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Author Topic: How does a gravitational sling shot accelerate a space probe?  (Read 19603 times)

Offline chris

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We often hear space scientists say that they've guided a probe past one or two planets on its way to get some "gravity assist" to speed its trajectory. But if a planet is attracting a probe by gravity and thus speeding it up, why doesn't the probe slow down again as it passes the planet and begins to move away again?



 

Offline dentstudent

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How does a gravitational sling shot accelerate a space probe?
« Reply #1 on: 06/11/2008 08:19:03 »
I think that it does, but the difference is minus the direction of the planet from which the sling-shot is received. If the planet were stationary, then the forces would equal out, but because there is the planet's rotational velocity around the sun, this has an effect on the residual sling-shot. Also the direction from which the satelite approaches the planet will have an effect - i believe that the sling-shots used are where the probe is catching up to the planet in the same direction as the planets orbit, and will therefore have a long pull effect from the planet. If the probe were to approach the planet in the opposite direction to the planet's orbit, there would be only a very small pull effect, and also a rather long slowing effect on the probe as it went past the planet.
 

Offline DoctorBeaver

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How does a gravitational sling shot accelerate a space probe?
« Reply #2 on: 06/11/2008 12:32:17 »
from http://www2.jpl.nasa.gov/basics/grav/primer.html

This article was concerned with the Voyager missions.



The planet's own motion is a key. A gravity assist with Jupiter involves not a stationary planet as considered above, but a planet with enormous angular momentum as it revolves around the Sun. In the diagram at right, Jupiter's motion along its solar orbit has been illustrated with a vector colored red (simplified, of course: Jupiter revolves along an arc, not a straight line. Imagine the Sun situated below the bottom of the diagram). The spacecraft acquires this Sun-relative vector, or a significant portion of it, during its interaction with Jupiter.

You can see how the red vector is added to VIN and VOUT. The resulting vector shows how the spacecraft's velocity, relative to the Sun, takes on a nice boost from Jupiter. Notice how rotation of the vector from Vin to Vout (the bending of the spacecraft's path by the planet's gravity) helps increase the result. This trajectory bending is the other key.

The spacecraft is a physical mass, so it has its own gravitation. That's how the spacecraft can tug on Jupiter and actually decrease the planet's orbital momentum by a tiny amount. In the exchange, the spacecraft acquires momentum from Jupiter ó a significant amount, compared to the momentum the spacecraft already had.
 

lyner

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How does a gravitational sling shot accelerate a space probe?
« Reply #3 on: 06/11/2008 17:54:43 »
A simple way to visualise this:
When a rocket gets near Jupiter it is almost captured by the gravitational field; you can imagine that it is briefly in orbit around it. The orbit is VERY elliptical - in fact, you arrange it to have a hyperbolic orbit (just 'grazing') so it can be a one-off interaction. It is following Jupiter around whilst it is captured so it leaves Jupiter at the same relative speed as it approached. So it ends up going at its original speed PLUS Jupiter's speed. Jupiter, of course, slows down a bit (immeasurably). Free momentum, if you like. I should mention that there is a certain amount of deflection of the spacecraft's path. But that is taken into account in the navigation.
Yes, you are right to say that its speed when it is actually passing very close to Jupiter is even higher. But you couldn't make use of this - the extra kinetic energy it gets when it's close is used up in 'climbing back out' of Jupiter's gravity.
« Last Edit: 06/11/2008 18:08:53 by sophiecentaur »
 

Offline Soul Surfer

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How does a gravitational sling shot accelerate a space probe?
« Reply #4 on: 06/11/2008 23:27:36 »
There is also another factor that you are not considering.  This is a three body problem.  You need to consider the sun in this equation.  mefore it entered jupiters main sphere of influence the spacecraft was in a particular orbit round the sun.  If jupiter was not there it would have contiued on its elliptical track.  the interaction with Jupiter has accelerated and decelerated the spacecraft as it approaches and recedes from it.  this may have changed its velocity a bit but more importantly it has significantly changed the direction of motion.  This change of direction has radically changed the orbit of the spacecraft around the sun even if the actual velocity has not changed significantly.
 

Offline DoctorBeaver

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How does a gravitational sling shot accelerate a space probe?
« Reply #5 on: 06/11/2008 23:48:12 »
Ian - the latest craft sent to Pluto (I can't remember the name offhand) got a gravity assist from Jupiter that increased its speed by 9,000kph to 54,000kph.
« Last Edit: 11/11/2008 08:09:06 by DoctorBeaver »
 

lyner

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How does a gravitational sling shot accelerate a space probe?
« Reply #6 on: 07/11/2008 11:30:43 »
speed and direction are, of course, both important. What the craft got from the exercise was, basically, some free energy. The clever thing is to choose the timing of leaving Earth so that the rendezvous with Jupiter is appropriate for the optimum conditions for 'navigation'.
Yes it is a multi body problem. It's lucky, though, that you can mostly ignore Jupiter except when the craft is relatively close - at which time you can ignore the Sun and talk in terms of an appropriate energy boost.
 

lyner

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How does a gravitational sling shot accelerate a space probe?
« Reply #7 on: 07/11/2008 14:28:42 »
SS
Quote
this may have changed its velocity a bit but more importantly it has significantly changed the direction of motion.
You mean 'speed' in that post. Velocity includes speed and direction.

