On 20 August, 1977, Voyager 2 was launched, followed 16 days later by Voyager 1. They are twin spacecraft, with the same sets of instruments but they were built to last only five years. They were designed to gather data on Jupiter and Saturn, and to be part of a bigger set of space probes that would go in pairs to visit all of the outer planets in the solar system. However, budgetary constraints meant that the other probes couldn't be sent, and the Voyager missions were extended to last 12 years, more than twice as long as originally intended.

Launch and Flight Path

Although Voyager 2¹ was launched before Voyager 1, it was on a slower trajectory towards the Jupiter system so Voyager 1 got there first. Voyager 1's launch had to be delayed twice, but on 5 September it went up in a flawless launch.

The Voyager missions were launched to take advantage of an alignment of the planets that only takes place every 175 years. The planets are arranged so that the gravity of one can help propel the spacecraft into the gravitational field of another, using a technique known as 'gravity assist' or, less reverently, 'inter-planetary snooker'. This means that the space probes didn't need as much fuel, and could therefore take more scientific instruments instead. They were also able to reach their destinations much quicker. Energy has to be conserved, in line with the First Law of Thermodynamics. There is only so much energy distributed around everything; if something gains energy, somewhere in the universe something must have lost energy.

Voyager 2 gained energy from its encounter with Jupiter's gravitational field, which allowed it to travel further and faster. This energy came from the orbital motion of Jupiter around the Sun. Before the encounter, Jupiter was travelling a little bit faster, and Voyager was travelling a little slower. Because, according to the first law of thermodynamics, energy in the solar system must be conserved, and because after the encounter Voyager was travelling faster than before, Jupiter was slowed in its solar orbit, by around 30cm per trillion (10¹²) years.

Voyager 1 reached Jupiter on 5 March, 1979, followed by Voyager 2 on 9 July. Voyager 1 also got to Saturn first on 12 November, 1980, and Voyager 2 visited Saturn on 25 August, 1981. Voyager 2 was then able to pass Uranus on 24 January, 1986, and Neptune on 25 August, 1989, exactly eight years after it flew by Saturn.

Close Encounters

The Voyagers sent back a vast amount of data on the solar system. Within their planned lifetimes they discovered rings of Jupiter and new satellites. They were able to send back data on weather phenomena on Saturn and Jupiter, and to study their satellites in detail. NASA considered information on Saturn's satellite Titan important, with its nitrogen-rich atmosphere, so it was incredibly lucky that Voyager 1 was able to gather so much information on it. If Voyager 1 had failed, Voyager 2 would have been diverted to study Titan, and would not have been able to continue to the other two gas giants, Uranus and Neptune.

The Voyagers weigh 825kg each, and were built to the same design as the Mariner spacecraft, which were used on some earlier missions to the planets. They each carry 11 scientific instruments, and are powered by Radioisotope Thermoelectric Generators (RTGs). They operate so far from the Sun that there is nowhere near enough energy for them to be solar powered². The Voyagers are 'three-axis stabilised'. This means that they keep a fixed orientation in space, unless they are in a manoeuvre. They remain fixed by looking at the Sun and another bright star - if they move more than a fixed amount then jets can be fired to restore attitude.

The Voyagers' narrow angle television camera is sharp enough to read a newspaper headline at 1km away.

What Went Wrong

On 5 April, 1978, Voyager 2's computer system automatically switched to the back-up receiver for data from Earth. The problem was that the back-up receiver had a faulty capacitor, so it couldn't lock to the frequency of commands from Earth. When the prime receiver was switched on it failed, so the entire mission had to be flown with a faulty receiver.

This put a huge strain on engineers back on Earth - if they broadcasted signals more than 96Hz out from the frequency Voyager 2 was listening at, they would be completely ignored. The other factor was that the frequency that Voyager 2 monitored also varied with the craft's temperature, so they had to predict what temperature the receiver would be at, at any given time. There was also a time delay for signal travel time over that distance.

On 26 August, 1981, just after the closest approach to Saturn, Voyager 2's scan platform jammed. Priceless data on Saturn and its satellites was lost, but the main worry was that an icy particle from one of the rings had hit the platform. Happily, the platform was able to slew (turn on its axis) again after a couple of days, and tests were run to find out what went wrong and what could be done about it. Engineers back on Earth had four years before the Uranus encounter to test it, and discovered that the slew rate was too high, and that caused the platform to jam. No chances were taken and the slew rate was kept 0.33° a second during the Neptune flyby - the maximum safe rate was 0.8° degrees a second.

What Went Right

Voyager 2 had to be reprogrammed to be able to function at Uranus and Neptune. Neptune receives one thousandth of the sunlight intensity of Earth, so picture exposures needed to be quite long. This also meant that pictures would be smeared unless the spacecraft maintained its positioning precisely - even turning the tape recorder on and off could spoil a picture.

By the time Voyager 2 reached Neptune, the intensity of the signals sent back to Earth was 36 times weaker than those sent from Jupiter. This meant that during the Neptune encounter, the three radio telescopes from NASA's Deep Space Network telescopes in Madrid, Spain, and telescopes in Australia were all used, as well as other NASA telescopes in New Mexico and Goldstone. Another factor is that the mission control at the Jet Propulsion Laboratory is in an earthquake zone - precautions were taken to make sure that in the event of a tremor no valuable data was lost.

Amazingly, it all went well. Voyager 2 opened up Uranus and Neptune's systems to us, and sent back a wealth of data, including information on both of their satellites, ring systems and magnetic fields.

Continuing Mission

After passing by Saturn, Voyager 1 left the plane of the ecliptic (the plane that all the planets are on, except Pluto) and started to journey toward interstellar space. Both of the Voyager craft carry a message from Earth, including recordings of music and positional data about where Earth is.

All being well, both the Voyagers will continue to send data about the outer reaches of the Solar System until 2020, 43 years after their five-year missions began. They won't encounter any more planets, and their instruments are being turned off one by one to conserve power. Assuming nothing untoward happens, both spacecraft will travel through the Oort Cloud, and then on through the galaxy. Here is a table³ of stars which Voyager 2 will pass in the next 300,000 years, and Voyager's distance to each star at closest approach.