In August and September 1977, two spacecrafts were launched from the Kennedy Space Center at Cape Canaveral in Florida. These spacecrafts were Voyager 1 and Voyager 2 which had both been sent on a journey to explore the Gas Giants and beyond. Today, over forty years after launch, the two crafts are still active. They are now on their way out of the Solar System, getting close to reaching the point at which the Sun no longer has any influence.
By the end of the 1960s, scientists had successfully managed to send spacecrafts to the Moon, Venus and Mars. Plans were in place to send a spacecraft to Mercury but, to get there, it would need a helping hand from Venus. A method called Gravity Assist meant that scientists could use the gravity of a planet to capture a spacecraft. Once in orbit of that planet, the space craft would begin travelling at the same velocity as the planet and when positioned in the right place, could be boosted off onto a different trajectory, taking it to another destination and using the velocity gained from the previous planet to get it to the next one quicker. This method was successfully attempted in 1973 by Pioneers 10 and 11, which we'll come to later, and by Mariner 10, also launched in 1973, which went from Venus and then to Mercury. As well as speeding up the craft's journey, it also reduced the amount of fuel needed, and by visiting two planets in one mission, did away with the need to have individual missions to visit different planets.
While all of this was going on, something that had not gone unnoticed was the positioning of the Outer Planets: Jupiter, Saturn, Uranus, Neptune and dwarf planet Pluto. They would soon be in a position which would make it technically possible to visit each planet with one spacecraft using the Gravity Assist method to slingshot the craft from one planet to the next. Seeing this as an opportunity not to be missed, NASA scientists began beavering away at plans for the "Grand Tour". This would be a program using four space probes, two launching in 1976 and 1977 to visit Jupiter, Saturn and Pluto and two to be launched in 1979 to visit Jupiter, Uranus and Neptune. However, budget cuts and the fact that NASA wanted to invest money in building the Space Shuttle meant that the Grand Tour could not go ahead and, in 1972, plans for it were scrapped. Instead, NASA set its sights purely on going to Jupiter and Saturn and proposed a mission involving two spacecrafts which would visit both planets. These spacecrafts were originally going to be known as Mariner 11 and Mariner 12, two crafts of the Mariner program. The Mariner program was a series of flyby missions to Mercury, Venus and Mars. Mariners 1, 2 and 5 were Venus missions, Mariners 3, 4, 6, 7, 8 and 9 were Mars missions, and Mariner 10 was the mission mentioned earlier that visited both Venus and Mercury. Mariners 11 and 12 were later renamed Mariner Jupiter-Saturn, and even later they became Voyager 1 and Voyager 2. Although their primary mission objective was to visit Jupiter and Saturn, scientists allowed for the possibility of sending them onwards to visit Uranus, Neptune and Pluto.
Although ambitious in their aims, the Voyager crafts weren't the first to visit the Outer Planets. Launched in 1973, Pioneer 10 and Pioneer 11 tested the water for them. Pioneer 10, launched first, proved that it was possible to get through the Asteroid Belt between Mars and Jupiter. It also measured huge amounts of radiation around Jupiter, something that scientists had to consider when designing Voyager to ensure their more sophisticated equipment wasn't destroyed by it. Pioneer 11 also visited Jupiter but went one step further and used the Gravity Assist method to go onwards to Saturn. Scientists intended to pass their Voyager probes through Saturn's rings, but wanted to make sure that doing so wouldn't damage the crafts. So, they decided to see what doing the same thing with Pioneer 11 would do. Fortunately, it didn't cause any damage, which is a good thing because when the experiment was tried out in 1979 (the year in which Pioneer 11 reached Saturn), Voyager was already on its way!
