Talk:Gravity assist/Archive 1

Archive 1

Terminology

Isn't 'gravitational slingshot' the name given to the actual body that does the 'slinging' ? The effect is called the 'gravitational slingshot effect' or, more commonly, the 'slingshot effect'. It certainly isn't " ... the use of the motion of a planet to alter the ...". MP (talk) 18:00, 10 February 2006 (UTC)

Can you provide references for this? I do not recall it being used in this fashion. - JustinWick 22:30, 11 February 2006 (UTC)

Merging

I think that this article should be merged with sling effect, but not with dynamical friction, since the latter deals with a more generic phenomena (the gravitational slingshot is more spaceflight related, dynamical friction to me seems something related to the kinematics of the universe, and requires a different article) // Duccio (write me) 08:55, 16 June 2006 (UTC)

Agreed, the other article is about a physical property, this is about the use of that property and others. But that article needs improvement. --Dhartung | Talk 20:38, 16 June 2006 (UTC)

agreed Sling effect and Gravitational slingshot are the same thing (note the 'sling' in both)~Sushi 06:12, 19 September 2006 (UTC)

"""agreed""" - JustinWick 00:17, 8 December 2006 (UTC)

Farscape

In the Science Fiction show, Farscape, I believe the character John Crichton was going to attempt a powered slingshot around the Earth in the first episode, and later (in the same episode I think) he piloted 'Moya' into a slingshot to escape from their pursuers. I think the characters made a big deal about it not being possible, and they were going to crash, too. I don't quite remember the specifics, so I won't add this to the Popular Culture section myself, but someone more familiar with the show might want to. --Gero 04:53, 29 January 2007 (UTC)

Simpler diagram and explanation

I find the diagrams and explanation of gravitational slingshots at http://www.mathpages.com/home/kmath114.htm much easier to understand, and suggest that a similar approach should be used here.

I also suggest some restructuring:

• Definition, as in the existing article.
• The problem which needs to be solved: even a Hohmann transfer would require too much fuel if the target is too distant or its speed is very different from that of the starting point (2nd paragraph of the current "Background" section.
• (Over-)simplified diagram and explanation of the concept, like the 1st diagram at http://www.mathpages.com/home/kmath114.htm, plus internal link to more general mathematical explanation at the end of the article.
• Examples: Cassini probe, Ulysses.
• Powered slingshots, but with the maths deferred to the end of the article.
• Limitations - similar to the current article, plus: limited windows because celestial bodies are rarely in the most convenient positions.
• More generalised explanation of unpowered slingshot, with maths.
• Maths of powered slingshot.
• In popular culture.
• See also, references, links

The main aim of this structure is to convey the main ideas to less technically inclined readers, then provide the technical details for those who are interested. Any comments?Philcha 20:41, 16 February 2007 (UTC)

What is "swing-by"?

I am discussing in the German Wikipedia about the term "swing-by". In our arcicle the flight of Apollo 13 is mentioned when they used the moon for their return. But in my opinion the term "swing-by" only means to use the gravity of a moon/planet to "steal" energy from it and to accelerate a spacecraft with this energy. Is this correct or wrong? --193.57.156.241 09:51, 1 March 2007 (UTC)

I have not heard the term "swing-by". Maybe you mean fly-by. According to the disambiguation page, when applied to spacecraft that does mean a slingshot maneuver. 64.160.39.153 09:25, 17 March 2007 (UTC)

It's an interesting point. Marinar 10 in 1974 is generally credited with being the first to use the slingshot effect. But the Apollo lunar missions were designed to use a "free return trajectory" that would swing around the Moon and get back to Earth without a major rocket burn if they did not go into lunar orbit. Apollo 8 was the first mission to actually do this. Apollo 13 made a burn behind the moon, a preplaned proceedure, known as a "post pericynthion abort." But this was only to reduce the time needed to return to Earth by about a day, as their supplies were limited. It could be argued that this is an example of a powered slingshot. It's a question of definition really. The orbital mechanics is the same. --agr 00:58, 8 April 2007 (UTC)

Gravitational slingshot for decelerating?

