Voyager: Frontiers of the Solar System – Spectrum of Science

Two of the most remarkable spacecraft ever launched owe their journey to space to a particularly auspicious coincidence in the solar system. In fact, about 60 years ago, the four giant planets slowly moved into a rare alignment that last occurred in the early 19th century. The planetary spectacle went largely unnoticed — until an aerospace engineering graduate student at the California Institute of Technology, Gary Flandro, saw the opportunity.

In 1965, the era of space exploration had just begun. Just eight years earlier, the Soviet Union had launched Sputnik 1, the first artificial satellite. Flandro was given the task of NASA’s Jet Propulsion Laboratory (JPL) to find the most efficient way to send a spacecraft to the major outer planets, that is Jupiter, Saturn, Uranus or even Neptune. To do this, he used one of the most popular precision tools of 20th-century engineers – a pencil – and plotted the orbits. In doing so, he noticed something intriguing: By the late 1970s and early 1980s, all four would be strung in a long arc toward Earth, like on a celestial string of pearls.

By colliding, a spacecraft on a suitable orbit could gain a little momentum at each of the giant planets with the help of its gravitational pull. Such a maneuver is called a swing-by. Flandro calculated that the repeated, accelerating diversions would shorten the travel time between Earth and Neptune from 30 to 12 years. However, there were 176 years between the forthcoming and the subsequent such constellation. In order to exploit the arrangement in the foreseeable future, a spaceship had to be launched by the mid-1970s.

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