On August 25th 2012, while the eyes of the global space community were focused almost entirely on the happenings in a crater on Mars, a significant event took place approximately 18 billion kilometres (11 billion miles) from Earth. Voyager 1 passed through the heliopause, the boundary between what is regarded as the “bubble” of space surrounding the solar system (heliosphere) which is directly influenced by the Sun, and “true” interstellar space.
That the spacecraft might be nearing the so-called “bow shock” area where the solar wind meets interstellar space was indicated by engineers and scientists working on the Voyager project in June 2012; however, it was not until September 2013 that NASA JPL felt confident enough in the data they’d received to confirm that Voyager 1 had in fact passed into interstellar space in August 2012, the first man-made object to have done so, some 35 years after having been launched from Earth in what was a highly ambitious programme of deep-space exploration.
The Voyager programme actually had its roots in a much more ambitious programme, the so-called Grand Tour. First put forward by NASA engineer Gary Flandro, The Grand Tour proposed the use of a planetary alignment which occurs once every 175 years, together with the potential to use the gravities of the planets as a means by which space probes could explore the outer planets of the solar system.
The idea of using gravity of the planets to help propel a space craft had first been realised by a young mathematician, Michael Minovitch, in 1961. With the aid of the (then) fastest computer in the world, the IBM 7090, Minovitch had been trying to model solutions to the “three body problem” – how the gravities of two bodies (generally the Earth and the Sun) influence the trajectory and velocity of a third (generally a comet or asteroid) moving through space; something astronomers and mathematicians had long wrestled with.
Through his work, Minovitch showed how an object (or space vehicle) passing along a defined trajectory close to a planetary body could, with the assistance of the planet’s gravity, effectively “steal” some of the planetary body’s velocity as it orbited the Sun, and add it to its own.
At the time, his findings were received with scepticism by his peers, and Minovitch spent considerable time and effort drawing-up hundreds of mission trajectories demonstrating the capability in order to try to get people to accept his findings. But it was not until 1965, when Flandro started looking into the upcoming “alignment” of the outer planets (actually a case of the outer planets all being on the side of the Sun, rather than being somehow neatly lined up in a row) due in the late 1970s, that Minovitch’s work gained recognition.
Recognising the opportunity presented by the alignment, Flandro started looking at how it might be used to undertake an exploratory mission. In doing so, he came across Minovitch’s work and realised it presented him with exactly the information needed to make his mission possible, and so the Grand Tour was born.
This mission would have originally seen two pairs of spacecraft launched from Earth. The first pair, departing in 1976/77 would form the MJS mission, for “Mariner (then the USA’s most capable deep-space vehicle)-Jupiter-Saturn”. These would fly by Jupiter and Saturn and then on to tiny Pluto; while a second pair of vehicles launched in 1979 which would fly by Jupiter, Uranus and Neptune.
Budget cuts at NASA following Apollo eventually saw the Grand Tour scaled-back to just two vehicles, Voyager 2 and Voyager 1, but the overall intent of the mission remained intact under the Voyager Programme banner, now led by Ed Stone. In the revised mission, both spacecraft would perform flybys of Jupiter and Saturn, with Voyager 2 using Saturn to boost / bend it on towards Uranus and from there on to Neptune, while Voyager 1 would approach Saturn on a trajectory which would allow it to make a flyby of Saturn’s huge Moon Titan, of significant interest to astronomers because of its thick atmosphere. This route would preclude Voyager 1 from reaching Pluto, as it would “tip” the vehicle “up” out of the plane of the ecliptic and beyond even Pluto’s exaggerated orbit around the Sun, and push it onto an intercept with the heliopause.