How Does a Gravitational Slingshot Work?
You have probably watched this kind of science fiction scene more than once: a Captain Stalid-Jargon and their crew flee extraterrestrials / escape a supernova / lack of fuel and are apparently out of options, about to get eaten / spray / stuck. But then, just ahead, they spot a planet! They therefore go to the right, the flamboyant rockets, then plunge and use its gravity to fight safe. Hourra! Picked triumphant music.
It therefore goes at least on the silver screen. But does this maneuver work in real life?
Yes! Well, not so much as it is done in the movies, but it is a real thing. It is widely known as a gravitational sling, although most scientists call it gravitational assistance, and it is an essential tool for most interplanetary missions.
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The idea seems quite simple. While a spacecraft approaches a massive object, for example, a planet, the gravity of the planet folds its trajectory, modifying the direction of the spaceship. But there is more than that: the spaceship can really use the gravity of the planet to accelerate Or Slow down after this maneuver, allowing easier trips to external or interior planets respectively.
Although the part that folds the trajectory seems quite obvious, this part of the speed or the ball is quite counter-intuitive. It is linked to the symmetry of gravity.
If you hold a rubber ball at a certain distance from the ground and drop it, the ball will accelerate as it falls, accelerating until the impact. Then he bounces, moving upwards and decelerating as he does. He will end up stopping, after which you can catch it or drop it again. But anyway, He cannot bounce higher than the height from which you dropped it. He gained kinetic energy – the energy of movement – when she fell but lost it again after the edge while she slowed down on the way to the path. This action is symmetrical, so at best (if you had a perfectly elastic ball and you had this experience in the void), it would bounce at the same height from which you dropped it.
The same goes for a spaceship that approaches a planet. The severity of the world you will accelerate when you fall, you will work on the closest approach (this is the “Slingshot” part), then you will lose this additional speed as you move away because the gravity of the planet always pulls you. As this gravitational handle departs, the spaceship will move compared to the planet at the same speed at which he initially approached.
So, if all the bonus speed is lost at the exit, how can we use this maneuver to speed up a spacecraft? The key lies in the expression “compared to the planet”. If you approach the planet, for example, 20 kilometers per second (km / s), you will leave with the same speed. But it’s your measured speed against the planet.
At the same time, above all, the planet is also in orbit around the sun. If you approach the planet from behind (that is to say in the sense of its movement), then, as the gravity of the planet gives you a boost, it is also, in a heliocentric sense, attracting you, adding part of its orbital speed to yours. This gives you a kick compared to the sun, accelerating you on your way to your destination. In essence, the spaceship obtains a net gain of speed by flying a little of the orbital kinetic energy of the planet.
In turn, this means that the planet slows down a little on its orbit around the sun – which seems dangerous! But do not fear: the planet slows down proportionally how much it is more enormous than the spacecraft. Given a probe with a typical tonne compared to a multisexillion-tonne world, the planet is not slow at all. You can launch a million probes about it and never be able to make the difference in its orbital speed. A bacteria that bounces you back while you are walking would have a much more important effect on you.
The reason why it is worth reaching the difficulty of gravitational assists is that the spaceships are launched by rockets, which can only accelerate a top speed. For our current rocket, these speeds are so low and the interplanetary distances so large that even the fastest and most direct trips take years (or even decades for destinations in the external solar system). You can load the spaceship with more fuel to burn to go faster, but there is also a limit to that. The fuel has a mass, and you would need to speed up this additional mass, which takes more fuel, which has more mass. This wrestling is described by what is called the rocket equation, and this means the amount of fuel you need to add to move even slightly more quickly very quickly.
So, shaving the time of your trip requires another method, such as the siphoning speed of a large juicy planet along the way! For example, the Cassini of Saturn probe, launched in 1997, was a huge spacecraft, the size of a school bus and had a mass of 2.5 metric tonnes without fuel. (The addition of the fuel he needed to fulfill his mission to Saturn, as well as the launch vehicle and other equipment, tipped the scales at 5.7 metric tonnes.) The mission planners therefore took advantage of Jupiter, sending the spacecraft that exceeds it on a sling maneuver which increased the speed that has shaved a lot of time out of the trip. In fact, just to go out in Jupiter in the first place, Cassini also carried out two Venus flies and one of the earth, stealing a planetary orbital energy.
Gravitational assistance also works in the other direction. The earth orbits the sun at more than 30 km / s, therefore shooting a probe in the sun or in the interior planets is extremely difficult due to all this lateral speed. Instead, mission planners prefer a more circuit route. They launch the spacecraft with enough speed in the opposite direction of the earth path around the sun to fall in front, let’s say, Venus, where it can then give even more from its orbital energy to the planet to fall towards the sun. Bepicolombo, a joint European space agency and a mission from the Japanese aerospace exploration agency in Mercury, did exactly that, passing the earth once and Venus twice to enter the vicinity of Mercury. Even then, he had to make a total of six Severity helps Mercury to match the orbital speed of the planet around the sun. The last assist took place in January 2025, and he will enter the Orbit of Mercury in November 2026.
Gravitational assists are an emblematic example of the reason why travel in space hard-he East Exactly rocket science, after all. Severity is the greatest culprit; The simple fact of moving away from the earth in the first place is most of the problem. It is therefore ironic that severity can facilitate most of the rest of the solar system.
