1. The Planetary Alignment That Cut the Trip from 30 Years to 12
Neptune is very far away. A spacecraft sent straight there, with no help along the way, would need about 30 years, according to NASA's history of the Voyager program[2]. Voyager 2 got there in 12[3].
NASA describes the secret this way: the layout of Jupiter, Saturn, Uranus, and Neptune, which comes around about every 175 years, lets a spacecraft on the right path "swing from one planet to the next" without needing a large rocket engine on board[3]. The trick it relies on is called a gravity assist. It was first demonstrated by NASA's Mariner 10 mission to Venus and Mercury in 1973 and 1974[3].
The timing was tight. The launch opportunities for a full tour ran from about 1976 to 1980[2]. Miss them, and the next good arrangement was a couple of lifetimes away.
2. How Gary Flandro Spotted the Once-in-175-Years Route
Flandro was a student at the laboratory, asked to look at possible paths for a mission to the outer planets[4]. Accounts place his breakthrough in 1964 or 1965; the exact year varies, but the important point is that the route was identified in the mid-1960s[2][1][4].
The discovery itself is described consistently. His calculations showed that Jupiter, Saturn, Uranus, and Neptune would line up favorably in the late 1970s, so that a spacecraft could use the planets' motion to sling itself from one to the next and visit all four in 10 or 12 years[1].
Gravity assists were already known, so the idea that was new was the string of planets, not the trick. Flandro himself says he was the first to notice that opportunity[4]. That is his own account. He also recalled the reaction: "I was told, 'This is impossible; stop wasting my time.'"[1]
3. How can flying past a planet make you faster?
It sounds like cheating. The spacecraft does not fire a big engine, and it does not touch the planet. It just swings by.
Here is the key. NASA's flight-basics guide says that, measured from Jupiter's point of view, a spacecraft leaves at the same speed it arrived. Only its direction changes[5]. But Jupiter is not standing still. It is racing around the Sun. Seen from the Sun, Jupiter's own orbital speed gets added to the spacecraft's speed, and the spacecraft keeps that extra speed on the way out[5].
A way to picture it: imagine stepping onto a moving walkway at an airport. Your speed compared with the walkway is whatever you choose, but compared with the floor, the walkway's speed is added in. A planet's gravity does something like this, except the "walkway" is the planet's whole orbit, and gravity bends the spacecraft's path instead of a belt carrying it. (This is only a picture to get the idea. The real thing is a curved orbit, not a belt.)
The speed has to come from somewhere. NASA explains that Voyager 2 left Jupiter carrying extra orbital angular momentum "stolen from Jupiter"[5]. The planet pays for it, but the bill is tiny. In NASA's words, the massive planet's loss is too small to be measured, while the tiny spacecraft's gain can be very great[5].
4. The Grand Tour Was Cancelled—but Voyager Kept the Route
Flandro's idea grew into the Grand Tour: several spacecraft sent to the outer planets by using the once-in-175-years alignment[2]. It never flew in that form. According to NASA's history, the Grand Tour lost out to other expensive new projects, including the space shuttle and the space telescope, in a shrinking budget[2].
A smaller plan took its place: two spacecraft aimed at Jupiter and Saturn. PBS tells the next part with a quote from JPL's director at the time, William Pickering. After the US Congress, which decides NASA's budget, turned down the full tour, JPL engineers designed two spacecraft that could reach Neptune anyway. "We just did it and didn't talk about it," Pickering recalled[1].
Choosing the paths was its own puzzle. NASA says more than 10,000 trajectories were studied before the two were chosen, the ones that allowed close flybys of Jupiter and its moon Io and of Saturn and its moon Titan[3].
Then came the surprise in the launch order. Voyager 2 left Earth first, on August 20, 1977. Voyager 1 followed on September 5, on a faster, shorter route[6]. The names are in the opposite order from the launches.
5. Why Voyager 1 Chose Titan and Voyager 2 Continued to Neptune
Voyager 2 went on the long road. NASA's fact sheet lists its closest approaches: Jupiter on July 9, 1979; Saturn on August 25, 1981; Uranus on January 24, 1986; and Neptune on August 25, 1989, passing about 5,000 km (3,000 miles) from the planet[6]. August 1977 to August 1989 is about 12 years.
It found a lot on the way. At Uranus: 10 new moons, two new rings, and a magnetic field tilted 55 degrees. At Neptune: six new moons, four new rings, winds of up to 680 mph (about 1,100 km/h), and on the moon Triton, a nitrogen-ice volcano[7]. After Neptune, the spacecraft headed south, below the plane in which the planets orbit[6].
Voyager 1 had the same arrangement of planets available to it. So why did it skip Uranus and Neptune? Because of Titan, Saturn's big moon. NASA says that "because of the specific requirements for the Titan flyby," the spacecraft was not directed to those two planets[8]. Titan turned out to have a thick atmosphere that completely hid the surface, made up of about 90 percent nitrogen[8]. A little over two weeks after Voyager 2, Voyager 1 left on an easier, faster trajectory that visited only Jupiter and Saturn[2].
You cannot visit everything. The same planetary lineup served two missions with two different goals.
6. Where the Two Voyagers Went After the Planets
The planet tour ended, but the trips did not. On August 25, 2012, Voyager 1 became the first spacecraft to leave the heliosphere, the bubble around the Sun, and start measuring the space between the stars[8]. Voyager 2 followed on December 10, 2018, becoming the second human-made object to enter interstellar space[7].
Try It Yourself: Three Ways to Follow the Voyager 2 Trip
- Check the timeline with a pencil. Write down Voyager 2's dates from section 5, starting with the launch in August 1977. Work out how long each leg took: launch to Jupiter, Jupiter to Saturn, Saturn to Uranus, Uranus to Neptune. Which leg was the longest? (Saturn to Uranus takes about four and a half years.)
- Look up when Jupiter and Saturn are in the evening sky. Both can be seen without a telescope when they are above the horizon. With an adult's help, a sky calendar from a library book or a planetarium website can tell you when to look for them where you live. Voyager 2 passed both.
- Read how it happened. NASA's history chapter on Voyager is free online, and so is the PBS page on Flandro's idea. See whether you can find the year each one gives for Flandro's idea.