There are times that fantasy has bled out too far into the hive mind, and people confuse a made up thing with an actual thing because of quirky similarities, nomenclature and possibly just a lack of understanding. So how do gravitational assists work? You probably know this involves flying your spacecraft dangerously close to a massive planet.
But how does this help speed you up? Sure, as the spacecraft flies towards the planet, it speeds up. But then, as it flies away, it slows down again. Sort of like a skateboarder in a half pipe. This process nets out to zero, with no overall increase in velocity as your spacecraft falls into and out of the gravity well. So how do they do it? The Magazine Shop.
Login Register Stay Curious Subscribe. The Sciences. Newsletter Sign up for our email newsletter for the latest science news. Sign Up. Already a subscriber? Want more? More From Discover. Recommendations From Our Store. But because the spacecraft is much, much smaller than the moon, the effect on the spacecraft's orbit is much greater than on the moon's orbit. For example, the Cassini spacecraft weighs about 3, kilograms, whereas Titan, the largest of Saturn's moons, weighs about 10 23 kilograms.
The effect on Cassini is thus about 20 orders of magnitude greater than the effect on Titan is. As an example, consider what happens when Cassini comes close to Titan during one of its orbits around Saturn. As the spacecraft approaches, Titan's gravity increases the velocity of Cassini relative to itself and, more importantly, changes the direction of Cassini's velocity.
As Cassini leaves Titan's neighborhood, its velocity relative to the moon decreases to its original value, but the direction of the velocity continues to change in the same direction. The result is that Cassini's velocity relative to Saturn has been altered. Depending upon the geometry of the flyby, the Saturn-relative velocity can be modified either in direction or magnitude or both. If the magnitude of the Saturn-relative velocity changes, then the orbital energy of Cassini changes with the resulting change in the orbital period.
Changes in either the magnitude or the direction of its Saturn-relative velocity modify Cassini's angular momentum, which results in a change in orbital orientation. As it left, it slowed down again, but it never slowed all the way to the speed it was before getting to Jupiter. It left the area near Jupiter faster and in a different trajectory. This technique was repeated at Saturn and Uranus. Gravity assists can be also used to decelerate a spacecraft, by flying in front of a body in its orbit.
When the Galileo spacecraft arrived at Jupiter, passing close in front of Io in its orbit, Galileo experienced deceleration, helping it go into orbit around Jupiter. How can a spaceship leave orbit?
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