We all know the Voyager probes that were sent further into space than any other human-made object. By now, we are quite familiar with their achievements, and because of that, it’s easy to take them for granted. However, the story of Voyager 1 & 2 is fascinating, worth books, not articles, and their story began long before their actual preparation. I suggest that today we shall look further into their fantastic story, even if space (for this article) is far too small for what we would need. 

NASA's Voyager 2 Sends Back Its First Message
Voyager 2

Let’s round things up a bit: what are the Voyager probes again? 

Voyager 1 & 2 are two spacecraft launched into space in 1977 (we will soon find out why that exact year). If we go way back in the history of the program, we can see that Voyager was not the first attempt at a mission of this type. The same thinking that was rooted in the Voyager plan popped up in the mid-60s, when an aerospace graduate student working as an intern at JPL, Gary Flandro, proposed the Grand Tour, a NASA program that would attempt to visit the outer planets.

Of course, just as JPL (Jet Propulsion Laboratory) promoted it back then, the program saved a lot of money by not sending probes to visit the planets individually, so it would have been quite an achievement. A plan was sent to the White House in 1969 and received a response in 1970. The White House was supportive of the project, but the costs were getting bigger and bigger, reaching 1 billion dollars. The plan was canceled in 1971. 

NASA, or more precisely, NASA engineers didn’t settle with the negative response and continued working on better plans. What happened then? In 1972, a project consisting of two probes was approved. Initially, the probes were imagined as part of the Mariner project, because of the similarities that the probes were going to have with the Mariner probes, and as a continuation of the Mariner already-existing mission.

However, the name was changed to Voyager because the project evolved much more than expected (as it is with everything NASA does, and that’s not a bad thing), and hence required a new dedicated program, the Voyager program. But wait, wait, how did this thing get approved? Well, NASA and JPL were smarter this time when making their proposals, and they promoted the mission as a Jupiter-Saturn exploration mission to reduce costs. And reduce costs they did: the mission had an estimated cost of 360 million dollars. Of course, this was just a little trick which obviously made things harder for the engineers, as the expectations were for the probes to last more than that, to be more precise, to survive longer than that. 

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Now, what’s the trick?

For sure, there must be something here. Why did the engineers at JPL push so hard for this mission to be done? Gary Flandro was the mastermind behind all this, and what he discovered back then in 1964 was groundbreaking. It absolutely changed the game. To be fair, we have to go even further back because as important as Gary Flandro was, he was not the one to lay the foundations for this. 

We’re in 1961, looking at a mathematician named Michael Minovitch. He was a 25 years-old graduate student who decided that he wanted to tackle one of the most interesting and fundamental problems in the history of physics: the three-body problem. It was something Newton himself couldn’t figure out. However, Minovitch had a powerful tool that Newton didn’t have, and that was an IBM computer. He was so excited about the new IBM computer at UCLA that he felt like giving a shot at this.

But what is this, more exactly? The three-body problem refers to the gravitational interaction between three bodies. Looking at how two bodies interact is easy, but when three appear, things get more complicated, and predicting how orbits work in this case is a problem to be solved by a computer. Now, the three-body problem could be easily solved by any computer, but back in 1961, solving the orbits was something absolutely remarkable. 

He continued his studies and came to incredible innovation in spacecraft propulsion. He figured out that if a probe would come close to a planet at a certain distance, it could basically steal some of its orbital velocity. This is an extraordinary thing, as it saves a spacecraft from a lot of fuel spent, basically enabling the probe to get a greater velocity by a simple physics trick, all-natural. 

An Extremely Rare Planetary Alignment Occurred When The Two Voyager Spacecraft Were Launched. 2
Gravitational slingshot (Credits: Rochester Institute of Technology)

On this matter, I’ve been looking at a straightforward explanation or way to understand this. Imagine we have a spacecraft of mass m and a planet of mass M. In the first phase, the spacecraft comes with a velocity, and the planet has its own velocity. The trick is, the moment the spacecraft enters the planet’s gravitational field is called a collision. And yes, it is just like a normal collision, and the things you know from physics that apply to collisions are the same.

So, what do we know about collisions? There are elastic and plastic collisions. In elastic collisions, each of the bodies moves after the collisions with their own new velocities and in their directions (given by how they hit each other). Basically, it’s a bounce. Throw two balls at each other, and you will have an elastic collision. Plastic is when two objects colliding result in a bigger one moving with its velocity. Basically, in this case, the two objects become bound to each other and now move with only one velocity (instead of two separate ones). What we have in our case is an elastic collision.

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You may ask, then where does the planet (the second body) go? I just said both of the bodies’ velocities get affected, right? Well, the thing here is that the influence of the spacecraft is way too small to create a significant difference. But yes, it does create a slight difference if you are accurate enough to observe it. On the other hand, the spacecraft is hugely influenced by the planet, as you could guess. So, back at collisions, what happens in an elastic collision? We know that the momentum is conserved and also that the kinetic energy is conserved. We shall leave the maths for some other time now, but then I’m asking you to take my word that the spacecraft can really gain velocity from this little trick. 

Then There Were Two: Voyager 2 Reaches Interstellar Space - Teachable  Moments | NASA/JPL Edu
Voyager trajectories Credits: JPL

In the summer of 1965, Flandro started studying the options he had for achieving his goal. He was one of the few to actually consider Minovitch’s ideas, as the scientific community was very skeptical about his calculations. Having this gravitational slingshot in mind, Flandro observed something astounding. Somewhere in the late 70s, the planets would be aligned in a way so rare that it would take another 176 years to happen again. 176 years! The planets were going to be all aligned on the same side of the Sun. 

And that is, in brief, the story of the Voyager launch. Until 1977 (the best year to start the journey, because of the positions of planets), the team of scientists used a solution of the three-body problem to compute the trajectories of the probes. They were very smartly launched in the same year, but delayed, with Voyager 2 going first, on August 20, and Voyager 1 on September 5. Voyager 2 was the one which had the strangest mission of the two, because it was the one planned to visit all of the outer planets, while Voyager 1 was planned to come in a very close flyby with Saturn and its moon Titan, and then headed for outer space.

Even though Voyager 2 was launched first, because of its trajectory Voyager 1 was still to reach Jupiter first, and in case Voyager 1 would’ve missed the Titan flyby, Voyager 2’s trajectory could’ve been changed to go for Titan instead of the icy planets, as that was the primary purpose of the mission. 


Much of you know about Voyager’s astounding success. Its achievements would take me another 2 pages (at least), so I’m only going to mention a few. In 1986, Voyager 2 reached Uranus, and on October 2, 1989, it ended its exploration of the Neptunian system. Voyager 2 is now studying the Interstellar Space and has been in space for 43 years, 8 months, and 19 days. Voyager 2 is still in full contact with Earth.

Voyager 1 is the most distant human-made object from Earth. It is now at a distance of 152 astronomical units (22.8 billion km) and still in contact with Earth. It entered the Interstellar Space in 2012, on August 25. Voyager 1 studied the weather, magnetic fields, and rings of Jupiter and Saturn and was the first probe to send detailed images of their moons.

Also Watch: New Horizons clicked a picture of Voyager 1 from billions of miles away

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