All the common examples describes the use of gravitational slingshot to gain speed. Is it possible to use gravitational slingshot to decelerate? If so, can anyone cite examples?Philcha 19:15, 6 April 2007 (UTC)

It certainly is possible. If you were entering the Earth-moon system you could do a reverse slingshot past the moon to lose speed. I don't know if it has ever been done for real though.WolfKeeper 19:44, 6 April 2007 (UTC)

Two examples: The first use of the gravitational sling shot was Marinar 10 in 1974 on the way to Mercury. This is an example of using the effect to reduce orbital velocity in order to get to a lower obrit. NASA's MESSENGER spacecraft to Mercury will use " one Earth flyby, two Venus flybys, and three Mercury flybys before orbit insertion on March 18, 2011." --agr 00:44, 8 April 2007 (UTC)

Thanks! I've added both examples.Philcha 21:12, 10 April 2007 (UTC)

Phrase in first sentence

I agree with other commentators who have indicated this article is difficult for the layperson (non-physicist) to understand. That said, we should resist the temptation to "dumb the article down" so much that it is inaccurate! My present concern is with a phrase in the first sentence, which talks about, "the use of the gravity of a planet or other celestial body". I agree the gravity of the planet is important, but as all the explanations show, the velocity of the planet is also required. That is, if the planet isn't moving, there's no gravity assist. Is there general agreement about the importance of the planet's velocity? Is there agreement the article's readers would benefit from having this piece clarified in the lead section? (sdsds - talk) 23:58, 15 July 2007 (UTC)

Not a slingshot

Individuals have often compared gravity assist with the effect of a slingshot. In terms of physics, though, gravity assist is a different example. Consider using a slingshot: a person would sling a projectile around a few times, stronger and better aimed each time, before letting it go. Upon letting go, the projectile's centrifugal force becomes its propulsive force. On the other hand, using a gravity assist, a spacecraft comes up and steals some angular momentum during a single flyby of a planet in motion, removing momentum from that planet. Gravity assist is really much more like a baseball connecting with a fast-moving bat than it is like a slingshot. Gravity Assist Primer - JPL

Taka2007 22:00, 15 October 2007 (UTC)

I agree this should be changed in most places on wikipedia where it is used. Special:Whatlinkshere/Gravitational_slingshot gives a list of those.... (sdsds - talk) 06:50, 16 October 2007 (UTC)

Considering the fact that Taka2007 self-evidently doesn't even know what a slingshot is, I would respectively suggest that he leave the article alone. It doesn't really matter if it makes sense, it is notably called the 'slingshot effect', and I'm certain we can find references to it being called that.WolfKeeper 11:20, 16 October 2007 (UTC)

I think Taka2007 is referring to a Sling (weapon) and "shot" (e.g. Lead shot) -- an easy enough mistake to make. Ewlyahoocom 02:11, 19 October 2007 (UTC)

Words clearly missing from lead

As it stands, the following makes little sense: "a gravitational slingshot or gravity assist is the use of the gravity of a planet or other celestial body to alter the path and speed of a spacecraft. Passing by such a body imparts some fraction of that to the spacecraft." Some fraction of what? The "that" doesn't seem to refer to anything in the previous sentence, so I guess that during the history of this article, something has been removed from the lead but the "that" reference to it has not been. Is it a reference to momentum or kinetic energy, for example? Either the word "that" needs to be replaced with whatever the "that" was meant to refer to, or something needs to be added to the previous sentence so that the "that" actually refers to something which has already been established. Unless I'm misreading the lead entirely, which is also possible? TheGrappler 03:05, 8 November 2007 (UTC)

I think it's meant to be "speed". I agree that it's unclear. --Doradus 05:41, 8 November 2007 (UTC)

I'm no physicist but... bouncing?

It seems to me the second paragraph of this page doesn't make sense: "A slingshot maneuver around a planet changes a spacecraft's velocity relative to the Sun, even though it preserves the spacecraft's speed relative to the planet (as it must do, according to the law of conservation of energy). To a first approximation, from a large distance, the spacecraft appears to have bounced off the planet (physicists call this an elastic collision even though no contact actually occurs)."

If the spacecraft 'appears to bounce off the planet' then surely it's velocity relative to the planet cannot remain constant?

