When we think of the pinnacle of human achievement, we inevitably think of great, grand, or even spectacular works of art. We think of the Pyramids of Giza, the Seven Wonders of the Ancient World, the Great Wall of China, or perhaps a Bach cantata or Michelangelo's ceiling fresco in the Sistine Chapel. Some people turn to the history of science and choose the ingenious equations of Kepler, Newton or Einstein that describe the world.
Yet it is impossible to leave out the fragile-looking Voyager-1 space probe, which weighs less than a tonne. It's not a spaceship, yet it is the furthest reaching man-made device. The fact that it still works today is truly an achievement.
The Grand Tour is underway
Even in the 1960s, before the moon landings, NASA staff were dreaming of exploring the solar system. They wondered when it would make sense to send probes to each planet. In 1964, an American aeronautical engineer, Gary Flandro, calculated that a special and very rare lucky conjunction was on the cards: four gas giants, Jupiter, Saturn, Uranus, and Neptune, would align as they moved outwards from Earth by the late 1970s. This planetary alignment only happens every hundred years or so, making it the perfect time to launch spacecraft. Why were they so happy about this interesting, but at first sight not very relevant, data? Well, because it would have solved the problem of distance in one fell swoop.
The fact is that these planets - especially Uranus and Neptune - are quite a long way from Earth. Normal propulsion would not have been sufficient to reach them. But now, thanks to the alignment, the gravitational force of each planet can be harnessed to bring the spacecraft to each of them in a so-called rocking manoeuvre. Thus, it is enough to go as far as Jupiter in a conventional way, since its gravitational force accelerates the space probe, and the probe, as it moves on, will certainly reach the next planet, where it will again 'play' this slingshot-like operation. The name of the solar system rocking horse project is the Grand Tour.
Today, when we measure the size of microchips in nanometres, it's hard to imagine how much harder work NASA engineers must have had to do. Approaching the Moon is mathematically much easier than calculating the swinging perimeter, so much more precise numbers were needed. To do this, they had to turn to the giants of the then-nascent computing revolution, and with the help of IBM, they got good news: it was possible to set the space probe on an optimal trajectory. The Grand Tour was within reach.
Gary Flandro was then working at the Jet Propulsion Laboratory (JPL), an independent group founded by Tódor Kármán (Theodore von Kármán) at the California Polytechnic Institute (CalTech), which supported NASA through missions. (The Jet Propulsion Laboratory and CalTech were the original names of the group.) The calculation of the coincidence allowed the mission to explore the solar system to be completed in less time and on a reduced budget.
The first steps
Initially, JPL envisaged launching four spacecraft: two would fly to Jupiter, Saturn, and Pluto, and two would fly the Jupiter-Uranus-Neptune path. The cost was estimated at a billion dollars at the time, and the Outer Planets Working Group set up for the purpose put the mission's duration at 13 years. As a result, the original plan to complete the interplanetary exploration in 7.5 years was halved, leaving only one pair, or two separate probes, on the drawing board. However, they added something else: the idea that the two probes, once they had examined the planets, could, because of the speed they had acquired, leave the Solar System and travel to other worlds, stars, and planets, as messengers to our intelligent species.
What sounded good in theory has in practice been a mixture of painful compromises and brave solutions. From the constant battle with the US Congress (i.e. a lower budget than requested) to the incorporation of the best tools of the age and deep space security, and even the crucial question of energy sources. It's fine to have batteries in the structure, and solar panels in near-Earth orbits are sufficient to power it, but as we move further away from Earth, solar power is diminishing. That's how they got to nuclear power, which could be used to recharge the batteries of the probes even in the more solar-poor regions of the outer gas giants. Voyager probes were also equipped with huge satellite dishes for communication - as we will see later, this became very important.
Message to extraterrestrials
The Voyagers have got another fantastic addition. A plaque has been added to the outer surface of the former Pioneer probes sent to Jupiter. The idea came from the late astronomer-scientist-scientist and writer Carl Sagan to send a message about our species, the home of the device, planet Earth and its inhabitants.
