# Mission To Pluto Uses Jupiter To Speed Up?

1. Jan 21, 2006

### uknova

I read a recently launched space craft is going to pluto and is swinging around jupiter to help it speed up.

I dont understand much about physics but surely the planets gravity will give as much kinetic energy as it takes away, once the craft is moving away from the planet.

How can it actually gain speed from swinging round a planet, i would of guessed it would speed up as it approached, then slow down to its original speed as it goes away from the planet?

Can anyone explain in simple terms?

2. Jan 21, 2006

### George Jones

Staff Emeritus
Remember that Jupiter is itself moving. A small amount of kinetic energy is transferred from Jupiter to the spacecraft. Since the mass of jupiter is so large, this does not have a noticeable on the motion of Jupiter. The mass of the spacecraft is tiny, and the transfer of kinetic energy does have a noticeable effect on its speed.

For answers with more detail, try googling "slingshot effect".

For example, see http://www.dur.ac.uk/bob.johnson/SL/1.html" [Broken].

Regards,
George

Last edited by a moderator: May 2, 2017
3. Jan 21, 2006

### Janus

Staff Emeritus
Relative to the Planet, yes, its speed would be the same before and after. Its direction will change however. Much the same as a ball bounced off a wall will be moving at the same speed after the bounce as it was before, but in a different direction.

Now imagine what would happen if you tossed the ball in front of a car driving down the road? Let's say you toss the ball at 20mph in the same direction as the car is moving at 70 mph. the car hits the ball with a relative speed fo 50 mph, and the ball rebounds with a relative velocity to the car at 50 mph. The new speed of the ball will now be 100 mph relative to you. you udsed the car to increase the speed of the ball relative to you, but its speed did not change relative to the car.

With probes we do the same thing. Except we use the planet's gravity to whip it around rather than bounce it off.

4. Jan 22, 2006

### tony873004

Here's a diagram showing how Jupiter bends the trajectory of New Horizons. Relative to Jupiter, the inward and outward velocities are the same, but relative to the Sun, they are not.

Color code:
Sun: yellow dot
Mercury: gray
Venus: white
Earth/Moon : blue/gray
Mars: red
Ceres, Pallas, Vesta: white
Jupiter: purple
Saturn: yellow
Uranus: green
Neptune: blue
Pluto/Charon: purple/white
New Horizons: orange

#### Attached Files:

• ###### pluto.GIF
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Last edited: Jan 22, 2006
5. Jan 23, 2006

### DaveC426913

And this is (*) why the old Star Trekkian "slingshot around the Sun" trick doesn't work. You can only slingshot around the planets to gain speed.

(*...one of the many reasons...)

(*gasp* Star Trek uses dodgy physics???)

6. Jan 23, 2006

### uknova

i thought i heard on a scientific documentary that in the future the sun could be used to accelerate a probe to 1/3 of the speed of light or something ridiculous, not possible ?

7. Jan 23, 2006

### franznietzsche

It works just as well as slingshotting around a planet. Relative to the sun there is no speed increase, relative to that nearby star the sun is moving towards already, there is.

8. Jan 23, 2006

### DaveC426913

It accomplishes nothing within the solar system - which is what I was sayin'.

9. Jan 23, 2006

### franznietzsche

Actually, depending on the orbital position of a particular planet, it could increase your speed toward that planet, as the planets are not stationary with respect to the sun.

10. Jan 24, 2006

### tony873004

I wouldn't call that a slingshot, though. I'd want to see some energy added to or subtracted from the orbit.

I agree with Dave. I don't see how anything (internal to the solar system) could be gained by a solar slingshot, unless we're splitting hairs and treating the Sun and the solar system barycenter as seperate, in which case there may be a negligable boost to be had.

Last edited: Jan 24, 2006
11. Jan 24, 2006

### Chronos

It's a math thing. A massive body can impart huge velocity to a less massive body if it has the proper trajectory. It's a simple geometry problem. Apply the approach velocity and mass, and skim just outside the Roche lobe of Jupiter and see what happens.

