A rare planetary alignment in the 1970s opened up the outer Solar System to exploration. It would allow all the outer planets to be visited, using the gravitational field of one planet to boost a spacecraft to the next one. NASA launched the two probes of the Voyager Mission in 1977 in order to make this Grand Tour.
Voyager 1 left Earth early in September, a few weeks after its twin Voyager 2. It could have been sent to Pluto – then still a planet – but instead it was going to observe Saturn's interesting moon Titan after visiting the two gas giants.
Earth and Moon
About two weeks into the journey, Voyager 1 was turned around to image her home and its natural satellite. This was the first photograph ever to capture the Earth and the Moon in the same frame.
The Jovian System
At the beginning of 1979 Voyager began photographing Jupiter. However the most intense period of observation was during the 48 hours around the craft's closest approach in early March. Although Pioneer 10 and Pioneer 11 had both observed the Jovian system, there were plenty of discoveries to make.
Jupiter's atmosphere was in a more active state than when either of the Pioneer craft had visited, and there were features which hadn't been seen before. For example, Voyager saw lightning bolts in the cloud tops, and polar aurorae in both visible and ultraviolet light.
Voyager also found the first evidence for Jupiter's magnetic field and discovered a Jovian ring. Unlike the elaborate bright ring system of Saturn, Jupiter's rings are faint and dusty.
Galileo discovered the four largest Jovian moons in the early seventeenth century, but they were still too small for ground-based telescopes to see surface details. Voyager photographed Io, Europa, Ganymede and Callisto, as well as Amalthea. It also discovered two new moons.
All of these discoveries fascinated the scientists and certainly the planetary scientists had to get busy with some new models of Jupiter's atmosphere.
However they weren't astonishing discoveries. The probe imaged Jupiter from an angle chosen to detect rings, if there were any. So the ring was a discovery, but not a big surprise. Everyone must have assumed that Jupiter had many small moons as yet unseen, so finding two of them was also not surprising. In fact, moons were still being discovered decades later.
But in the probe's images there was a real bombshell, something completely unexpected: active volcanoes on one of Jupiter's moons, Io. Voyager recorded nine of them, the first extraterrestrial ones.
The Saturnian System
Pioneer 11 had already wowed the astronomers with its images of Saturn's rings in what was amazing detail at the time. It also found two new moons and evidence for the planet's magnetic field.
But Voyager's views of the rings were still a revelation. Not only was a new ring discovered, but also the complex structure of the ring system was revealed. The probe found three new moons, all of them shepherd moons, moons whose gravitational fields keep particular rings in order.
On Saturn itself the probe found lightning in the atmosphere. It was also clear that Saturn was a windy planet, with winds at the equator of around 1100 mph (500 meters per second). There is a gradual decrease in wind speed as you move away from the equator.
Observation of Titan was somewhat disappointing, as it was impossible to see the surface through the smoggy atmosphere. Titan is the only Solar System moon with a thick atmosphere. The first evidence of an atmosphere came from the 1940s with Gerard Kuiper's discovery of methane. Voyager data showed the moon had a thick atmosphere which was about ninety percent nitrogen, and several kinds of hydrocarbon molecules could be identified.
The other main moons were photographed and appeared to be made mostly of water ice. This was another notable surprise, because in those days astronomers thought that water was scarce in the Solar System. It's only in the 21st century that water was also discovered on Mars, the Moon and Mercury.
The extended mission
Voyager 1's primary mission ended after it left Saturn in November 1980. The extended mission became the Voyager Interstellar Mission in 1989. Both spacecraft collected data on the solar wind, the Sun's magnetic field, and other information that would help to determine where the boundary of the heliopause was and characteristics of interstellar space.
The Pale Blue Dot
On Valentine's Day 1990 Voyager 1 was about the same distance from the Sun as Pluto, though in a different direction. It turned back towards the planets and took the pictures that would make the Solar System “Family Portrait”. Among the images you could just make out Voyager's last view of home, the pixel that contained Earth. Carl Sagan famously christened it the “pale blue dot”.
Voyager 1 entered the space between the stars in 2012, but that's another story.