August 20th 2017 marks the 40th anniversary of the launch of Voyager 2, which with its sister craft Voyager 1 (launched on September 5th, 1977) are humanity’s furthest-flown operational space vehicles, with Voyager 1 being the most distant human made object from Earth, at some 140 AU (AU= astronomical unit, the average distance between the Earth and the Sun; 140 AU equates to about 20.9 billion kilometres / 13 billion miles).
Despite being so far away from Earth, both craft are still sending data back to Earth as they fly through the interstellar medium in the far reaches of the solar system (the Pioneer 10 and Pioneer 11 craft which pre-date the Voyager programme by some 5 years, ceased transmissions to Earth in January 2003 and September 1995 respectively, although Pioneer 10 is the second most distant human made object from Earth after Voyager 1).
The Voyager programme stands as one of the most remarkable missions of early space exploration. Originally, the two vehicles were to be part of NASA’s Mariner programme, and were at first designated Mariner 11 and Mariner 12 respectively. The initial Mariner missions – 1 through 10 – were focused on studying the interplanetary medium and Mars, Venus and Mercury (Mariner 10 being the first space vehicle to fly by two planets beyond Earth – Venus and Mercury – in 1974). Mariner 11 and Mariner 12 would have been an expansion of the programme, intended to perform flybys of Jupiter and Saturn.
However, in the late 1960s Gary Flandro, an aerospace engineer at the Jet Propulsion Laboratory (JPL) in California noted that in the late 1970s, the outer planets would be entering a period of orbital alignment which occurs once every 175 years and which could be used to throw a series of probes out from Earth, which could then use the gravities of the worlds they encountered to “slingshot” them on to other targets. This led to the idea of a “Grand Tour” mission: sending pairs of probes which could use these gravity assists to fly by Jupiter, Saturn, Uranus, Neptune and Pluto in various combinations.
Funding limitations eventually brought an end to the “Grand Tour” idea, but the planetary alignment was too good an opportunity to miss, and so elements of the idea were folded into the Voyager Programme, which would utilise Mariner 11 and Mariner 12. However, as the mission scope required some significant changes to the vehicles from the basic Mariner design, they were re-designated as Voyager class craft.
(As an aside, the Mariner class is the longest-lived of NASA’s space probe designs; as well as the ten missions of the 1960s and 1970s which carried the design’s name, the Mariner baseline vehicle – somewhat enlarged – was used for the Viking 1 and Viking 2 orbiter missions to Mars, and formed the basis of the Magellan probe (1989-1994) to Venus and the Galileo vehicle (1989-2003) which explored Jupiter. And uprated and updated baseline Mariner vehicle, designated “Mariner Mark II”, formed the basis of the Cassini vehicle, now in the terminal phase of its 13-year study of Saturn and its moons.)
Each of the Voyager mission vehicles is powered by three plutonium-238 MHW-RTG radioisotope thermoelectric generators (RTGs), which provided approximately 470 W at 30 volts DC when the spacecraft was launched. By 2011, both the decay of the plutonium and associated degradation of the thermocouples that convert heat into electricity has reduced this power output by some 57%, and is continuing at a rate of about 4 watts per year.
To compensate for the loss, various instruments on each of the vehicles have had to be turned off over the years. Voyager 2’s scan platform and the six instruments it supports, including the vehicle’s two camera systems, was powered-down in 1998. While the platform on Voyager 1 remains operational, all but one of the instruments it supports – the ultraviolet spectrometer (UVS) – have also been powered down. In addition, gyro operations ended in 2016 for Voyager 2 and will end in 2017 for Voyager 1. These allow(ed) the craft to rotate through 360 degrees six times per year to measure their own magnetic field, which could then be subtracted from the magnetometer science data to gain accurate data on the magnetic fields of the space each vehicle is passing through.
However, despite the loss of capabilities, both Voyager 1 and Voyager 2 retain enough power to operate the instruments required for the current phase of their mission – measuring the interstellar medium and reporting findings back to Earth. This phase of the mission, officially called the Voyager Interstellar Mission, essentially commenced in 1989 as Voyager 2 completed its flyby of Neptune, when the missions as a whole was already into their 12th year.
Voyager 2 was launched on August 20th, 1977. Of the two vehicles, it was tasked with the longer of the planned interplanetary missions, with the aim of flying by Jupiter, Saturn, Uranus and Neptune. However, the latter two were seen as “optional”, and dependent upon the success of Voyager 1.
This was because scientists wanted the opportunity to look at Saturn’s moon Titan. But doing so would mean the Voyager craft doing so would have to fly a trajectory which would leave it unable to use Saturn’s gravity to swing it on towards an encounter with Uranus. Instead, it would head directly towards interstellar space.
So it was decided that Voyager 1, which although launched after Voyager 2 would be able to travel faster, would attempt the Titan flyby. If it failed for any reason, Voyager 2 could be re-tasked to perform the fly-past, although that would also mean no encounters with Uranus or Neptune. In the end, Voyager 1 was successful, and Voyager 2 was free to complete its surveys of all four gas giants.
Along the way, both missions revolutionised our understanding of the gas giant planets and revealed much that hadn’t been expected, such as discovering the first active volcanoes beyond Earth, with nine eruptions imaged on Io as the vehicles swept past Jupiter. The Voyager missions were also the first to find evidence that Jupiter’s moon Europa might harbour a subsurface liquid water ocean and to return the first images of Jupiter’s tenuous and almost invisible ring system. Voyager 1 was responsible for the first detailed examination of Titan’s dense, nitrogen-rich atmosphere, and Voyager 2 for the discovery of giant ice geysers erupting on Neptune’s largest moon, Triton. In addition, both of the Voyager vehicles added to our catalogues of moons in orbit around Jupiter and Saturn, and probed the mysteries of both planet’s atmospheres, whilst Voyager 2 presented us with our first images of mysterious Uranus and Neptune – and thus far remains the only vehicle from Earth to have visited these two worlds.
The flyby of Neptune also sealed Voyager 2’s future. Scientists were keen to use the flyby of the planet to take a look at Triton, Neptune’s largest moon. However, because Triton’s orbit around Neptune is tilted significantly with respect to the plane of the ecliptic, Voyager 2’s course to Neptune had to be adjusted by way of a gravitational assist from Uranus and a number of mid-course corrections both before and after that encounter, so that on Reaching Neptune, it would pass over the north pole, allowing it to bent “bent” down onto an intercept with Triton while the Moon was at apoapsis – the point furthest from Neptune in its orbit – and well below the plane of the ecliptic. As a result, Voyager 2 passed over Triton’s north pole 24 hours after its closest approach to Neptune, its course now pointing it towards “southern” edge of the solar system.