Space Sunday: Voyager at 40

Voyager: 40 years on. Credit: NASA

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.

A drawing of the Voyager vehicles. Credit: NASA/JPL (click for full size)

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.

A plume rises 160 km (100 mi) above Loki Patera, the largest volcanic depression on Io, captured in March 1979 by Voyager 1. Credit: NASA/JPL

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.

This is the last full planet image captured of Neptune. Taken by Voyager 2 on August 21st, 1989, from a distance of 7 million km (4.4. million mi). A true colour image white balanced to reveal the planet under typical Earth lighting conditions, it shows Neptune’s “Great Dark Spot” and surrounding streaks of lighter coloured clouds, all of which persisted through the period of Voyager 2’s flyby. More recent Hubble Space Telescope images suggest the “Great Dark Spot”, initially thought to be a possible cloud / storm formation, similar to Jupiter’s Great Red Spot, has vanished, leading to speculation that it may have actually been a “hole” in a  layer of Neptune’s layered clouds. Credit: NASA/JPL

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.

Global colour mosaic of Triton, taken in 1989 by Voyager 2, showing the moon’s southern pole (bottom) and equatorial latitudes, above which a clearly visible terminator cuts, where Neptune’s shadow falls across the moon. With a radius of 1,350 km (839 mi), about 22% smaller than Earth’s moon, Triton is one of only three objects in the Solar System known to have a nitrogen-dominated atmosphere (the others being Earth and Titan). Triton has the coldest surface known anywhere in the Solar System (38 K, about -391 degrees Fahrenheit). It is so cold that most of the moon’s nitrogen is condensed as frost, making it the only satellite in the Solar System known to have a surface made mainly of nitrogen ice.

Voyager 2 revealed Triton to be another enigma: a moon with a nitrogen-rich atmosphere, and vast southern polar cap believed to contain methane ice overlaid by an icy and perhaps carbonaceous dust deposited from huge geyser-like plumes, some of which were found to be active during the flyby.  This chaotic landscape is believed to the result of “cryovolcanic” eruptions of very cold liquids from the moon’s interior, which rapidly froze on exposure on the surface. The slightly less chaotic equatorial region of the moon appears to be dominated by bluish-green banding thought to be relatively fresh nitrogen frost deposits. It contains an odd terrain formation referred to as the “cantaloupe terrain”, the origins of which remain unknown.

While Voyager 1 was the second of the two probes to be launched from Earth, it’s mission profile meant it was also the faster of the two vehicles. It reached Jupiter in January 1979, some six months ahead of Voyager 2, and then passed by Saturn in late 1980, almost a year ahead of Voyager 2. At its launch, Voyager 1 was the fastest human-made object to depart Earth, a record it held until the launch of the New Horizons mission to Pluto in 2006. However, because of the planetary assists it received from Jupiter and Saturn, Voyager 1 remains the fastest human-made vehicle built and launched to date, travelling at a velocity of approximately 17 km per second (11 mi/s) relative to the Sun.

“Family portrait”: On February 14th, 1990, the cameras aboard Voyager one were turned back towards the Sun to take a series of images capturing the majority of the major planets in the solar system. Some 60 images were capture from 32-degrees above the plane of the ecliptic, with the Sun, Earth and Venus at its centre (Mercury was too close to the Sun and too small to be seen by the camera systems, even when using their darkest filters to capture the image of the Sun. Taken from a distance of 6.4 billion km (4 billion mi), the Sun is 40 times smaller than when seen from Earth but still 8 million times brighter than Sirius, the brightest star seen in Earth’s skies. The inset images are the individual frames capturing the planets as seen by Voyager. Shortly after this set of images was record and transmitted to Earth, the probe’s cameras, their work completed, were turned off to conserve power. Credit: NASA/JPL (click for full size)

In 2005, it was confirmed that Voyager 1 had passed though the region of termination shock on the “leading edge” of the solar system. This is the point where the solar wind slows down to subsonic speeds as the space dominated by the Sun’s activity starts to give way to the interstellar medium in a region of space called the heliosheath (which together with the heliopause, forms an elongated “bubble” of complex interactions between the solar and interstellar winds called the heliosphere). At the time the probe passed through termination shock, it was some 94 AU from the Sun.

