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You may remember that back in the 1970's there was a so-called "planetary alignment", when the four gas giant outer planets of the Solar System were more or less lined up. This was a very rare occurrence indeed - the distances between the planets are so enormous and their orbits so different that it takes centuries for all four to appear in the same region of sky when viewed from Earth. If you don't quite follow what I'm saying, perhaps the table on the right will help.

NASA made use of this rare opportunity to launch two spacecraft to the outer Solar System in 1977 - Voyager 1 and Voyager 2, which were identical in construction and design but were sent on slightly different trajectories. Voyager 2 visited all four outer planets whereas Voyager 1 only went past Jupiter and Saturn.

Both spacecraft used the "slingshot" method to gain speed ; this entails approaching a planet close enough to "steal" some momentum from it but not so close that the planet's gravity captures the spacecraft. Because of the universal law that momentum must always be conserved (momentum being the product of Mass and Velocity), even the tiniest bit of momentum from a massive body like Jupiter translates into a huge increase in speed for something as light as a spacecraft. The mass of both the planet and the spacecraft remain constant, so any momentum change results in the planet slowing down by an infinitesimally small amount whereas the spacecraft speeds up significantly. This has become an established technique for accelerating spacecraft so that they can cover the enormous distances in the outer Solar System in years rather than decades. Apart from getting a free velocity boost, the other major advantage of this manoevre is that no fuel is needed - fuel is heavy and must be carried into space from Earth, so any technique that can be used to lighten a spacecraft's fuel load is crucial.

The Voyager spacecraft

Voyager 2 sped past Neptune, the outermost planet, in August 1989. Voyager 1 did not go anywhere near Neptune but crossed the planet's orbit early that same year. From that moment on, both spacecraft left the familiar "planetary" part of the Solar System behind and began heading out into space. Because Voyager 2 had been diverted slightly to fly past Uranus and Neptune, its overall velocity relative to the Sun was less than that of Voyager 1 ; Voyager 1, travelling faster, thus pulled steadily away from its sister ship.

Of course, the Solar System goes a long, LONG way beyond the orbit of Neptune. The Kuiper Belt (the source of most short-period comets) consists of millions of small, icy bodies and extends from the orbit of Neptune to about 50 Astronomical Units from the Sun, while the much larger Oort Cloud (where long-period comets come from) is reckoned to be as much as a light year away. However, scientists have long held the view that the beginning of interstellar space is not the limit of the Sun's gravitational influence (which may be as much as two light years out), but rather where the Sun's magnetic field ends. The Sun is surrounded by an immensely powerful magnetic field called the Heliosphere (similar to that of the Earth but far larger), the extent of which has always been a matter of speculation.

Four spacecraft have travelled beyond the Outer Planets

The Sun, Solar System and Heliosphere

About a year ago, signals from Voyager 1 started to indicate a change in the pattern of particles its instruments were recording. Particles that come from the Sun comprise what is called the Solar Wind, and those that arrive from space (ie. not the Sun) are called Cosmic Rays. Prior to August 2012 almost all of the particles Voyager detected were from the Solar Wind, but during August and September 2012 the spacecraft started detecting increasing numbers of Cosmic Rays, indicating that it had entered the transition zone near the edge of the heliosphere. But at that time no change was detected in the direction of the magnetic field lines, meaning that Voyager 1 was still inside the heliosphere.

On 12 September 2013 NASA announced that detailed analysis of Voyager's data revealed that the spacecraft had exited the heliosphere earlier in the year, thus officially becoming the first man-made object to enter interstellar space. The interstellar plasma detected by Voyager's instruments from 9 April to 22 May 2013 showed a sudden sharp increase in density, more than 40 times that observed in the heliosheath (the outer layer of the heliosphere) ; at the same time the levels of Solar Wind particles dropped dramatically. Many heated scientific debates took place but it was finally agreed that the data was conclusive enough to announce that, 35 years and 7 months after being launched, Voyager 1 had at last reached interstellar space.

There are several reasons why it took so long to arrive at this conclusion:

A distance chart showing just how far (or not) Voyager 1 has travelled.
Note the exponential scale below the line very carefully

This is indeed an historic milestone for mankind, but allow me to introduce a note of caution here - Voyager 1 has NOT left the Solar System as some news reports have suggested. It has merely exited the magnetic field around the Sun, which in cosmic terms is tiny. At the speed Voyager 1 is travelling (17 kilometres per second) it will take another 300 years to reach the inner edge of the Oort Cloud and a further 30 000 years to exit the Solar System altogether!

If you're not sure about that, look at the picture on the right but note that the distances shown are not to scale - each number represents an Astronomical Unit (about 150 million kilometres), so Saturn is ten times further from the Sun than the Earth and the inner edge of the Oort Cloud is a thousand times further away! Voyager 1 has travelled only about one tenth of the distance to the Oort Cloud. The nearest star is nearly a million times further away! A truly sobering thought and an apt demonstration of the sheer vastness of space ...