Actually, I would take issue with the details of your idea.
Without Jupiter, you could choose to point your craft in a range of directions (limited by the energy you have on board). It is the energy (KE / speed) which you get from Jupiter which gives you greater choice of directions / orbits.
Merely increasing your speed without a change of direction, at the time, still lifts you into a higher energy orbit; your apogee increases but you would return to the same point later in your orbit.
The direction in which you leave Jupiter will affect the eccentricity of the resulting orbit.  The argument is that, in orbit, you have so much PE and so much KE and the total energy stays the same. At your apogee, your KE  is at a minimum and your PE is a maximum. A radial course (the extreme case) would give you maximum radius but the KE at the top would be zero, so you would come straight back into the Sun. A tangential course would give the least PE gain (lowest orbit). I think the orbit times would be the same in all cases(?not sure about that).

To lift into a higher circular orbit, you need two boosts - a small one (not much KE needed) at the apogee, to keep you there.

Multiple slingshots, using several planets, could give you escape velocity from the Sun but I don't think Jupiter is enough (is it?).
« Last Edit: 07/11/2008 14:36:57 by sophiecentaur »
 

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Offline ZHUYH

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How does a gravitational sling shot accelerate a space probe?
« Reply #8 on: 11/11/2008 07:01:03 »
Shrunk
Hi  everyone!
 "gravity assist"  maybe  vortex force.

the sun, planets and the moon, radius ,the rotation cycle, quality, spin, and several satellite tables:
 Name     radius r --- ----- rotation cycle T --- quality M --- Scroll of Ft = KG ω Mm / r ^ 2 --- number of satellite
 Earth ----- 1.0 -------- 1.0 -------- 1.0 -------------- 1.0---------- ----- 1
Sun ----- 109 -------- 25 --------- 3 * 10 ^ 5 ----------- 1.01-------- ------ 9
Moon ----- 0.272 ------ 30 --------- 0.012 ------------ 0.0054------------ 0
Mercury ----- 0.38 ------- 59 --------- 0.055 ------------ 0.0065------------ 0
Venus ----- 0.95 ------- 244 -------- 0.82 ------------- 0.0037------------ 0
Mars ----- 0.53 ------- 1.026 ------ 0.11 ------------- 0.38-------------- 2
Jupiter ----- 11.2 ------- 0.41 ------- 318 -------------- 6.18------------ -- 61
Saturn ----- 9.4 -------- 0.44 ------- 95 --------------- 2.44---------- ---- 31
Uranus --- 3.8 -------- 0.7 -------- 14.5 ------------- 1.43------------- -22
Neptune --- 3.9 -------- 0.76 ------- 17 --------------- 1.47------------ -- 12
 Pluto --- 0.50 ------- 6.4 -------- 0.17 ------------- 0.106------------- 1

Contrast planets, we can easily find : Quality similar to the planets, large vortex force of the owner of the satellite and more, but less or no satellite. Vortex force of Jupiter's largest, with the largest satellite in planets; Saturn's spin of the second row, the number of satellites also ranked second; Although the quality of Mars than Venus, Mars due to the vortex force of less than 1%, so not only no natural satellites, and satellite access even more difficult than Mars.
 

lyner

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How does a gravitational sling shot accelerate a space probe?
« Reply #9 on: 11/11/2008 13:13:16 »
Quote
"gravity assist"  maybe  vortex force.
How would that be?
The dynamics of this method are described very accurately using Newtonian mechanics and the practical results agree with theory. To include your force, you would have to DECREASE the other forces and energies involved. But there is ample evidence that the existing forces and energies do the job so where does your idea fit in? They can't both be working at once.
In addition to inventing a complete, self-consistent system (and it doesn't appear that that you have),  you need to explain the shortcomings of the accepted model.
Go on then!

 

Offline ZHUYH

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How does a gravitational sling shot accelerate a space probe?
« Reply #10 on: 12/11/2008 02:03:37 »
This new force is a force accompanying Gravity. Vorticity can explain many phenomenons such as Sunís and Earthís oblateness, planetsí precession(Earth, Venus, Mercury), Lense-Thirring effect, Pioneers 10 & 11 deceleration, Earthís rotation deceleration(0.0016sec per hundred year), Moonís orbit change, solar systemís angular momentum transfer, and planetsí rings and satellites formation. A formula derived from Vorticity can well explain polar orbiting satellitesí precession: Ft=KGqMmωcosθ/R^2 (q=1-ωp/ωe, ωp=satellite orbital revolution velocity, ωe=Earth spinning velocity).
  "gravity assist"  maybe  vortex force.

