In 2013 Voyager 1 slipped into interstellar space, the space between stars. But what was a space probe doing twelve billion miles from home? The answer goes back to the early 1960s when competition in space was one front of the Cold War between the USA and the Soviet Union.

Interplanetary missions – rocket science?
Round 1 went to the Soviet Union in 1957 when it launched the first artificial satellite, as well as Round 2 when Yuri Gagarin was the first man in orbit.

Getting an interplanetary probe to its target (including the Moon) was proving difficult for both superpowers. Even by the end of 1964, the two countries had between them 21 complete failures out of 28 Moon attempts, no successful Mars mission, and of twelve Venus missions, only NASA's Mariner 2 was successful. The USA was slightly ahead here.

While some people were hard at work trying to figure out how to get around the inner Solar System, others were already looking towards the outer planets. Distance was a major problem. Getting a probe to Neptune would take thirty years, even longer to Pluto (still counted as a planet then). The amount of fuel needed – which would also increase the weight of the spacecraft – was immense. A favored option was nuclear power, and in the USA the Jet Propulsion Lab (JPL)was working on this.

Interplanetary missions – maybe it's math?
In 1961 a mathematician named Michael Minovitch went to work at JPL to compute orbits. He got interested in solving the classical mathematical three-body problem. The three bodies of interest to him were two planets and a spacecraft. Minovitch formulated equations and tested them using computers.

His solutions involved using what is now called gravity assist or a gravity slingshot. If a spacecraft approached a planet at the correct angle, the planet's gravity field could accelerate it in the desired direction.

As Minovitch studied various orbits and their effects, he found that in the late 1970s there would be a line-up of planets that occurs only every 175 years. The alignment would let spacecraft visit all the outer planets, using the gravitational energy of one to get to the next one. A 40-year mission could be reduced to ten years, and use very little energy beyond that of the initial launch. He presented his work to JPL before he left, but there didn't seem to be any follow-up.

In 1964 when Gary Flandro came to JPL, he was assigned to investigating possibilities for outer planet missions. When he began to consider gravity slingshots, he rediscovered Minovitch's alignment. Flandro calculated a number of routes through the outer planets that could use the alignment. His paper, citing Minovitch's work, was published in 1966. Flandro is generally given the credit for the “grand tour” of the outer planets.

From theory to mission
The gravity assist was a good idea, but could they make it work? A spacecraft had to be in the right place at the right time and pass a planet at the correct angle, or it would go shooting off on the wrong trajectory and miss its next destination.

Francis M. Sturms Jr and Elliott Cutting at JPL compared the navigational accuracy needed for a gravity slingshot with what could actually be achieved. They found a good match, and applied the method to the Mercury mission Mariner 10. It was successful. The probe picked up gravitational energy from Venus, which reduced the launch energy. As a bonus, Italian scientist Giuseppe Colombo calculated an orbit for the probe which let it make three flybys of Mercury during its mission.

Pioneers
Before there was a Grand Tour, NASA sent Pioneers 10 and 11 to Jupiter and Saturn. Pioneer 10 was launched in March 1972 and its twin just over a year later. Pioneer 10 was the first spacecraft to go through the asteroid belt, and the pair of them provided useful data to refine the Voyager planning, including learning that Jupiter's radiation field was stronger than had been previously believed.

After the primary mission, the Pioneers continued to collect and return data about the outer Solar System for many years. They are silent now and on the way to the boundaries of the Solar System.

Grand Tour
A grand tour of the outer planets was in the planning. It wasn't as grand as originally envisaged, but they would launch two probes in 1977. Voyager 1 would visit Jupiter and Saturn, then Saturn's moon Titan. (This did mean that it couldn't also go to Pluto.) Its twin Voyager 2 would go to Jupiter and Saturn, and all being well, could be sent on to Uranus and Neptune.

When the probes completed their primary missions, each would continue to collect data about the outer Solar System and cross into interstellar space. This meant that they had to be well-equipped and very durable. Their instruments are powered by radioisotope thermoelectric generators which use the heat from radioactive decay to generate electricity. Their output diminishes with time, and the missions end when there isn't enough energy to send data back.

Greetings
The Voyagers are also carrying an unusual bit of equipment. At the behest of Carl Sagan, each has a Golden Record with sounds of Earth on it. Previously, Sagan had come up with the idea of the Pioneer plaques, which proclaim who we are. Even he realized that the chances of their ever being found were practically nonexistent.

References:
(1) K Cooper, “Space Travel's Unsung Hero” Astronomy Now May 2013, pp.22-25
(2) AJ Butrica, “Voyager: The Grand Tour of Big Science” https://history.nasa.gov/SP-4219/Chapter11.html