12. Jan 24, 2006

### Labguy

If the orbital bypass was right, an object could gain some from the Sun's rotational angular momentum. Also, another "gain" in the solar system might not be velocity (read "speed") but could easily be a change in the direction (vector?) of the orbit. Head toward Saturn vs head for Neptune, etc.
As per definition though, a change in the vector is also a change in velocity. Velocity = speed + directional coordinates.

13. Jan 24, 2006

### tony873004

It's not just the change in velocity that makes it a slingshot. Earth's velocity is constantly changing due to its changing direction, and changing speed from its elliptical orbit. But we don't refer to this as a slingshot. It's just an elliptical orbit. The kinetic energy + potential energy is constant the whole time relative to the Sun.
But the planets are all changing their directions, hence velocities, constantly. But we don't refer to these as slingshots. We just call them orbits. After "slingshoting" the Sun, the size, shape, orientation and energy of your orbit remains unchanged. If you happen to be heading towards Saturn rather than Neptune after perihelion, its because Saturn and Neptune have moved. A spacecraft "slingshoting" the Sun will still be tracing the same ellipse it was tracing prior to the "slingshot". I know some spacecraft that head to Venus for a slingshot gravity boost sometimes circle the Sun an extra time to get their intercept timing correct, but this is not considered a slingshot. It's just an extra orbit.

I'm not sure how the Sun's rotational angular momentum would come into play. At least not in any significant manner. I guess the spacecraft would pull a tide on the Sun, and since it outraces its tide, its tide would pull back on the craft and cause it to lose energy. I'm not sure if this is what you are referring to as it is extremely insignificant. Or perhaps frame dragging, but again, extremely insignificant.

14. Jan 24, 2006

### Labguy

I don't recall ever posting anything that was extremely insignificant, except this perhaps...

15. Jan 24, 2006

### tony873004

Then is there something I missed? Is there any significant energy that a spacecraft could draw from the Sun's angular momentum?

16. Jan 24, 2006

### Labguy

There is energy to be gained, but I wouldn't know what you consider significant..?
Are the "shepherding satellites" around Saturn that keep rings in place (F-Ring, I think) significant? Those satellites each trade speeding up then slowing down with respect to each other and with Saturn. Thier significance is the way they weave the F-ring into a spiral, takes energy to do that.
If an object approached the Sun ~parallel to the equatorial plane and approached from a direction toward the surface rotation, it would get a kick, and this could be used for a vector change. To me, that's significant.

17. Jan 24, 2006

### tony873004

Let's just say anything above 10-6 meters/second (1/1000 of a mm/second). I doubt the tide raised by a spacecraft making a close pass of the Sun would alter the craft's velocity by more than this amount. The effect would probably be many magnitudes less than this.

But the Saturn satellites are slingshoting each other. They're not getting a slingshot boost from Saturn. If one of them disappeared, the orbit of the other one would stabalize. A Saturn satellite getting a boost from another Saturn satellite would be comprable to a spacecraft getting a boost from a planet, not from the Sun.

This is what I don't understand. I don't see why it would gain anything from such a passage. Sure, it's going to change direction, but that's just part of being in orbit, not part of being slingshot.

Last edited: Jan 24, 2006
18. Jan 25, 2006

### Labguy

I should have said that "kick" meant a boost in speed/velocity in addition to the agreed-on vector change. It should get a small amount of added velocity from the Sun's angular momentum of rotation if it approached the way I described. This would be above and beyond the simple change in vector from the Sun's gravity. But, I have no idea at all how to calculate how much of a "kick" there would be, but there would have to be some.
BTW, many stray objects (comets, spare rocks, small spacecraft, etc.) passing like this (around the sun) or even around Jupiter for a slingshot, don't end up in an "orbit" but are a one-time pass to oblivion and are put into hyperbolic or parabolic paths never to return. Such as non-periodic comets and the old Voyager spacecraft. They're gone forever.

19. Feb 1, 2006

### OSalcido

isnt the sun orbiting the center of the galaxy? wouldnt the slingshot effect if used with the sun be able to boost a small craft to insane amounts of speed?

20. Feb 1, 2006

### Labguy

No, because we (the Earth) and anything we would launch are also orbiting the center of the galaxy at the same speed as the Sun, +- small variations.

Last edited: Feb 1, 2006
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