(True, but the paragraph you quoted says "speed", not "velocity". --Doradus 05:46, 8 November 2007 (UTC))

It's exactly the same, due to conservation of energy, it's an elastic collision. (Masses haven't changed, potential energy is the same at any given distance, hence speed is same.)WolfKeeper 15:35, 14 July 2007 (UTC)

I find the last paragraph of http://www.space.com/scienceastronomy/solarsystem/gravity_assist.html a much more believable explanation of the conservation of energy as it relates to gravitional slingshots. It states that the increase in the velocity of the spacecraft is accompanied by a tiny reduction in the velocity of the planet. According to that page, cassini's fly-by of earth in 1999 will alter the earth's position by 15 inches over the next billion years.

That's from the point of view of the Sun though. From the point of view of the planet, the speed is *necessarily* exactly the same, due to conservation of energy.WolfKeeper 15:35, 14 July 2007 (UTC)

OK, I'm not saying you're wrong - i'm just trying to understand this. I would have thought the principle of the conservation of energy was a statement about the total momentum and not 'relative speeds'. Consider the following.

(Conservation of energy and momentum applies in all inertial reference frames. --Doradus 05:46, 8 November 2007 (UTC))

Imagine I'm a stationary pool ball and another pool ball hits me a slightly oblique angle. Previously it was coming towards me at a certain speed (quite fast). After the collision we're both travelling along slightly diverging paths. So, after the collision it's travelling away from me at a different (lower) speed. So it's speed relative me has changed - right? Ignoring friction, etc, I guess the principle of the conservation of energy tells us the the total momentum of both balls is the same after the collison as before it. Masses haven't changed, total momentum hasn't changed, but the speed of one ball relative to the other has?

Where am I going wrong here?

You need to (or at least it's easiest to) look at it from the center of mass frame. In the case of a planet and a probe; the center of mass frame is overwhelmingly aligned with the planet, because the planet is stupidly heavier (and it's similar with the bat and ball, because the bat is typically rather heavier). In the case of the snooker balls, that approximation is not at all valid, so you need to look at it from the point of view of the frame where the balls are moving in opposite directions with equal and opposite speeds. Then it all works out fine.WolfKeeper 23:27, 15 July 2007 (UTC)

Right... like a bat and a ball. Can't really claim to understand the details but am prepared to accept what you say is true. (How gracious of me :-) Found the bit on the wikipedia page about elastic collisions where it says "The velocity of the center of mass does not change by the collision" kind of helpful, sort of knew this was true from high school (and from snooker balls) but hadn't heard it expressed like that before. I do think that including on the page the fact that, when viewed from the point of view of the sun, the planet has been slowed down by a tiny, tiny amount will help people understand. Also, I think emphasising the idea of different frames of reference (with links to other wikipedia pages on the subject perhaps) will help non-phyicists. cheers dude.

Pop culture cruft

In my opinion the "In popular culture" section is typical of pop culture sections and is in need of pruning and close maintenance. Gravity assist is used so often in sci-fi that this section is already bloated and almost certain to grow out of hand if every example of its use is included. We have notable real-life examples, why load up an otherwise scientific article with a lot of fiction? Kevin Forsyth (talk) 16:12, 16 October 2008 (UTC)

• The removal of this section seems like an act of vandalism; pop culture is important because it reaches the public. Anyone who watches a cartoon with a gravity assist could look here for a scientific explanation of the phenomenon. Albmont (talk) 13:15, 24 March 2009 (UTC)

Miscellaneous discussion

In the second paragraph under the heading "explanation", it says "with respect to the planets surface" This is not correct as all planets rotate as well as revolve. I think it would be more correct to say "with respect to the panets center of mass" or "with respect to the poles of the planet"66.177.105.109 (talk) 12:16, 16 July 2010 (UTC) I am unclear regarding the following dates; they appear to create a contradiction: "That discovery was made by Michael Minovitch in 1961. The gravity assist was first used in 1959 and relied on research performed at the Department of Applied Mathematics of Steklov Institute." If the discovery ws made in 1961, how could it have been used (except accidentally) in 1959? —Preceding unsigned comment added by 74.171.140.198 (talk) 10:59, 30 June 2008 (UTC)

Yes, this is unclear and the problem seems to be that Minovitch did not discover the application of gravity assist to spacecraft. It was known well before 1961. The document "Instrumented Comets - Astronautics of Solar and Planetary Probes, " 8th International Astronautical Congress (Barcelona 1957) by Krafft A. Ehricke states, "This method of gravitational navigation does not, of course, violate the law of conservation of energy, since the orbit of the perturbing body is changed likewise by the vehicle, the changes in orbital energy and orbital elements being, of course, in proportion to the perturbing masses. Thus, obviously the Moon or a planet is not perceptibly perturbed by the vehicle. If after an encounter, orbital energy and eccentricity are increased and if the eccentricity of the original vehicle was high, then the new orbit can be changed into a hyperbola with respect to the given center of attraction. This instantaneous change in orbital energy during the encounter can be determined very easily." And he goes on to describe the mathematics. It is clear by his description and figures that an understanding of gravity assist was already established before Minovitch.

