How does gravity assistance work?
When you send a space probe into space, to explore a particular planet, for example, you are not sending it directly towards that planet.
Already, during the duration of the probe’s journey (which can take several years), the planet will have moved. We must therefore aim not at the current position of the planet, but the position of the planet when the probe arrives in its orbit.
Then, a second reason, you have to take into account the difficulty of sending a rocket to another world. It’s not as easy as throwing a basketball at the hoop. Rather, it should be seen as throwing a self-propelled ball from a moving car to an also moving basket with players with baseball bats located between the basket and you. The exercise is therefore difficult in itself and it is about compensating for all the movements and interactions involved in order to reach your target, and preferably the first time.
Finally, a third reason is that you rarely want to get involved in a scientific project if you know that you will not be able to see the outcome of it during your life, meaning by that that the Mission duration should be kept as short as possible so that scientists live long enough to see the mission succeed.
When it comes to sending a space probe to Saturn, Jupiter or Pluto, the journey itself lasts already 5, 10, even 15 years!
The first two problems are easily solvable. For the latter, the travel time of the probe must be reduced. For that, the probe must go faster.
One solution for this is to use the gravitational assistance of the other planets: the probe will pass close to a third planet – not the one we want to study – and regain speed at that time.
How does gravity assistance work? That’s the subject of this post.
Principle of the maneuver
The basic principle of gravitational assistance, as briefly explained, is to pass close to a planet, then to take advantage of the gravitational pull to gain speed, and finally to start again faster than one arrived.
At first glance, it might seem logical to think that the probe arriving at the planet is accelerating, but at best should only take off again as quickly as it arrived.
However, with gravitational assistance, we are able to move away much faster than during the approach phase. The probe has therefore gained speed and therefore energy.
For example, the Voyager 1 probe launched in 1977 approached Jupiter in 1979 with a speed of about 15 km / s (54,000 km / h). At its closest to Jupiter, it was traveling at a speed of 38 km / s (136,800 km / h). But when she pulled away, she did so with a speed of about 22 km / s (79,200 km / h). So we see that the probe accelerated sharply, then slowed down a bit, but in the end, it took off again noticeably faster than it arrived, which is the goal.
How is it possible ?
A phenomenon not as trivial as that
How can we leave faster than we arrived? Where does the energy acquired by the probe come from?
Already, yes: by approaching the planet, we gain speed. A lot of speed, and when you leave, you inevitably lose some… but not all! Just see the example with Voyager 1 and Jupiter above.
To understand, we must not forget that Jupiter is moving forward in its orbit: it is in motion! When we approach him, we are partially trapped by his gravity.
So, if we could land on Jupiter, we would still share the speed of Jupiter itself! And this, this speed, once it’s acquired by the probe, it can hold it. The probe therefore does receive a certain amount of kinetic energy simply by “clinging” to Jupiter (by the force of gravity)!
He will then jump much higher, and higher and higher if he can jump several times in a row on different moving branches.
This principle is used by acrobats and acrobats in a circus : the latter use the strength and energy of a teammate to increase their own inertia and gain speed or height.
The same goes for swinging: during each phase of descent and ascent, you bend or extend your legs to change your moment of inertia and be able to go higher. Without knowing it, in fact, when we do this, we inject energy into our movement: stretching or retracting your legs requires muscle energy, and in the end, this energy is converted into kinetic (speed) and then potential energy. .
Ditto for Tarzan: we had to put the branch in motion! the energy that Tarzan gains comes from the energy source that set his branch in motion. So it doesn’t come out of nowhere.
Ditto for Tarzan: we had to put the branch in motion! the energy that Tarzan gains comes from the energy source that set his branch in motion. So it doesn’t come out of nowhere.
To come back to the planet, the probe approaching it from behind “hoists” up to the moving planet in its orbit to gain energy then moves away from it to start again, retaining as much of this energy as possible. .
the energy captured by the probe is obtained when the planet pulls the probe towards it.
Now, don’t forget that if the planet attracts the probe, the probe also attracts the planet! The planet is therefore also moving a little towards the probe.
the whole being in orbital motion, it is therefore as if the planet is slowing down very slightly in its orbit, as the probe accelerates!
What the probe gains in speed the planet loses. The probe therefore steals orbital energy from the planet.
Of course, the probe is infinitely less massive than the planet. The probe can therefore accelerate to more than 70,000 km / h, the speed lost by the planet is absolutely imperceptible.
And with a motionless star, like a star?
Gravitational assistance as described above therefore consists of stealing orbital energy from a planet to allow a space probe to go faster.
If we try to do the same thing with a stationary star (in the frame of reference), the star has no orbital speed and it will not be possible to steal it. For example, it is not possible to go faster by sending a probe towards the sun to circle it and away from it faster.
There are, however, other methods that work for stationary stars.
For this we must realize that the force of gravity, although very weak in absolute terms, is immensely strong when we consider the gravity produced by a planet or a star, quite simply because these stars are immensely massive. This is to say that it takes a lot of fuel to overcome these forces. In a typical rocket, the mass of fuel thus represents 90 ~ 95% of the total take-off mass: only the remaining 5% corresponds to the mass of the rocket and the mass of the payload.