Gravity Assist or Boost.

Good answer, But,,

That's what i also was thinking that it gains extra speed because as in the example jupiter is also moving around the sun at 30K MPh. Ok Understand, BUT....

Is not the probe also moving that fast already? ,,, or even faster actually since it is "catching up" with Jupiter to get this boost?

So Jupiter moving 30K MPh, probe moving 33k MPh to catch up to jupiter, hmmmm?

Not really because the craft comes in at an angle (see the resultant velocity picture on the second link)

Just a hunch without reading the links below. I would guess as long as the planet was stationary, the gravitational acceleration and deceleration would be symetrical. With a moving planet being the variable in the equation, the approach of the satelite would be at a pre-perpendicular, perpendicular and then an increasingly acute side angle called a ray. As forces increase the speed of the satelite, the planet is steadily moving off at a right angle and the perpendicular forces will be less as the two objects move away from each other. The optimal angle of approach can only be symetric to the angle of escape if the planet is stationary. It would probably symetrical between the gain of gravitational force and the loss of force due to the larger planet moving away and not finishing the dance. This symetric gain and loss would be the result of the perfect timing of NASA calculations. When the force cannot be the same at escape, you get to keep some of the borrowed gravitational force because the planet doesn't hang around long enough to apply the same force for the same amount of time.This is if the approach angle is calculated right. All fly by boosts seem to be behind the planet after it moves along it's orbital path. I would also assume planets with the oppisite rotation couldn't be used at all. On a fly by, the planet has to be turning toward the sun on its leading edge and the trailing edge away which is where the satelite should pass to get this boost.

Just An Observational Opinion So, Don't Take My Word On It.

Very few people know this, but gravity boosts need THREE bodies to work. The 'slingshot' effect involves a planet, one of the planet's moons, and the spacecraft.

If the spacecraft passes between the planet and its moon, they both pull on it forward as it approaches, and pull back after it passes between them (more precisely, after it passes the line that runs from one's centre of mass to the other's).

The spacecraft accelerates as it approaches and decelerates as it leaves; but it decelerates less than it will have accelerated because it will leave in less time (remember, it will be moving faster) than it took to approach. The net result is the 'slingshot' effect: the planet and the moon will have given the spacecraft a net increase in speed. Correspondingly, the planet will lose energy (it'll move more slowly in its orbit around the Sun), as will the moon (it'll orbit more slowly around the planet).

The effect works the other way, too. If the spacecraft passes beyond the moon's orbit, the moon ends up passing between it and the planet. The spacecraft and planet 'slingshot' the moon, and the spacecraft slows down.

All of the above works only if the spacecraft has a mass much smaller than that of the moon and that of the planet.

As for your diagram, a two-body system will only have the trajectories of the two bodies modified. Neither the planet nor the spacecraft will end up moving faster than before, but both of their trajectories will have changed. Inthe planet's case, no one will notice because of its mass being much greater than the spacecraft's. But the spacecraft will leave at the same speed as before, but in a different direction.

DZ

Very few people know this, but gravity boosts need THREE bodies to work. The 'slingshot' effect involves a planet, one of the planet's moons, and the spacecraft.

This one I will beg to differ from. For I know craft have used both Mercury, and Venus for this effect and neither have moons. Even tiny Mars has been used for this effect also, and it's moons are soo small there not really should be called a moon, they are soo small they cant even pull themselves into a round shape.

Mercury, Mars, etc.

My apologies, NSS is right ... and not. Same for me.

The slingshot effect DOES need three bodies. But in the case of Mercury and Venus, which have no moons, the Sun is the third body. For Mars, its two moons are the third bodies.

In Mars case, the Sun has an effect, too. But generally speaking, it's too far awy to have much of one compared tothe moons. That's especially the case for massive, far-away planets like the gas giants.

DZ

Dr. Edward Belbruno is a resident scholar at Princeton University and the principal of a company that calculates optimum trajectories for spacecraft with modest engines. It as he who saved the day for several otherwise lost programs. He recently lectured for us on low energy trajectories to the Moon and Lagrange points.

What makes these trajectories so effective is that they harness the gravitational pull of more than one celestial body and thus plays off two or more forces to place the arrival of the space craft at the right place, at the right time and, most important, at a speed slow enough to be captured by the arrival target.

The only downside that I know of using this method, is that it takes many times longer and the spacecraft must travel a much longer distance before it arrives. The upside is that small engines can be used to move significant payloads.

If you Google his name, I am certain you will get so many answers, you will be dizzy with information. If you choose to delve deeply into his formulas, you'd best be an advanced student of Advanced Calculus and Celestial Mechanics.

Frankly, I only understood 10% of what he said.

L.J.

Your diagram reference frame is Jupiter, which is moving. The motion of the probe is symmetrical with Jupiter but not with the solar system.

I think I remember that it had to be timed just right so there would be a second planet in the "vicinity". The probe approaches and accelerates toward the first planet, passes it and then starts to decelerate but before long the accelerating pull of the second planet equals and then exceeds the decelerating pull of the first planet. The probes approach speed to the second planet is now higher than it was approaching the first planet, so it's leaving velocity will be that much higher.

I hope this makes sense, it seems to do so, to me anyway.

on the approach side the force is increasing.

On the "escape" side the force is decreasing (i.e., not keeping up with rate of acceleration until it's too late). Therein lies the difference.

Absent retro propulsion in the probe, the notion that zero momuntum would be reached at point three would hold only in the case of an orbital insertion, or collision landing, trajectory.