In fact, gravity assists are so important for space travel beyond Earth\u2019s neighbors that they can determine whether a mission occurs at all. The Voyager missions, one of NASA\u2019s highlights, were a product of the fact that the outer planets were aligned sufficiently that we could use each one\u2019s gravity to send Voyager 2 on to the next, known as the “grand tour”. \u00a0Fifty years after Voyager 2 launched, we have not been back to the ice giants, despite all our advances in space technology, partly because<\/a> the opportunity for such a double assist has not returned.<\/p>\n How does a gravity assist work?<\/p>\n The simple explanation for gravity assists is that, in passing, a spacecraft uses a planet\u2019s gravity to speed it up, or slow it down, as required. Authors who don\u2019t want to be distracted from describing a mission\u2019s objectives use language like this all the time, but it\u2019s probably left a lot of people puzzled.<\/p>\n \u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0<\/p>\n For example, if you visit a museum and pass by one of those models of a gravity well where you can roll balls or coins around a surface stretched like space-time by a major mass<\/a>, you can\u2019t use them to accelerate. Your ball gets faster as it drops towards the well, but loses that speed again as it passes out, along with a little lost to friction.<\/p>\n The reason passing a spacecraft around the edge of a planet\u2019s gravity well is different is that planets are in motion around the Sun. That motion is as important as the gravity. From the perspective of someone on a planet visited by a spacecraft, things look very much like that museum exhibit \u2013 the path of the craft is shifted by the encounter, but its speed does not change. To everyone else, however, including observers on other planets, the planet\u2019s gravity pulls the spacecraft along with it.\u00a0<\/p>\n A stationary planet would not give a spacecraft extra velocity, as the planet frame reveals, but the motion relative to the Sun gives the mission extra speed.<\/p>\n A little of the planet\u2019s angular momentum is transferred to the spacecraft, but being so much more massive, the planet experiences no noticeable change. Meanwhile, for the spacecraft, the effect is large. Depending on the relative alignment of the spacecraft and planet, the encounter can either increase or decrease speed relative to the Sun.<\/p>\n The acceleration such an encounter provides is not infinite, which is why many missions now require several assists. To become the fastest-moving craft<\/a> humans have ever made (relative to the Sun), the Parker Solar Probe has undergone seven fly-bys of Venus, each raising its speed a little higher. Extra speed can also be gained by firing rockets when closest to the planet, maximizing the gain in kinetic energy for the amount of fuel burned.<\/p>\n The long and winding road<\/p>\n Gravity assists are so useful that it\u2019s often worth going in the opposite direction from your target, just so you can grab the extra velocity an assist can provide.<\/p>\n Voyager 2\u2019s use of each giant planet in turn made sense, but it was the exception. The need to use Jupiter and Saturn required Voyager 2\u2019s launch at an angle compared to a direct mission to Uranus, but it was always making its way out of the Sun\u2019s gravity well.\u00a0<\/p>\n That\u2019s not the case for subsequent missions. Galileo, Cassini, and Juice, on the other hand, all went inward to Venus to eventually reach planets further from the Sun than Earth. It\u2019s also common for craft to make fly-bys of Earth, returning to where they came, to build up speed. These missions need to use the inner planets for gravity assists, when the Voyagers didn\u2019t, because their goal is to orbit Jupiter or Saturn, not just pass by<\/a>.<\/p>\n Juice is only well-positioned to check out 3I\/ATLAS because it’s not on a direct route to Jupiter, instead having recently done a fly-by of Venus.<\/p>\n Image credit: ESA<\/p>\n![\"A](\"https:\/\/www.newsbeep.com\/us\/wp-content\/uploads\/2025\/11\/GravAssis.gif\" "\\"A")
<\/p>\n![\"Infographic](\"https:\/\/www.newsbeep.com\/us\/wp-content\/uploads\/2025\/11\/ESA_s_Mars_and_Jupiter_missions_observe_comet_3I_ATLAS.jpg\" "\\"Infographic")
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