Derek Lawden in England also clearly described the application of gravity assist well before Minovicth. Lawden is typically cited as being the first. Additionally the idea was developed in Science Fiction well before that. This link gives some examples that far predate 1960 http://www.daviddarling.info/encyclopedia/G/gravityassist.html

The number of authors who recognized the gravity assist phenomena on comets is vast. For more information on all of this see: AIAA paper 2001-0176, "From instrumented comets to grand tours - On the history of gravity assist" by Dr. Gary Flandro. Quorsav (talk) 16:25, 17 February 2009 (UTC)

I may be misinformed about this phenomenon, but didn't Apollo 13 make at least a partial slingshot around the moon in order to return to Earth? —Preceding unsigned comment added by 98.28.179.6 (talk) 01:38, 23 June 2008 (UTC)

I dont understand this - maybe some words are missing:

Upon arrival, the gravity of Jupiter bent the up and over the planet back towards the sun.

All this required was the amount of fuel needed to send Ulysses to a point near Jupiter, well within current technologies.

---

What's the limit boost from a slingshot? lysdexia 09:08, 28 Oct 2004 (UTC)

The limit is the planet's own velocity. If the body were a point, and the spacecraft came within an infinitesimal distance of it, then the boost would be exactly the planet's velocity. --Doradus 21:09, Jun 18, 2005 (UTC)

Twice the planet's speed, WRT the Sun.

               Sun's FoR           Planet's FoR
                       _                    _
Initial:        .  <--(_)            .-->  (_)

                       _                    _
Final:    <-----.  <--(_)         <--.     (_)
                ^                           ^
                spacecraft                  planet

—wwoods 00:04, 19 Jun 2005 (UTC)

Sorry, my bad. --Doradus July 5, 2005 21:20 (UTC)

I added some text to the article. User:Ray Van De Walker

Some of what you added is pretty questionable, so I removed it for now. Specifically, this:

The proof is that the path of light is always a geodesic, an "inertial straight line". A mass following the local geodesic experiences no forces. At the Schwarzschild radius, light travels in a circle, so an inertial straight line is a circle. At the Schwarzschild radius the inertia of the "orbiting" spacecraft's mass therefore does not oppose the centripetal force of gravity: A stable orbit is impossible. As orbits approach the Schwarzschild radius, the orbiting spacecraft would require increasing amounts of energy to escape. At some point, the needed escape energy will be greater than the energy that could be added by a slingshot.

From my pretty basic knowledge of general relativity, that seems quite nonsensical. First of, geodesics for light and matter are different. Second, light geodesics are not circles at the Schwarzschild radius - IIRC, they can be are circles at 3/2 of the event horizon radius. Depending on your defintions, the Schwarzschild radius may be the same as the event horizon radius.

--130.232.31.109 21:50, 9 Jun 2005 (UTC)

Limits to slingshot use

Perhaps someone more able can clarify this:

"The main practical limit to the use of a slingshot is the size of the available masses in the mission."

Surly the most obvious limit is the relative speed of the planet and the projectile. As the speed of the projectile increases, the potential benefit of a gravitational slingshot is reduced. And, of course, a projectile moving faster than a planet will actually be slowed down.

No, there's no reason it would necessarily be slowed down. --Doradus 04:19, 4 November 2005 (UTC)

"Another limit is caused by the atmosphere of the available planet. The closer the craft can get, the more boost it gets, because gravity falls with the square of distance. If a craft gets too far into the atmosphere, the energy lost to friction can exceed that gained from the planet."

Is this really significant? I can see that a point mass is the ideal subject for a gravitational slingshot, but given the scales on which these things are happening, a planet is still a very small thing. Anyway, would it not be better to say that it is the overall size of the planet rather than its atmosphere? Gas giants are pretty much all atmosphere and rocky planets have a vanishingly thin layer of gas.

