How do sling shot space craft, manage to gain energy from Planets.

[Mike Smith Cosmos]

You would think that any energy gained on the way IN would be lost on the way OUT. But the host planet manages to give up some energy. How ? Why ? Where exactly does the exchange or transfer of energy occur ?

[J.C.MacSwell]

You would think that any energy gained on the way IN would be lost on the way OUT. But the host planet manages to give up some energy. How ? Why ? Where exactly does the exchange or transfer of energy occur ?

Consider what you described, the spacecraft having the same final kinetic energy, from a different frame. Consider it from a frame in which the spacecraft has much less speed initially...

Edited August 3, 2013 by J.C.MacSwell

[Strange]

If you think of it from the perspective of the planet, the spacecraft arrives and leaves at the same speed. However, the planet is moving (in its orbit) and so, from a stationary point of view, the spacecraft is moving faster when it leaves. And the planet is moving ever so slightly slower. So the kinetic energy comes from the planet.

 

Wikipedia has a good explanation: http://en.wikipedia.org/wiki/Gravity_assist#Explanation

[Mike Smith Cosmos]

If you think of it from the perspective of the planet, the spacecraft arrives and leaves..............

and J.c. MacSwell said

 

Consider what you described, the spacecraft having........

 

Wow ! If I thought about it enough , we should theoretically [,possibly], be able to slingshot ourselves around the Universe.

Not sure how/whether it works with the Sun [for the slingshot out of the solar system], and/or the Galaxy , [ for a visit to another Galaxy in the local group say Andromeda ] . Assuming of course we could live that long or at least imagine living that long !

 

So it appears to get its extra speed /energy, it gets it from the movement of the planet around the Sun .

 

Perhaps for a Sun sling shot, It is the movement of the Sun around the Milky way Galaxy center ? [can give the extra energy to get out of the solar system ?] Presumably , that is how the Pioneer craft got shot out of the solar system and still going strong. Unless it was its last sling shot off NEPTUNE ?

 

Then for hopping out of the Milky Way galaxy , perhaps using whatever rotation the milky way has about the Local group center .

 

Interesting to know what happens when you try to sling shot yourself out of the whole Universe, presuming it is turning somehow ?

Edited August 5, 2013 by Mike Smith Cosmos

[EdEarl]

Vikings and Pioneer got energy from several planets, not the Sun. Just because the planets are not moving fast enough to escape does not prevent them from adding energy to a spacecraft moving faster than the planet.

 

Light cannot escape the universe; thus, a spacecraft cannot.

[Enthalpy]

Perhaps for a Sun sling shot, It is the movement of the Sun around the Milky way Galaxy center ? Presumably , that is how the Pioneer craft got shot out of the solar system and still going strong.

For a slingshot as we use to understand it, the spacecraft should come from outside the Solar system - more accurately, its energy should suffice that it is not bound in the Sun's gravitation well. So aliens sending a craft from a different solar system could take advantage from the different speed of our Sun for a slingshot. It would need patience.

 

Escape a galaxy by slingshot : this is the astronomical model for some objects observed outside any galactic gravitation well. Smaller objects get speed and are ejected. Same story for a globular cluster, which can eject an object from time to time.

 

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What can be - and is - used by us is the Oberth effect, including at our Sun.

http://en.wikipedia.org/wiki/Oberth_effect

It tells that near a massive body, when you add a small speed to the minimum that allows to escape this body's gravitation, you keep a big speed after escaping, because speed sums act through their square as an energy. That's daily life when sending probes far from Earth, and it's used from our Sun as well: "for instance" the Pioneer, Voyager and New Horizons keep much speed after escaping Sun's gravitation.

 

As a special case, Solar sails near our Sun would benefit both from the Oberth effect and from more intense light, so their Sun escape scenarios involve to first dive near the Sun and accelerate there.

 

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(Every?) probe we sent farther than Jupiter used the slingshot effect at Jupiter, because our chemical propulsion is too weak for the Sun's gravity. In some cases we could make direct shots, but this would mean huge launchers and tiny probes.

 

In other cases like Ulysses escaping the ecliptic plane, or Messenger going to Mercury

http://en.wikipedia.org/wiki/Ulysses_(spacecraft)

http://en.wikipedia.org/wiki/MESSENGER

the energy is out of our reach with chemical engines only and demands a slingshot. Solar sails woud have done Ulysses' mission better and faster, if they were available.

 

Some missions (Cassini to Saturn) even pass by Venus and Earth before Jupiter to further save mass.

http://en.wikipedia.org/wiki/Cassini%E2%80%93Huygens

In this case, the initial hydrogen rocket for the shot to Jupiter was cancelled, but the indirect trip permitted a less efficient solid rocket.

