# I've heard gravity can "fling" stuff, how does this work?

1. Oct 3, 2015

### Coolamebe

So I've heard things like the sun (or I guess any star" would be able to "fling" objects away from the solar system, I have no idea how this would work. Would this be similar to quasars, how the black hole "flings" matter away that reaches the escape velocity?

2. Oct 3, 2015

### Staff: Mentor

Welcome to the PF.

3. Oct 3, 2015

### Coolamebe

Thanks a lot for that, I was trying to google it but I guess "flinging" isn't specific enough haha. Thanks.

4. Oct 3, 2015

### Staff: Mentor

LOL. Yeah, "gravitational flinging" probably won't return many hits.

BTW, I think gravitational slingshots are generally used with the planets, and not with the Sun. There are a couple reasons for this -- after you do the reading, can you make a couple guesses why that is true?

5. Oct 3, 2015

### Coolamebe

Well my guesses for using the planets instead of the sun would be:

When we are exploring the solar system, we care about the position of the planets relative to the suns position. When we gravitationally slingshot our spacecrafts, we add the velocity of the planets relative to the sun to the spacecrafts velocity relative to the sun. But if we were to gravitationally slingshot using the sun as the "slingshot" it wouldn't work because relative to the sun, the sun isn't moving, it has no velocity relative to itself. So we don't add any velocity to the spacecraft.

As the sun is so hot, we can't send the spacecraft close enough to the sun to add enough velocity.

Also is this how the comets orbit's are so elliptical and have points very close to the sun and very far away from the sun? Do they gravitationally slingshot themselves?
And does this happen with rogue planets, so if a rogue planet came close enough to be affected by the sun's gravity, would it just be attracted and become a part of the solar system or would it be slingshotted away?

6. Oct 3, 2015

### Staff: Mentor

They don't get gravitationally slingshotted. First, think about what happens here on Earth if you throw an object. If it weren't for air resistance the object would continue at the same horizontal velocity but its vertical velocity would change under the influence of gravity, causing it to accelerate downwards until it hits the ground.

Now, what if we throw our object fast enough so that as gravity accelerates it down, it moves so fast that the curvature of the Earth causes the Earth to 'fall away' from the object as it travels. The object would continually 'fall' towards the Earth, but it would never be able to hit it because the surface of the Earth falls away from it. What we have here is a perfectly circular orbit.

Now, if we throw the object just a little faster, this orbit turns into an elliptical orbit, where the object recedes from Earth during part of the orbit and approaches the Earth during the other part. It still can't hit the surface because it has no way to get rid of its speed and thus fall to the ground.

If we throw our object really hard, it will be flying too fast for gravity to ever bring it back to the Earth. Gravity still acts on it, but it can't slow it down enough.

The same thing happens with comets. They almost always fall in a manner that causes them to swing around the Sun instead of crash into it and they are rarely able to gain enough speed during their orbits to reach escape velocity.

It would almost always pass by and continue on without being captured. In order to be captured it would have to have some sort of gravitational interaction with the other objects in the solar system to slow it down.

7. Oct 4, 2015

### sophiecentaur

The essential requirement for a slingshot orbit is that it gives the spacecraft extra Energy, at the expense of the planet it is 'slung' around (not the Sun). If a craft flies close to the Sun, it will gain Kinetic Energy by following an elliptical orbit. Problem is that, once the craft gets back to Earth Orbit radius, it hasn't actually gained any energy from the exercise. However, a craft, leaving Earth will have its existing orbital energy (as it had on Earth) and its motors will give it some extra energy and this can take it near another planet. When it is near enough for that planet to influence it - and if you time things right - it will 'catch up' the planet and whilst in near orbit to it (a hyperbolic planetary orbit btw) it will end up travelling 'forward' of the planet and at a speed that is increased by almost double its original speed relative to the planet. (I should really be talking in terms of Momentum and Velocities but that could make it harder). This only works if the planet is in a different orbit from that of the Earth (they all are, in the Solar System so no problem). The extra energy imparted to the craft can be used to take it further out and into another slingshot round another planet. This condition doesn't occur every day and you have to choose your time. Voyager(s) used both Jupiter and Saturn for their gravity assisted journeys out of the Solar System. Many images on Google.
You can also use a slingshot orbit to brake your motion when returning to Earth orbit (this also reduces the need for so much fuel). I always imagine future inter-stellar journeys using the method to 'stop off' at a star of interest in this way. They would need to study the planetary system long and hard so as to time their arrival suitably.
As Drakkith mentioned, a rogue planet can be captured in this way if it gets close to a suitable planet, disturbing its orbit too, of course - the sizes would be comparable - and the Solar System could gain another member. The original planet would be bumped to a higher or lower orbit. But you would most likely get very elliptical resulting orbits for the two and that could cause a big disturbance among all the rest. Owch.