|Jupiter and its four Galilean moons - Io, Europa, Callisto and Ganymede - as seen by Voyager 1. NASA image.|
Voyager 1 flew by Jupiter on March 5, 1979, opening an extraordinary decade of exploration by this spacecraft and and its twin Voyager 2 after their launches toward the outer solar system in 1977. Both spacecraft flew by Jupiter and Saturn, and Voyager 2 went on to visit Uranus in 1986 and Neptune in 1989. Today both spacecraft are still sending back data after having effectively flown outside of the solar system.
When Voyager 1 flew by Jupiter, it sent back eye popping images of Jupiter and its four major moons, Io, Callisto, Ganymede and Europa. For the first time, Jupiter’s complicated atmosphere could be seen with full clarity. The Voyager images transformed the four Galilean moons in the eyes of Earthbound observers from anonymous spheres into worlds of their own: Io was a reddish pockmarked moon that at first glance resembled a pizza, Ganymede was marked with craters and grooves, Europa appeared as an icy but relatively smooth world, and Callisto also was icy but covered with craters. Voyager made many more discoveries about the Jovian system, including a ring around Jupiter and new smaller moons orbiting the biggest planet in our solar system.
The day after the spacecraft’s flyby, Voyager Project Scientist Ed Stone said: “I think we have had almost a decade’s worth of discovery in this two-week period."
Three days after its closest approach to Jupiter, the departing Voyager 1 looked back and shot an image of a crescent Io, mainly to verify the accuracy of Voyager’s navigation system. When a member of Voyager’s navigation team, Linda Morabito, examined the image, she found what looked like a cloud rising from the moon's surface. As the smallest of Jupiter’s four Galilean moons, Io doesn’t have an atmosphere, so Voyager scientists soon reached the then surprising conclusion that Io has active volcanos, the first body other than the Earth known to have them.
In retrospect, I look at Voyager 1’s flyby of Jupiter the first real “wow” event of humanity’s reach into space. The most exciting thing about the space probes that came before Voyager was that humans had actually succeeded in exploring worlds beyond the Earth.
Their discoveries didn't really impress the public. Humanity’s reach for the Moon culminated with landings in places that resembled slag heaps instead of the mountainous spires that many people had expected, our first robotic visits to Mars dashed our hopes of finding life, Venus was shrouded by clouds, and Mercury was a cratered world that resembled our own Moon. Two Pioneer spacecraft had flown by Jupiter in 1973 and 1974, and their flights proved that spacecraft could survive the journey through the high radiation environment surrounding Jupiter. But their rudimentary imaging systems gave us only the smallest hints of the beauty of Jupiter and its moons.
These “meh” moments were one reason space exploration seemed to fall out of public favour in the 1970s.
I’m not saying that our early explorations were not important or that their discoveries did not excite scientists, but to the general public, the headlines were that humans had walked on the moon and robots had given us close-up views of other worlds, not what these explorers had actually found.
Finally, in March 1979, Voyager 1 provided images and other data that showed that our solar system contained many surprises. These results drew attention from the public, and more media were on hand when Voyager 2 flew through the Jovian system later in the year. Even bigger crowds and more media coverage were in evidence when the Voyagers provided stunning close-up views of the Saturnian system in 1980 and 1981.
I joined the media hordes at the Jet Propulsion Laboratory for Voyager 1’s flight through the Saturnian system, and the only disappointment was the opaque cloud layer that shrouded the largest Saturnian moon, Titan. More attention was paid to the spectacular features Voyager found on other moons and the strange findings from Saturn’s rings, including shepherd moons.
Titan eventually gave up its secrets nearly a quarter century later when the Cassini spacecraft arrived for a long visit to Saturn with the Huygens lander that touched down on Titan's surface and found lakes of hydrocarbons and other astonishing features. Jupiter and its moons have also been explored by the Galileo spacecraft and is now being investigated by the Juno spacecraft, which is sending back amazing images of its own.
Not long after the Voyager planetary flybys came to an end, NASA launched the Hubble Space Telescope in 1990. Although Hubble couldn't operate fully until 1994 due to the problems with its main mirror, it began to send back an amazing stream of images and other data that have helped upend our view of the universe. Hubble images, including those of the marks left on Jupiter by Comet Shoemaker-Levy 9, the ‘Pillars of Creation’ in the Eagle Nebula, the Hubble Deep Fields and many others, have captured the imaginations of many people.
Hubble has found that the universe is a larger, more colourful and more amazing place than what we had previously thought. Other space telescopes such as Chandra and Spitzer have made discoveries of their own, and the Kepler and TESS space telescopes are finding a variety of unexpected planets orbiting distant stars. In recent years a string of rovers have revealed striking geography on Mars and found tantalizing hints of life on the Red Planet. The New Horizons spacecraft amazed many people on Earth with its surprising views of Pluto in 2014, and it started off 2019 with the first view of a Kuiper Belt object.
Today we see the universe as a very different place from what we imagined forty years ago, and new questions regularly arise about its nature. Today's discoveries are just the latest in a string that followed Voyager 1's astounding findings when it arrived at Jupiter four decades ago.