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.
The Voyager mission commenced on August 20, 1977 with the launch of Voyager 2. with Voyager 1 following on September 5, 1977. Since then, their mission has become legendary, returning some of the most amazing images of the giant planets of the solar system and their myriad moons, together with a considerable amount of data about each of them.
In the 1990s, the faster-moving Voyager 1 “overtook” the older Pioneer deep-space probes in terms of its distance from the Earth. Pioneer had been a programme launched in the early 1970s to explore the asteroid belt, Jupiter, Saturn and (it had been hoped) probe the outer reaches of the heliosphere. Sadly, both Pioneer 10 and Pioneer 11 are no longer active, the former having lost power to its communications array on January 23rd, 2003 when some 12 billion kilometres from Earth, and the latter having ceased communication with Earth at the start of December 1995.
It’s incredible to think that the Voyager mission is 36 years old and is still going – and that the primary phase of the mission lasted just 12 years, ending in 1989 when Voyager 2 passed beyond Neptune, heading out along the plane of the ecliptic towards its eventual encounter with the heliopause (which is thought to be not too far away). Voyager 1’s interplanetary mission phase was even shorter – just 3 years 3 months and 11 days. The rest of its time has been spent on the “interstellar” phase of the mission – 32 years and eight months.
One of the reasons it has taken a while to confirm that Voyager 1 has officially passed into interstellar space and beyond any influence of the Sun is that while the heliopause has long be theorised, no-one has actually been quite sure of where it actually is.
This is perhaps demonstrated in the fact that it was originally thought that Voyager 1 had crossed the termination shock – the point at which the solar wind slows to subsonic speed relative to the Sun – in late 2003, but this was later revised by just over a year, which passage through the termination shock seen as being in December 2004. Even then, a consensus opinion that the vehicle was indeed in the so-called heliosheath – a region of “turbulence” between the termination shock and the heliopause where solar wind and interstellar space are interacting – was not reached until 2005 (as a side note to this, Voyager 2 passed the termination shock in 2007).
Another reason for caution has been due to the more recent Interstellar Boundary Explorer (IBEX) mission, launched in 2008 with the express purpose of studying the boundary between interstellar and solar space. Operating in a high orbit averaging some 191,345 kilometres (118,896 miles) above the Earth, IBEX has produced data which has done much to revise thinking on the nature of the heliopause region.
Now Voyager 1 is entering into the true interstellar medium, and if 32 years sounds a long time since its last planetary encounter, it will be another 40,000 before the next. Sadly, by that time, the vehicle’s power systems will have long-since died – it is thought that at the current rate of depletion, the onboard RTG powering Voyager 1 will be unable to supply electrical energy to any of vehicle’s instruments by around 2025. Even so, it is the fastest-moving man-made object currently in existence, moving at some 17 kilometres (11 miles) per second – and will continue to do so long after its power system has expired.
Should either Voyager spacecraft encounter intelligence out among the stars, each carries a golden record on board which contains pictures and sounds of Earth, along with symbolic directions for playing the record and data detailing the location of Earth.
And as they pass out into the deep void beyond the influence ouf our Sun, they are becoming our first physical interstellar ambassadors.
All images courtesy of NASA, unless otherwise indicated.