations. Each disk is protected into a aluminum blister, which is forecast to last for 72,000 years, with instructions for playing its content. The Voyagers also gained foot into the popular culture, with movies and TV series altogether with a musical opus and a song. The 'We offer friendship across the stars. You are not alone' message to possible aliens was sent by NASA by 2017 to the Voyager 1 at the occasion of its 40th anniversay, after a competition and the message was sent by Star Treck’s OG Captain Kirk. That looping message will now also accompany the craft. The Voyager probes won't come within a few light years of another star for some 40,000 years. Both the Voyager missions, thus, eventually, not only could perform that tempting 'grand tour' of the gas giants of our solar system, as, moreover, they came to be transformed into missions bound to leave the domain of the Sun! At about 41 years in 2018, both Voyagers turned NASA’s longest running missions. Each Voyager could last billions of years

Neptune seen by Voyager 2. picture courtesy NASA/JPL

one of both Golden Records which the Voyagers carried off like a message to potential ET intelligence. NASA/JPL