https://ntrs.nasa.gov/api/citations/20170009181/downloads/20...

> A very rare planetary alignment would occur in the late 1970’s allowing a spacecraft to visit all the outer planets (Jupiter, Saturn, Uranus, Neptune and Pluto) using gravity assists at each planet to send it on to the next. This unique alignment would not occur again for another 175 years!

https://pds-ppi.igpp.ucla.edu/archive1/VGLE_1001/DOCUMENT/MI...

> During the late 1970's, alignment of the outer planets affords a variety of multi-planet launch opportunities. From the standpoint of Jupiter flyby distance, the 1977 opportunity (Bourke et al., 1972; Schurmeier, 1974) is the most favorable for reaching Saturn via a gravity-assist swingby of Jupiter. Two advanced spacecraft will be launched during a one-month launch period which opens on August 20, 1977. Each spacecraft will be launched by a Titan IIIE/Centaur D-IT from Launch Complex 41 at the Air Force Eastern Test Range. Final Earth-departure injection velocity will be delivered by a solid rocket portion of each spacecraft. After solid rocket burnout and jettison, the remainder of each spacecraft, having a mass of approximately 825 kg, begins its cruise to Jupiter.

https://commons.erau.edu/cgi/viewcontent.cgi?article=2830&co...

https://solarsystem.nasa.gov/missions/voyager-2/in-depth/

> The two-spacecraft Voyager missions were designed to replace original plans for a “Grand Tour” of the planets that would have used four highly complex spacecraft to explore the five outer planets during the late 1970s.

> In 1974, mission planners proposed a mission in which, if the first Voyager was successful, the second one could be redirected to Uranus and then Neptune using gravity assist maneuvers.

> Although Voyager 2 had fulfilled its primary mission goals with the two planetary encounters, mission planners directed the veteran spacecraft to Uranus—a journey that would take about 4.5 years.

> In fact, its encounter with Jupiter was optimized in part to ensure that future planetary flybys would be possible.

> The Uranus encounter’s geometry was also defined by the possibility of a future encounter with Neptune: Voyager 2 had only 5.5 hours of close study during its flyby.

https://www.scientificamerican.com/article/salvaging-nasas-g...

> When the twin spacecraft were launched, NASA was taking advantage of a rare alignment of Jupiter, Saturn, Uranus and Neptune that occurs once every 175 years to send probes on a "Grand Tour" of the solar system. The alignment allowed the spacecraft to harness the gravity of each planet and swing from one to the next using relatively minimal amounts of fuel. NASA first demonstrated the technique with its Mariner 10 mission to Venus and Mercury from 1973 to 1975.

https://www.secretsofuniverse.in/voyagers-planetary-alignmen...

> ... 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.

http://www.gravityassist.com/IAF3-2/Ref.%203-148.pdf

... and then probably the most important paper in all of this: Utilization of Energy Derived from the Gravitational Field of Jupiter for Reducing Flight Time to the Outer Solar System http://www.gravityassist.com/IAF2/Ref.%202-123.pdf published in 1966.

> 4. Conclusions

> The 1975-1980 time period is characterized by an abundance of interesting multiple planet trajectories which efficiently utilize energy derived from a close approach to the planet Jupiter. The trajectories discussed here are characterized by very short flight times in comparison to those for direct flights from Earth to the corresponding target planets. Although higher launch energies are suggested for some of the multiple planet flights, the additional expense of this energy might be offset by the great savings afforded by the short flight times. This is due to the expense of providing adequate vehicle reliability for the extended flight duration characteristic of direct trajectories and to the high costs involved in maintaining tracking, orbit determination, and other flight related activities for protracted periods.

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... And so, the Voyager probes became some very fast moving probes.

Note that the Voyager probes are 825 kg... while New Horizons was 478 kg at launch time.

You can go faster for getting to 550 AU... with a less massive probe. That's why New Horizons is as fast as it is. For that 3.6 AU/y rate - we haven't launched anything that will overtake them yet. That's going 1/3 the way from Earth to Saturn "direct" each year... twice as fast as Cassini took to get to Saturn. ESA just launched JUICE... it's going to to take 8 years to get to Jupiter (5.2 AU). The current speed of Voyager could do it in a year and a half... but that's with all the gravity assists that it got to where it is now.