When ready for launch, the two Voyager crafts were identical to each other. They both weighed 1,592 pounds (722 kilograms) and carried the same instruments. These instruments include a magnetometer, cosmic ray detector, plasma detector, photopolarimeter, infrared interferometer, spectrometer, radiometer, ultraviolet spectrometer, low energy charged particle detector and plasma wave detector. All exciting stuff! Also part of the Voyager craft is a high gain antenna, the bit that looks like a satellite dish in the picture on the left. This antenna sends sends and receives signals between Voyager and Earth and is always, regardless of where Voyager is, pointed towards Earth. Voyager also carries a couple of cameras to send back any pictures it has taken during its trip. All of this equipment is powered by Voyager's own power supply (which I suppose is handy - you'd have to have a pretty long cable if it relied on a power supply on Earth). As radioactive materials onboard Voyager decay, they produce heat, which produces the electricity to power Voyager. It is expected that Voyager has enough power to last until about 2025 although by then most of its equipment will have been inactive for a long time.
The Voyager crafts are quite unique in that Voyager 2 (pictured right) was actually launched before Voyager 1! Voyager 2 launched on 20th August 1977 and Voyager 1 launched on 5th September 1977. The reason for this was that Voyager 1 was set to get to o Jupiter first so mission planners decided to name the craft that was going to get there first as Voyager 1 and the craft to get there second as Voyager 2. Not long into its journey, Voyager 2's main radio receiver was shut down, and due to a blown fuse, could not be restarted. This means that for the entirety of its mission, contact with Voyager 2 has been through its back-up radio receiver!
Just as planned, Voyager 1 reached Jupiter in January 1979 with its closest approach being on 5th March 1979. Voyager 2 arrived in April 1979, just as Voyager 1 was leaving, and made its closest approach to the planet on 9th July 1979. As well as taking some fantastic pictures of Jupiter, including the one on the left, they also observed nine volcanic eruptions on Jupiter's moon, Io. This was a significant discovery as it proved that Earth was not the only body in the Solar System with geological activity. As well as observing thin rings around Jupiter, the two Voyager crafts also discovered three new moons: Metis, Adrastea and Thebe, the closest, second closest and fourth closest moons of the planet.
After leaving Jupiter, both Voyager crafts reached Saturn shortly afterwards. Voyager 1 arrived in November 1980 and Voyager 2 in August 1981. Again, high quality images of the planet were sent back to Earth, and the planet's atmosphere was analysed. More information was provided about Saturn's magnificent rings, including observations of rings that appear to be twisted in strands and also spokes, clumps and kinks in the rings. 7 new moons were also discovered by the two Voyager probes including two shepherd moons of the F-ring. But it was at Saturn that Voyager 1 and Voyager 2 parted company. Saturn's moon, Titan, was of great interest. Mission planners had to make a decision whether to take a look a Titan, the only moon in the Solar System with an atmosphere, or send Voyager 1 on a long journey to Pluto. Visiting Titan would mean that Voyager 1's exploration of the planets would have to end as its trajectory afterwards would send Voyager 1 in a completely different direction to the orbital plane of the planets. However, missing out Titan would allow for the possibility of sending the craft to go onwards to Pluto (although still missing out Uranus and Neptune on its way). As Voyager was designed primarily as a mission to explore Jupiter, Saturn and their moons, and with Titan being a completely unique and mysterious moon, it was decided that Voyager 1 should pay it a visit and leave Pluto for another probe. After which, it would shoot off onto its journey out of the Solar System, a journey which it is still making today. Voyager 2 however would benefit from a different mission extension - to go on to visit Uranus and Neptune. NASA granted additional funding to the Voyager Mission, allowing Voyager 2 to visit the two remaining Gas Giants with all systems active.
Voyager 1's observations of Titan revealed the moon to be covered by a thick atmosphere made up of 90% nitrogen (the only other body in the Solar System with an atmosphere containing mostly nitrogen is Earth!). Surface temperatures were measured at -180°C (-292°F) and it was thought, although not observed, that there may be lakes of ethane or methane at the moon's surface. Some scientists like to speculate the Titan could be a cold version of an early Earth. In recent years, Titan has been revisited by the Cassini-Huygens probe enabling people to catch their first glimpse of the unique moon's surface. After its encounter with Titan, Voyager 1, as planned, left the planets of the Solar System and began its journey towards the edge of the Solar System and into Interstellar Space.