Gaius Cornelius 8 July 2005 16:49 (UTC) Likely the atmosphere of Earth is significant even when the gravity assist maneuver comes no closer than 1000 kilometers of Earth's surface. Not very significant, I agree. At 50 kilometers, above the surface of Earth, air friction losses would exceed any delta v gained. Worse, the spacecraft might disintegrate due to the stress. I don't think a point gravity would allow "unlimited" magnification of thrust, but perhaps twice as much thrust.66.177.105.109 (talk) 12:48, 16 July 2010 (UTC)

I'm a bit lost -- what is your objection here? The statement is true as it stands. Planets definitely are not point masses from the slingshot perspective; especially powered slingshots, where point masses would allow an unlimited magnification of thrust. --Doradus 04:19, 4 November 2005 (UTC)

First use?

The article lists both Luna 3 and Mariner 10 as the first use in different sections of the article. As a reader, I don't know which is correct, so I leave it up to someone knowledgeable to correct that. —Preceding unsigned comment added by Taed (talk • contribs) 12:07, 2 October 2010 (UTC)

Change in Kinetic Energy

The statement is made: "kinetic energy is proportional to the vehicle's velocity at the time of the burn". I don't believe this is exactly true. I believe it is approximately true for the case where v >> DELTA_v. For Kinetic Energy (scalar): DELTA_E = (1/2)m[v'^2 - v^2]

       = (1/2)m[(v + DELTA_v)^2 - v^2)
       = (1/2)m[2*v*DELTA_v + DELTA_v^2]

Look at the two terms in the square brackets. The first term is the "proportional to v" term. The second term is non-zero. 128.183.48.149 (talk) 16:12, 16 August 2011 (UTC)

A very helpful explanation: elastic rebound & the double ball drop experiment

I've visited this article perhaps three times in the last year, but I was not able to wrap my head around this concept until I appreciated that the slingshot effect works identically to an elastic rebound. While this concept is mentioned in our article, the explanation at [1] is much stronger. It begins by discussing how elastic rebound works (which is a highly intuitive process), and then it shows why the gravitational assist is simply a variety of elastic rebound. It would be great if the wikipedia article could be reorganized in this fashion.

The above-mentioned webpage happens to be a subpage of the "Slingshot Effect" website, which is already the first item of our ==External links== section ... however, because this was so helpful, I gave it a separate line in the External Links section. 75.4.201.146 (talk) 21:47, 10 September 2011 (UTC)

The tennis ball is an orange

The tennis ball in the picture with the train is actually an orange. Consider:

• Orange color
• Sunglasses
• Sun halo and rays around it

These are all orange iconography, not tennis ball iconography! — Preceding unsigned comment added by 67.149.59.83 (talk) 16:21, 28 December 2012 (UTC)

Speed Relative to a Planet

Isn't this line incorrect? I'm pretty certain, but don't feel I'm qualified to edit the page:

"A slingshot maneuver around a planet changes a spacecraft's velocity relative to the Sun, even though it preserves the spacecraft's speed relative to the planet (as it must do, according to the law of conservation of energy)."

Isn't the speed of the spacecraft relative to the planet changed as you're either stealing energy from the planet or donating?

From the planet's perspective the satellite would increase velocity, but the rest of the solar system would decrease in velocity, as the planet is now travelling slower. Sure the changes of the percieved velocity of the Sun and planets would be nigh-imperceptible, but the energy would be conserved.

Seems like the Explanation section makes the same erroneous claim.

No, the statement is correct. The spacecraft first falls toward the planet, picking up speed, and then receeds, thus losing the exact same amount of speed with respect to the planet. No energy changes hands in the frame of reference of the planet. The situation is different in the reference frame of the Sun. --Doradus 00:52, 18 January 2006 (UTC)

That's right, there is no change in planet's frame of reference but I guess in order to get the boost in the Sun's frame of reference the planet must slow down. Since the planet is so heavy, the slowing down is miniscule but there must also be conservation of energy in the Sun's frame of reference so the slingshot is in fact taking energy from the planet. I think the explanation is not clear enough. Poszwa 16:46, 20 January 2006 (UTC)

