The Voyager mission, with its Voyager 1 and Voyager 2 spacecraft, began in 1977 and is still going, making it one of the longest and most successful space missions ever.
They have provided us with an incredible insight into the secrets of our solar system and now promise information on the limits of the Sun’s influence as they journey to interstellar space.
Launched on board Titan-Centaur rockets, both spacecraft began their journey from Cape Canaveral in Florida in 1977.
Voyager 2 launched first, on 20 August 1977. Sixteen days later, on 5 September 1977, Voyager 1 launched.
As originally designed, the Voyagers were to conduct close-up studies of Jupiter and Saturn, Saturn's rings, and the larger moons of the two planets.
To achieve this objective the spacecraft were built to last 5 years.
Having achieved its initial objective though the mission was extended to include the additional fly-bys of Uranus and Neptune by Voyager 2.
The Voyager mission is run by NASA’s Jet Propulsion Laboratory in Pasadena, California.
The planetary mission, having taken in the four outer giants and forty-eight of their moons, completed in 1989.
The spacecraft had lasted 12 years, much longer than originally planned.
In 1989 it was decided that the Voyagers should continue to explore beyond the outer planets to the outer limits of the Sun's sphere of influence and possibly beyond into interstellar space.
In recognition of changing the field of study from the planets to outer space, the mission name was changed to the Voyager Interstellar Mission.
Before heading to interstellar space it was suggested by Carl Sagan, a member of the Voyager imaging team, that Voyager's unique perspective of the solar system should be captured on film. And so it happened. An image of the solar system from its outer reaches. At a distance of 6.4 billion kilometres (4 billion miles) Voyager 1 turned its cameras back on the planets and on 14 February 1990 took the first ever 'family portrait' of the solar system which showed Venus, Earth, Jupiter, Saturn, Uranus and Neptune. Missing from the picture were Mercury, which was too close to the Sun to be visible; Mars, which showed only a thin crescent of sunlight, and Pluto, which was too dim. These images, later arranged in a large-scale mosaic, make up the only family portrait of our planets arrayed about the sun. Carl Sagan described Earth as “a pale blue dot” and wrote:
“That's here. That's home. That's us. On it everyone you love, everyone you know, everyone you ever heard of, every human being who ever was, lived out their lives.…There is perhaps no better demonstration of the folly of human conceits than this distant image of our tiny world."
The spacecraft are identical in design and each weighs 773 kilograms.
They were stabilised around three axes by gyroscopes which enabled precise altitude control to keep the 3.6 metre antenna pointing toward Earth.
They were laden with scientific instruments from radio astronomy antennae to magnetometers and from charged particle detectors to spectrometers.
There is also a pretty decent imaging platform for capturing high quality planetary images including two TV style cameras with a number of available filters, one camera has a 200mm wide angle lens at f/3 and the other, a higher resolution 1500mm lens working at f/8.5.
Like many spacecraft, they were powered by radioisotope generators, three in the case of the Voyagers which run on Plutonium-238. Essentially the Plutonium would slowly decay and provide just over 450 watts of electricity at 30 volts. Due to degradation of the systems and the slow decay of the Plutonium, the generators are now producing around 55% of the power they produced at launch.
The launch was scheduled to coincide with a favourable planetary alignment which meant the outer planets could be visited with minimal extra effort and crucially with a minimum amount of fuel.
On its launch, Voyager 1 took in Jupiter and Saturn before heading out of the solar system while Voyager 2 went on after its rendezvous with Saturn to visit Uranus and Neptune.
Not only was this planned to give some unprecedented opportunity to study the outer planets but it also enabled the spacecraft to be accelerated through gravitational slingshot to speeds high enough to escape the Solar System.
Earth is the pale blue dot in the middle top row of this 'family' portrait. Credit: NASA/JPL.
Following two very successful fly-bys of the outer planets the Voyager craft both started their long and lonely journey out of the solar system.
Most of the non-essential instruments have now been powered down leaving just a handful of navigation, communication and scientific instruments running.
The interstellar mission
Their mission now has become one of mapping the environment in the depths of the solar system.
At present we don't know where our solar system ends, indeed its boundary is difficult to define but consider the boundary to be the point where the energy from the Sun becomes indistinguishable from the energy from interstellar space.
This region is known as the heliopause but its limits are unknown. Voyager 1 and Voyager 2 are travelling at over 3 AU (Astronomical Units) per year.
1 AU is 150 million kilometres, the distance between the Earth and the Sun, and given their great speed it is hoped that the heliopause will be identified before the Voyagers' generators expire around 2025.
What of the future of perhaps our most famous interplanetary turned interstellar probes? Well we certainly won't see them here on Earth again as they are heading out of the solar system, never to return. Voyager 1 is on a course which, in about 40,000 years, will take it within 2 light years of a star called AC+79 3888 which lies in the constellation of Camelopardalis.
Voyager 2 will continue on its path and pass Sirius in about 296,000 years, so don't hold your breath.
Whilst an alien encounter seems unlikely, some consideration was put into the possibility of a foreign civilisation intercepting the Voyager crafts and so, like an interstellar message in a bottle, they each had a golden record (more accurately, a 12-inch gold-plated copper audio visual disc) attached (see image, right).
If intercepted then it was hoped that the sounds and images found would give an indication of the diversity of life and cultures on Earth.
The contents of the record, which include 115 images, musical selections and greetings in fifty-five languages, were selected for NASA by a committee chaired by Carl Sagan.
Each record comes with a stylus to enable the aliens to play it. Instructions are also included which, given knowledge of some fundamental physical properties of the Universe, any advanced civilisation should be able to play it.
Its incredible that just 20 years after the first man made satellite (Sputnik 1) was launched (1957), two spacecraft embarked on what is perhaps mankind's greatest voyage to date, the journey into interstellar space.
I very much doubt any civilisation out there is likely to intercept them but I think its an incredible testimony to human ingenuity and hope that somewhere in the depths of space they will find a new home.
This artist's concept shows NASA's two Voyager spacecraft exploring a turbulent region of space known as the heliosheath, the outer shell of the bubble of charged particles around our sun. The two Voyager spacecraft will soon reach nterstellar space, which is the space between stars. Image credit: NASA/JPL-Caltech