Whereas Voyager 1's trajectory from Titan sent it on it on a journey away from the planets, Voyager 2 was able to continue its tour. After its visit to o Saturn, Voyager 2 went on to Uranus. It reached the ice-cold world in 1986 and made its closest approach to the planet on 24th January of that year. As the first, and so far only, mission to visit Uranus, Voyager 2 has provided most of the information we know about the planet. One of the most interesting discoveries about Uranus was about its magnetic field. Well, it's interesting if you get excited about magnetic fields. Uranus was known to orbit on its side, as if something had knocked the planet over early in its formation. Firstly, it was not known whether Uranus had a magnetic field or not. Voyager 2 confirmed that it has. However, Uranus's magnetic field is tilted about 60 degrees from the axis around which it rotates. This causes its magnetic tail to be twisted. In other words, if you got loads of iron filings to follow Uranus, they would resemble a corkscrew. I'm not too sure what the significance of this discovery is, but like I said, it would probably interest you if you get excited about magnetic fields as it's just about the most interesting a magnetic field can get! Voyager 2 also revealed that Uranus's radiation was at about the same level as Saturn's and that the planet's temperature at its cloud tops, whether in sunlight or darkness, is the same (-213c / -350F / 60K). This is quite significant because, due to the planet's sideways orbit, a night on Uranus lasts for 21 Earth years. Voyager 2 also visited some of Uranus' moons. It discovered ten moons (Cordelia, Ophelia, Bianca, Cressida, Desdemona, Juliet, Portia, Rosalind, Belinda and Puck) and visited the planet's previously discovered moons. Amongst them, it found that Miranda (pictured above) was one of the oddest of all the moons of the Solar System. It appeared to be a mish-mash of all kinds of surfaces, possibly caused by the moon being broken up during a collision at some point and then reforming.
After visiting Uranus, Voyager 2 set off for Neptune. The final major planet of the Solar System received its only visitor in 1989. Voyager 2 made its closest approach of Neptune on 25th August 1989. It revealed the planet to be a deep blue sky-like colour and discovered the "Great Dark Spot". Thought to be similar to the Great Red Spot on Jupiter, which has been observed for about as long as it's been possible to see it (about four hundred years), the Great Dark Spot differs in that it is no longer appears to be present on Neptune, at least in recent observations from the Hubble Space Telescope although another dark spot elsewhere in Neptune's atmosphere as since been observed. Voyager 2 discovered the both Uranus and Neptune's atmosphere is mostly made up of hydrogen, helium and methane. The presence of methane filters out red light which is why the two planets appear blue, with Neptune being the bluer of the two. As well as visiting Neptune, Voyager 2 paid a visit to the planet's moons. It discovered six new moons (Naiad, Thalassa, Despina, Galatea, Larissa, Proteus) and sent back images of Neptune's largest moon, Triton. It also discovered that Triton, like Jupiter's moon Io, was geologically active, with eruptions of nitrogen gas coming from below the moon's surface.
Something else worth noting about Voyager 2 is relevant to the discovery of Pluto decades earlier in the Twentieth Century. Observations of Uranus and Neptune towards the end of the Nineteenth Century confused astronomers. The two planets were not always where they were expected to be when observed. This led astronomers to believe that there was another planet more distant than Neptune having a gravitational influence on the two worlds and affecting their orbits. The search for this planet, known by some as "Planet X", began and eventually Pluto was found. However, Pluto was too small to have the effects on Uranus and Neptune that it should have had. Voyager 2 measured Neptune's mass and found that initial scientific calculations of it were wrong. It was this miscalculation that was causing the discrepancies in the orbits of Uranus and Neptune and not a more distant planet. However, it could also be argued that if scientists had calculated Neptune's mass correctly, astronomers wouldn't have had to look for a ninth planet, and Pluto may never have been discovered.