 

lyner

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How does a gravitational sling shot accelerate a space probe?
« Reply #11 on: 12/11/2008 08:01:43 »
OK but, if this is an extra force then the other forces must be smaller. These (conventional) forces are accurately described in the absence of rotation so where do they go when there is some rotation?
 

Offline ZHUYH

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How does a gravitational sling shot accelerate a space probe?
« Reply #12 on: 14/11/2008 03:01:12 »
quote
[The dynamics of this method are described very accurately using Newtonian mechanics and the practical results agree with theory.]

  Must comply with the conservation of angular momentum and energy  ,The additional angular momentum and energy   may be the  vortex force.
 

lyner

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How does a gravitational sling shot accelerate a space probe?
« Reply #13 on: 14/11/2008 11:15:55 »
ZYUYH
But, in your other thread, you have quoted the vortex force as only being worth 1/2000,000 of the gravitational force. That won't supply enough energy to make a slingshot manouver worth doing.
 

Offline ZHUYH

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How does a gravitational sling shot accelerate a space probe?
« Reply #14 on: 17/11/2008 06:56:17 »
the vortex force as only being worth 1/216,000 of the gravitational force[earth]
Fn = GMm / r ^ 2 
Tangential component [vortex force] Ft = kGMm ω Cos α / r ^ 2
 By the formula, we can see that:[earth]

 Ft = k ω Cos α Fn
Cos α=1,ω = 1/ 24◊3600[cycle / sec] , K = 0.4 for one factor,the unit:  sec/cycle 
Ft = [0.4/ 24◊3600]Fn

   =Fn/216,000
  is  not:            1/2000,000

 
 

lyner

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How does a gravitational sling shot accelerate a space probe?
« Reply #15 on: 17/11/2008 08:50:50 »
Apologies; I see that I slipped an extra  zero in my post. The question still stands, though.
Can you tell me how this tiny force can make a significant difference to an interplanetary orbit?
 

lyner

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How does a gravitational sling shot accelerate a space probe?
« Reply #16 on: 17/11/2008 10:07:48 »
Having re-read some of the posts I can see that it would the the 'spin force' from Jupiter which should be considered - could you tell us what that would be?
But are you saying that the 'conventional' effect wouldn't happen at all?

Why not establish some cred by using 'omega' to represent angular velocity, as normal and give it units of radians per second?
If you don't, you could be out by a factor of Pi, in fact it looks as if you are. Any comment?
« Last Edit: 17/11/2008 14:31:52 by sophiecentaur »
 

Offline ZHUYH

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How does a gravitational sling shot accelerate a space probe?
« Reply #17 on: 18/11/2008 07:38:29 »
"On 4 December 1974, a year and a half after launch, Pioneer 11 passed to within 34,000 km of Jupiter's cloud tops, taking fantastic images of the Great Red Spot and measuring the mass of the moon Callisto. Using a gravity assist, it continued on to Saturn.

Pioneer 11 passed by Saturn on 1 September 1 1979, reaching to within 21,000 km of the cloud tops. The first probe ever to orbit Saturn,"
www.fxcm.com [nofollow]

At a rough estimate  :

  Pioneer  11, for example, if it passed  1,000,000 km from Jupiter's orbit in a year ,rely on Jupiter's rotation  the vortex ,Pioneer  11 can get about 45 m/s
speed.
 

lyner

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How does a gravitational sling shot accelerate a space probe?
« Reply #18 on: 18/11/2008 09:22:53 »
Quote
Pioneer  11, for example, if it passed  1,000,000 km from Jupiter's orbit in a year ,rely on Jupiter's rotation  the vortex ,Pioneer  11 can get about 45 m/s
speed.
And are you saying that the normal increase in KE, which you'd expect from the change in GPE, doesn't happen?
 

Offline Soul Surfer

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How does a gravitational sling shot accelerate a space probe?
« Reply #19 on: 18/11/2008 09:51:54 »
ZHUYH  how do you explain why the scientists got the results dead right without having to make use of this mysterious vortex force you are talking about.  It is true that rotating gravitating objects do distort space time but the distortion for normal speed objects is tiny and is what gravity probe B (look this up) is trying to measure using one of the most precisely made satellites ever put into orbit and several years of observation.

The oblateness of jupiter and the earth is also adequately explained using perfectly conventional dynamics
 

lyner

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How does a gravitational sling shot accelerate a space probe?
« Reply #20 on: 18/11/2008 12:43:50 »
But, don't you realise, SS, that the mundane, accepted (and largely correct) explanation to any phenomenon is just not glamorous enough.

Amazingly, if Zhuyh's idea ever gets generally accepted (to whatever degree it actually applies) it will be dropped like a stone and dismissed as 'ordinary Science'.
Some other fringe idea will take its place.
You can't argue with them because the knowledge which puts people in space and allowed the Information Explosion is too much like hard work. It requires rigour and strict method; not favourites with the fringe-Science brigade.
 

Offline Soul Surfer

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How does a gravitational sling shot accelerate a space probe?
« Reply #21 on: 18/11/2008 23:09:17 »
LOL
 

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