Nine months before the launch of the Voyagers, NASA approached Sagan for the same reason. They reasoned that if these twin probes were to reach unimaginable distances, they should carry news of us, of our species, but more than the plaques on the Pioneers. The message of humanity thus became a gold-plated copper plate, less exposed to the radiation of space than magnetic tape. It was still only the second half of the 1970s, so there was only room for an hour and a half of sound (natural sounds, hits of the time, classical music) and nearly 100 images and moving pictures. It also gave strangers a chance to learn about the basic mathematical theorems that underpin our science, the everyday life of human beings, and a welcome speech in 55 languages (including Hungarian). A user's manual was also scratched into the gold disc so that those who found it could easily play it.
The Voyagers' launch
1977 proved to be one of the friendliest hurricane seasons since 1965. So, nothing could stand in the way of the Voyagers' launch. Moreover, the probes designed for the Grand Tour had already been made much more robust than their predecessors, based on the deep space experience of the Pioneer-10, -11 probes. Funnily enough, in 1977, Voyager-2 was launched from Cape Canaveral, followed two weeks later by Voyager-1.
Over the next 22 months, Voyager-2 slowed from a speed of 36 kilometres per second to 10 km/s as it reached Jupiter. The computers and the programmers, physicists and engineers who worked with them made no mistake: the gas giant's massive gravity well accelerated the space structure to 28 km/second. Two years later, a similar scene took place on Saturn, but this time the final speed was 36 km/s. Voyager-2 experienced its last rocking manoeuvre 3 years and 7 months after launch, and its distance from the Sun was increasing - it was ready to leave the Solar System.
NASA was a little worried about collisions in the asteroid belt, but fortunately, they were spared. In the meantime, periodic tests ensured that the systems were working properly.
Voyager-1 discovered active volcanoes on Jupiter's moon Io and found traces of water on the moons Europa and Ganymede (setting the targets for future missions). He also found craters on Calisto, but the most spectacular Jovian phenomenon, the great red spot, was a counter-clockwise rotating gigantic storm, larger than the Earth. Thanks to Voyager, two more tiny little moons have been added to Jupiter's catalogue (Metis, Adrastea). It was the first time that a spacecraft, not a human, had discovered new celestial bodies. Voyager-1 passed near Saturn on 12 November 1980. Contrary to initial plans, Pluto was not visited.
The two probes were planned on different routes from the start. Voyager-2's mission was to study Saturn, with its special rings, and its moon Titan, which was of great interest. The engineers found a gap between the rings and guided the probe there to avoid the debris that makes up the rings, minimising the chance of a collision. This was successful, and in return, the structure sent back to Houston photographs of unprecedented quality. It has also discovered several new Saturn moons and shared puzzling images of the moon Enceladus with Earth's control centre.
Voyager-1, meanwhile, was heading towards Titan, which has a thick atmosphere. Unfortunately, X-ray imaging beyond visible light failed precisely because of the thick atmosphere, and Voyager-2's plans to reach Uranus and Neptune were cancelled because of its orbital modification. However, the probe began to move away from the Solar System as it emerged from the planes of the planets.
At the wind and beyond
It was also Carl Sagan who suggested that Voyager-1 take a final photo of the planets at the edge of the Solar System. By this time, its engine power had been so reduced that it was necessary to shut down all the instruments in sequence. The images were taken, including the pin-sized Earth, which put our importance into perspective. On 14 February 1990, a series of images called "Family Portrait" was taken. Eight years later, on 17 February 1998, Voyager-1 sprinted past the previous long-distance record holder, Pioneer 10, and officially became the furthest human object ever to travel. Unfortunately, since 2007, several of its instruments have had to be switched off.
Where is Voyager 1 now?
On 5 December 2011, NASA announced that Voyager 1 had entered a new region called "cosmic purgatory". Within this region, charged particles streaming from the Sun are slowly turning inwards, and the magnetic field of the solar system is doubling due to the pressure of interstellar space.
Voyager-1 said goodbye to the heliosphere in 2012, officially leaving the Solar System and entering interstellar space. But its system is still sending out weak signals and data, which anyone can see on NASA's website. It is expected to go silent in 2025 as it continues its journey towards Alpha Centauri, the closest star to Earth.
Voyager-2, also reaching record speeds, is not to be feared either: a 2018 signal showed that it too has left our solar system. Maybe another spacecraft will find it one day, and a great bond between two spacefaring species will be born.