After its rendezvous with Neptune and its moons, Voyager 2 had completed its tour of the Outer Planets. Like Voyager 1 almost ten years earlier, it would now begin its journey away from the planets and towards the edge of the Solar System. Today, both crafts are still making this journey. They are headed in different directions to each other and will never be reunited. What both crafts are looking for is the heliopause. This is the area that is believed to be the edge of the Solar System - the place where the Sun no longer has any influence. Once past this point, the crafts will be in interstellar space. Voyager 1 is today the most distant man made object in existence. The previous record-holder was Pioneer 10 but the faster-travelling Voyager 1 achieved a greater distance away from Earth on 17th February 1998. As the two crafts also travel in opposite directions, the distance between them is the greatest distance separating any two man-made objects. A way of imagining Voyager 1's distance is by considering how long it takes to receive radio communications from the craft. If a craft was on the Moon, it would take just over 1 and a half seconds to communicate with Earth. If it was at the Sun, it would take 8 minutes to send signals back to Earth. Voyager 1 is so far away that it currently takes over 14 hours to communicate with Earth and as it gets further away, this time will increase even more! Voyager 2 is about six years behind Voyager 1 in distance seeing as it had its tour of the planets extended. Exactly when the Voyager craft reach interstellar space is not known. It is expected that the crafts will continue to function until 2025, over forty-five years after launch, although by then a large number of its systems will be switched off.
Although the Voyager mission will end at some point in the future and signals from the crafts will be impossible to receive by Earth, either due to loss of contact or loss of power of the crafts, the Voyager probes will continue to travel through space. And who knows, they may one day arrive at another planet or get picked up by some passing aliens! Neither of them is actually heading towards a particular planet or star, and the nearest either of them will come to one will be in about 40,000 years when Voyager 1 will be 1.6 light years away from the star AC+79 3888 (exciting name, eh?). But on the off chance that either of the Voyager crafts do bump into an alien being, the people of Earth have kindly made attempts to communicate with them, or at least entertain them. Attached to both Voyager probes is a disc known as the Voyager Golden Record, which is kind of a Space Age message in a bottle. This record contains recordings of sounds, music and greetings from Earth. It also includes images, quite advanced technology for the 1970s! Sounds are of random things from Earth - animals (such as whales, dogs, chimps), vehicles (tractors, busses, trains), humans (laughter, footsteps, heartbeat, a kiss) weather (thunder) and geological occurrences (volcanoes, earthquakes). Music is also varied, including examples of traditional music from around the world to classical music by Bach, Beethoven and Mozart, and rock and roll from Chuck Berry! Rather interestingly/bizarrely, EMI refused permission to allow the track "Here Comes The Sun" by The Beatles on the record. So, while the Fab Four may have conquered the world in the 1960s, they're not allowed to conquer space. Greetings are spoken in 55 different languages, including languages that are no longer even spoken on Earth, such as Ancient Greek and Latin. There's even a greeting in Welsh, just in case any aliens happen to speak it! As well as sounds, there are also 115 images on the record. These images are just as varied as the sounds and include diagrams and photographs of Earth and the Solar System, human beings, animals, places, buildings and vehicles. View this page for a full list (link will take you to an external site). Suspecting that aliens probably don't have record players (they're even fairly rare on Earth now!), a cartridge and needle have also been included with Voyager's golden records (but no speakers or a TV screen). The front of the disc even contains instructions on how to play it! Whether anybody/anything can actually make sense of these instructions is another thing.
The Voyager Mission is one of the most important, ambitious and successful in the history of space exploration. Between them, the two crafts have successfully visited all four of the Outer Planets. Voyager 2 is still the only craft to have visited Uranus and Neptune and most of what we know about these planets and their moons is thanks to it. Before the launch of Voyager, we knew of the existence of 31 moons in orbit of the four planets. The Voyager probes discovered 26 moons, the largest number of moons to be discovered in a single mission, increasing the total to 57 (although this figure is now much larger). They also discovered geologically active (volcanic) moons, confirming that Earth isn't the only geologically active body in the Solar System. They observed rings around each of the four Gas Giants, proving that it wasn't just Saturn that had them. Of course, the Voyager mission holds the records for the longest running mission, the oldest mission to be still active, and the most distant man-made object in space and, while the crafts stay active, they will continue to break records and make even more new discoveries.