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planetary alignmentThe rough alignment along a solar radius of the outer planets, Jupiter to Neptune, that takes place approximately every 179 years. The most recent planetary alignment took place at the beginning of the 1980s, making possible the two Voyager missions to Jupiter, Saturn, and beyond (see Voyager probes). The planetary alignment was thought to provide the only cost-effective way to visit the outer planets because using Jupiter's gravitational field to accelerate a probe, changing both its velocity and direction, saved fuel and took years off the journey time. But improved mathematical techniques for calculating accurate flightpaths, including gravity assists, mean that space scientists do not necessarily have to wait for planetary alignments to send probes to the outer Solar System. See also gravity assist.
Collins Dictionary of Astronomy © Market House Books Ltd, 2006