To clarify: it's not right, but it's a close enough approximation. Even in the absence of an atmosphere, tidal forces due to the difference in gravitational attraction on different parts of the spacecraft will result in the loss (or gain; see below) of small amounts of energy by the craft. Also, the bit about 'as it must do, according to the law of conservation of energy' is a bit silly: does that mean if the craft hits the planet head-on and comes to a complete stop, it's violating the law of conservation of energy? What if the craft scrapes against it/partially enters an atmosphere on the way by and is slowed down? And my craft launched off of a planet; clearly I didn't maintain the same speed relative to it. (In fact, if I recall correctly, doing a gravitational slingshot around (say) a neutron star that was rotating quickly enough in the correct direction would actually speed you up a fraction, due to tidal effects.) None of this violates the conservation of energy... you are taking energy out of one part of a system and putting it into another. —Preceding unsigned comment added by 72.5.239.5 (talk) 20:04, 6 May 2011 (UTC)

It's not correct, and I disagree about it being close enough. It's not as though it's some deliberate approximation, it's completely extraneous and incorrect. The conservation of energy with the craft gaining speed is most easily shown with the conservation of momentum (ignoring small losses due to atmospheric friction, etc). Momentum is defined as p=m*v. The planet slows down fractionally, but because of its enormous mass results in a large change in momentum. This momentum is transferred to the craft, whose small mass means it gains a large Δv. Momentum is conserved, energy is conserved, and the craft does gain speed relative to the planet. I'm removing it from the article. SKoOctopus (talk) 22:06, 19 February 2013 (UTC)

Archimedes Screw Graviton Model Provides Simple Understanding Of Slingshot Effect

If one imagines that gravity is due to clockwise spinning right-handed Archimedes screw particles, which emanate out from the Earth in a straight line, then this explains the force relative to our planet when they interact with another body. When Juno approaches Earth towards the Sun, the gravitons also have a velocity of the Earth relative to the Sun. This 'extra' motion is then transferred to Juno as it passes close by, which accelerates the spacecraft relative to the Sun. 176.24.226.120 (talk) 11:27, 14 August 2013 (UTC) Alan Lowey

Can someone find a better illustration for the Cassini probe than this...

...  ?

There should be corners in the trajectory at the planetary flybys and the trajectory should look mostly straight in the space between planets. 70.109.180.73 (talk) 22:19, 2 April 2013 (UTC)

I wonder if a poor illustration communicates the wrong thing and is worse than no illustration at all. 70.109.180.73 (talk) 22:19, 2 April 2013 (UTC)

While you may have a point about "corners in the trajectory", the rest is not "mostly straight", due to orbital dynamics (see Hohmann transfer orbit). It was based on this image published by NASA itself: http://saturn.jpl.nasa.gov/photos/imagedetails/index.cfm?imageId=776 so unless you can find an even more reliable source, I would not change it. cmɢʟee୯ ͡° ̮د ͡° ੭ 16:51, 4 April 2013 (UTC)

Actually I'm dubious about the "corners in the trajectory" point as well, that's not how gravitational assists work. Gravitational assist boosts the orbital velocity of a spacecraft, creating larger, more elongated orbits. To have rapid changes in probe direction would defeat the point of using the gravitational potential energy to raise the probe's orbital velocity. As far as I can see the NASA image is a reasonably accurate representation. ChiZeroOne (talk) 17:26, 4 April 2013 (UTC)

Corners and straights?

You're right, ChiZeroOne. I'm embarrassed to have made this mistake in my reply. Perhaps I was unduly influenced by this graphic that shows corners and mostly straight segments, especially in the dashed line: http://voyager.jpl.nasa.gov/gallery/images/spacecraft/tour.jpg — did NASA err in this graphic? cmɢʟee୯ ͡° ̮د ͡° ੭ 20:45, 16 April 2013 (UTC)

I still need to do the math, but I am pretty sure both diagrams are correct. Taking the Jupiter flyby as an example: In the case of Cassini, it did not require a huge boost of energy to get to Saturn's orbit, so the deflection angle is not that large and not immediately visible in the diagram. In the case of Voyagers, both probes picked up enough energy at Jupiter to exceed escape velocity from the solar system, so the deflection angles would have to be much higher. The reason the trajectories appear straight in the Voyager diagram is very probably because the trajectories between planets were hyperbolic, which appear almost straight when looking at short segments.Basically, the Cassini path is essentially a series of elliptical trajectories stitched together while the Voyager paths are a series of hyperbolic trajectories.