On December 13th, 2010, it was confirmed that Voyager 1 had was passing through the point at which the radial outward flow of the solar wind from the Sun is turned “sideways” relative to its outward flow due to the inward pressure of the interstellar wind. This indicated that, at a distance of 116 AU, the vehicle was approaching the heliopause: the region surrounding the Sun where its dominance of local space completely gives way to that of the interstellar medium. A year later, Voyager 1 detected Lyman-alpha radiation from the Milky Way for the first time, further proof that it was reaching the very edge of the Sun’s influence. Six months after that, readings from the vehicle significantly changed, with a marked increase in the detection of charged particles originating in interstellar space: it had entered the heliopause.

A simplified image of the heliosphere and its component elements and their interaction with interstellar space (shown to the left) from 2013, showing the (then) relative position of Voyager 1. Credit: NASA/JPL

Confirmation that Voyager 1 had passed through the heliopause and entered interstellar space came in March 2013 – although the period at which the vehicle had done so was in late 2012; it just took time to gather sufficient data that it had in fact done so. As a result of the reading gathered by the vehicle during its passage through the heliosheath and heliopause, scientists were able to confirm that the heliosphere acts as a massive radiation shield, protecting the planets of the solar system from atomic nuclei travelling at close to the speed of light, in much the same way that Earth’s magnetic field protects us from solar wind (which would otherwise strip away our atmosphere).

However, as odd as it might sound, while Voyager 1 may have passed beyond the Sun’s influence and into the interstellar medium – it hasn’t actually left the solar system per se. It is still around one-seventh the distance to the aphelion of Sedna, a large minor planet orbiting the Sun between 76 AU and 937 AU. It will not enter the Oort cloud, the source region of long-period comets and regarded by astronomers as the outermost zone of the Solar System, for another 300 years.

Perhaps the most famous aspects of the Voyager spacecraft are the Golden Records. Conceived by astronomers Frank Drake and Carl Sagan, they contain sounds and images selected to portray the diversity of life and culture on Earth, and are intended for any intelligent extraterrestrial life form, or for future humans, who may find them. You can listen to audio and see images from the records on-line (click on the record image, top left of the plaque image for audio, and the square for images), as a part of the multi-media showcase for the Golden Records. Credit: NASA/JPL

Voyager 2 is now adding to the sum of Voyager 1’s examination of the heliosphere. It passed through termination shock in August 2007 at a point much closer to the Sun that Voyager 1, because it was headed “southwards” and away from the plane of the ecliptic, and towards the point at which the heliosphere is more flattened as it surrounds the Sun. It is currently measuring the heliosheath, and is expected to pass through the heliopause some time in 2018/2019, and join Voyager 1 in interstellar space.

Both of the Voyager craft should have sufficient electrical power to continue taking measurements and returning data to Earth through until around 2030, and their reserves of manoeuvring fuel should last about the same amount of time. While not aimed at any specific star, Voyager 1 will pass by Gliese 445 at a distance of about 1.6 light years in about 40,000 years. By coincidence, Voyager 2 should pass within 1.7 light years of Ross 248 in roughly the same time frame.

Voyager Mission Planetary Firsts (1979-1990)

  • First spacecraft to fly by all four planets of the outer solar system – Voyager 2
  • First mission to discover multiple moons of the four outer planets (both spacecraft): three new moons at Jupiter, four new moons at Saturn, 11 new moons at Uranus, six new moons at Neptune
  • First spacecraft to fly by four different target planets –  Voyager 2
  • First spacecraft to visit Uranus and Neptune – Voyager 2
  • First spacecraft to image the rings of Jupiter, Uranus and Neptune Voyager 2
  • First spacecraft to discover active volcanoes beyond Earth – Voyager 1
  • First spacecraft to detect lightning on a planet other than Earth (at Jupiter) – Voyager 1
  • First spacecraft to find suggestions of an ocean beyond Earth (Europa) – both spacecraft
  • First spacecraft to detect a nitrogen-rich atmosphere found beyond our home planet (Titan) – Voyager 1

Heliophysics Firsts (2012-present)

  • First spacecraft to measure solar wind termination shock—the boundary where solar wind charged particles slow below the speed of sound as they begin to press into the interstellar medium – Voyager 1
  • First spacecraft to detect the limit of radial outward flow of the solar wind – Voyager 1
  • First spacecraft to leave the heliosphere and enter interstellar space – Voyager 1
  • First spacecraft to measure full intensity of cosmic rays—atoms accelerated to nearly the speed of light—in interstellar space  – Voyager 1
  • First spacecraft to measure magnetic fields in interstellar space – Voyager 1
  • First spacecraft to measure density of interstellar medium—material ejected by ancient supernovae –  Voyager 1

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