 

As a drawback, Jupiter's period around the Sun is 11 years, so the window opens only from time to time. Better propulsion like my Solar thermal engine would bring this flexibility.

Edited August 5, 2013 by Enthalpy

[Airbrush]

You would think that any energy gained on the way IN would be lost on the way OUT. But the host planet manages to give up some energy. How ? Why ? Where exactly does the exchange or transfer of energy occur ?

 

The spacecraft gains a LOT of energy from the planet's immense gravity on approach, but only from the exactly correct angle, and the planet loses a TINY negligible bit of energy slowing its' orbit. But the spacecraft does not lose energy on the way out, just like in a sling shot the stone does not lose any energy on the way out. Or something like that.

Edited August 6, 2013 by Airbrush

[imatfaal]

 

The spacecraft gains a LOT of energy from the planet's immense gravity on approach, but only from the exactly correct angle, and the planet loses a TINY negligible bit of energy slowing its' orbit. But the spacecraft does not lose energy on the way out, just like in a sling shot the stone does not lose any energy on the way out. Or something like that.

 

 

I think it is more along the lines that the space craft is dragged along with the moving planet thus gaining speed. The increase of speed into the planet and decrease out are not the important part they should be close to even - the speed up comes from the fact that the miniscule satellite gets a boost from a moving planet. Jupiter is moving at 13 kilometres a second - so receiving a drag from that is very beneficial. I think you could use trade winds as an analogy (but more on distance than speed) - you don't sail into the trade winds for the momentary boost of getting into that water and you can still benefit after spending extra energy to get back onto your route because whilst you are in the trades you are boosted across the ocean at great speeds (and reliability).

[Delbert]

In simple terms I think the reason is the craft spends more time approaching the planet and less time retreating. Presumably it's the change of direction which is instrumental in achieving this imbalance. Something to do with the solution to the three body problem, I understand.

 

Just noticed that there's a few Three Body Problem game downloads on the jolly old interweb.

[Mike Smith Cosmos]

Am I to take it that ONLY orbital velocity can increase the incoming space craft with kinetic energy increase . OR is the angular velocity of the body ( planet, sun, Galaxy ) able to give increase of velocity so as to increase kinetic energy ?

 

How does the spacecraft know the [planet,sun, galaxy,cluster] is moving, rotating beneath it, [moving relative to what].?

 

 

Is the Gravitational Field .sweeping along the space craft as the sling to give the space craft its added kinetic energy.

 

Its existing velocity outside the dotted blue line is added to and lost ,on the way in and out, is that correct ?

 

 

What is happening to the gravitational field as it SPINS , does it assist, neutral or subtract from velocity increase ?

 

What effect does the Gravity of the spinning Milky way Galaxy, have on the space craft ? Does it give it an extra boost if it is spinning in the direction of the space craft as it goes behind [ the planet, sun , or body ] ?

Edited August 8, 2013 by Mike Smith Cosmos

[Janus]

Am I to take it that ONLY orbital velocity can increase the incoming space craft with kinetic energy increase . OR is the angular velocity of the body ( planet, sun, Galaxy ) able to give increase of velocity so as to increase kinetic energy ?

 

How does the spacecraft know the [planet,sun, galaxy,cluster] is moving, rotating beneath it, [moving relative to what].?

Let's assume that we are looking down on the space craft and planet from above the orbital plane, and that from our persepective, both are moving to the left.

 

The spacecraft is ahead of the planet by 500,000 km and moving at 3 km/sec relative to the Sun. The planet is moving at 5 km/sec. If we switch to the perspective of someone at rest wtih respect to the planet, we would say that the spacecraft is approaching the planet from the left at 2 km/s.

 

We'll assume that the trajectory of the space craft puts it in a parabolic orbit around the planet. Thus from the respect of the planet, it starts from 500,000 km to the left of the Planet traveling to the right at 2 km/sec. It then falls in towards the planet, whips around it and heads back out, this time going right to left. When it is 500,000 km to the left of the planet it is moving at 2 km/sec to the right. It's speed with respect to the planet hasn't changed, but its direction has.

 

Now go back to the Sun's persepective. It started out moving to the left at 3 km/s which is equal to the 5km/s planet velocity minus the 2 km's spacecraft-planet relative velocity. After swinging around the planet and returning to its starting distance, it is moving at the 5 km/s planet speed plus the 2 km/s spacecraft-planet relative speed. So now it is moving at 7 km/sec to the left relative to the Sun.

 

Of course, this neglects any effect that the swing around had on the planet. In reality, the planet gets a little kick to the right and losses a little orbital velocity in the process, so the final velocity of the spacecraft will be just a smidge under 7 km/s and the planet a bit under 5 km/sec. If the planet is very massive compared to the spacecraft, it results in a very small difference.