46 Years Ago, a Rare Alignment of Our Planets Allowed For An Iconic Space Mission

The summer of 1977 was a great time to be a space nerd. Star Wars Episode IV: A New Hope was a summer blockbuster. NASA was testing its futuristic Space Shuttle in the Mojave Desert. And, on August 2 and September 5, Voyager 2 and 1, respectively, blasted off from Florida on their way to tour the enigmatic giant worlds of the outer solar system. The summer of 1977 changed our view of outer space forever.

The twin Voyagers carried the same array of instruments — spectrometers, cosmic ray detectors, and cameras — to tell scientists on Earth about distant worlds; they also carried matching “Golden Records” with recordings of sounds, music, and voices to tell distant worlds about life on Earth.

Altogether, each Voyager carried slightly less computing power than a modern smartphone. By today’s standards, they're bare-bones machines, and in some senses, their electronics were outmoded even by the time they launched. But sometimes simplicity works: the Voyagers have outlived many of their original designers. And it's hard to imagine not knowing the things Voyagers 1 and 2 revealed about the outer reaches of our Solar System: that Jupiter’s Great Red Spot is a gargantuan hurricane, that Europa’s ice is cracked because of tides churning beneath it, that Io is volcanically active on a terrifying scale, or that Titan has hydrocarbon seas and rivers beneath its methane smog.

Thanks to the Voyagers, NASA knew it was worth launching the Galileo mission to Jupiter and the Cassini mission to Saturn.

“I remember seeing the image of the moon Io for the first time and thinking that the Caltech students had engineered a brilliant stunt — they must have substituted a picture of a poorly made pizza for the picture of Io!” recalls Voyager program co-investigator Alan Cummings in a post for NASA. “All that orange and black on Io changed our thinking about the moons in the Solar System. I think most of us thought they would all look more or less like our own Moon. But, wow, how wrong was that!”

Two Long One-Way Trips

Voyager 1 swept past Jupiter in 1979, using the planet’s tremendous gravity to power a slingshot outward toward Saturn and its haze-shrouded moon Titan (mission planners had to choose between a flyby of Titan or Pluto, and they chose Titan). From there, the tug of Saturn’s gravity “bent the spacecraft's path inexorably northward out of the ecliptic plane.” Voyager 1 was on its way out of the Solar System.

Voyager 2 also flew past Jupiter for a gravity assist in 1979, then past Saturn in 1980, but its path also carried it past the Solar System’s two most distant worlds, “ice giants” Uranus and Neptune. To this day, Voyager 2 is the only spacecraft we’ve sent to either of the ice giants.

“The planet Uranus turned out to be a fuzzy blue tennis ball, with an atmosphere not at all as exciting as Jupiter or Saturn,” recalls Suzanne Dodd, now the Voyager program manager, in a post for NASA. “So initially, it felt a little disappointing, but then there was the moon Miranda. That was shocking – a jumble of different geologies on the same body. It was the jewel of the encounter.”

After flying past Neptune in 1989, Voyager 2 carried on its own way out of the Solar System, curving south (relative to Earth’s poles) while its sister headed north.

How The Planets Aligned

The trips were only possible because of a rare alignment of the planets. Our Solar System’s massive outermost worlds lumber slowly along wide, long orbits: Jupiter takes about 12 years to make a lap around the Sun, while Uranus takes 84; Neptune orbits the Sun in such a wide circle that its orbit takes a staggering 165 years to complete. But once every 175 years, the planets happen to pass the same point in their orbits at the same time, so that from Earth’s viewpoint they all line up in a roughly straight line.

Aerospace engineer Gary Flandro, working in NASA’s Jet Propulsion Laboratory, realized that such an alignment was due to happen in the late 1970s and that NASA could take advantage of it to explore the outer Solar System. The outer planets’ rare alignment meant that a spacecraft could reach all four of them on a single curving trajectory, using each planet’s gravity to get a speed boost and help set the course for the next world. Each spacecraft could save fuel and reach its destinations in a fraction of the time.

Based on Flandro’s calculations, the original version of Voyager would have been a fleet of four spacecraft, dispatched in pairs to the outer worlds: two to Jupiter, Saturn, and Pluto, and two more to Jupiter, Uranus, and Neptune. But the price tag for that pair of missions would have been about $1 billion at the time (equivalent to a little over $5 billion today), and NASA’s planetary science missions were competing for funding against the newly-approved Space Shuttle program — part of a longstanding budget rivalry between crewed spaceflight and planetary science.

Eventually, the pared-down version involved two spacecraft, Voyager 1 and Voyager 2. And NASA chose to overlook poor little Pluto in favor of Saturn’s moon Titan.

The planetary alignment also meant that in 1990, Voyager 1 could point its camera back toward Earth and capture a “family portrait” of our Solar System. That portrait included the now-famous Pale Blue Dot: a color image of Earth from 4 billion miles away, looking tiny and fragile amid the vastness of space.

Candice Handsen, now a senior scientist at the Planetary Science Institute, and then part of the Voyager imaging team, recalls that her colleagues printed out Voyager 1’s wide-angle mosaic of the Solar System, with the more focused color images of individual planets as insets, and hung them along a wall in the Von Karman Auditorium at JPL.

“Jurrie [Van der Woude] said that he had to replace the picture of Earth rather often — people always wanted to touch it,” writes Handsen.