Still looking for Cassini numbers, but voyager orbital elements are available at http://voyager.jpl.nasa.gov/science/hyperbolic.html . Jaxcp3 (talk) 15:38, 14 August 2013 (UTC)

Powered Slingshot

I have had it suggested that the powered slingshot should not work, as the acceleration/deceleration felt by the satellite as it approaches the planet should be the same as that felt as recedes from the planet. The acceleration due to gravity is the same regardless of mass. Therefore the overall speed should not be changed regardless of the satellite's mass. I am unsure of how to make a simple arguments as to why powered slingshots work. In a linear sense the argument about acceleration kind of makes sense, but, as these are vector quantities, direction is also important? Could someone help me out? Kenneth McKee 9th Oct 2006

I think you should look at everything in terms of potentials rather than forces. The added rocket burn increases the overall mechanical energy of the rocket at periapsis without affecting its potential energy. Thus the change in energy as the rocket escapes the slingshotting body is unaffected by the rocket burn. Of course this energy is being measured in the reference frame of the planet, so this may not be a helpful explaination. Maybe this will inspire someone to create a better answer? - JustinWick 09:53, 7 December 2006 (UTC)

There's a physics puzzle I heard of once, where you compare a rocket launched from ground level, with one rolled down a (frictionless) U shaped ramp that goes below ground level- at the bottom the rocket lights and rides up the ramp into the air. If you do the calculation, the one that rolls down the ramp goes higher! The reason is that the propellant is burnt at lower potential energy, where it has more speed (kinetic energy), which it gives up to the rocket- in other words it goes backwards less fast relative to the ground after being exhausted, and so the rocket goes forward faster and ultimately gains more altitude due to conservation of energy. This is *exactly* the same thing as the powered slingshot, gravity forms the frictionless ramp.WolfKeeper 10:43, 7 December 2006 (UTC)

That's not a bad way to put it. I wonder if this should be addressed in the article? Cool physics tidbit, my classes didn't cover that particular gem. - JustinWick 23:13, 7 December 2006 (UTC)

Also remember that the way one looks at this effect depends greatly on what frame of reference is chosen. Strictly speaking, the planet's frame of reference is an accellerating reference frame, so it's dangerous to treat it as an inertial one without taking into account "ficticious forces." - JustinWick 23:33, 7 December 2006 (UTC)

The sentence on aerobraking in the background section seems wrong. The article on aerobraking discusses several spacecraft that have used this manuver. Perhaps some rewording is in order? Fetch 16:23, 19 January 2007 (UTC)

The statements made about kinetic energy in this section are all true and correct, but they have nothing to do with why the powered slingshot works. It works simply because speeding up at closest approach reduces the time gravity will slow you down on departure in comparison to the time it sped you up on approach. The statement about rocket speed being subtracted from propellant speed being the reason for conservation also is technically correct but the implied argument that this explains the slingshot breaks down when rocket is travelling faster than propulsion speed. — Preceding unsigned comment added by 2600:1003:B120:9200:27AC:C907:4A05:4BF5 (talk) 08:10, 10 October 2013 (UTC)

Repeate

This bit "Another close terrestrial analogy, demonstrating deceleration, is provided by a tennis ball bouncing off a moving train. Imagine throwing a ball at 130 mph toward the back of a train that is moving away at 50 mph. The engineer of the train sees the ball approaching at 80 mph and then departing at 80 mph after the ball bounces elastically off the back of the train. Because of the train's motion, however, that departure is at 30 mph relative to the station." appears in the article twice in different sections. 10:49, 10 October 2013 (UTC)194.176.105.153 (talk)

1959 or 1961?

Used in ’59, discovered in ’61? If usage was accidental, please could that be said explicitly. If usage deliberate, then the dates clash. JDAWiseman (talk) 09:47, 18 November 2013 (UTC)

The gravity assist maneuver was first used in 1959 when the Soviet probe Luna 3 photographed the far side of Earth's Moon. The maneuver relied on research performed at the Department of Applied Mathematics of Steklov Institute.<ref>Egorov, Vsevolod Alexandrovich (1957) "Specific problems of a flight to the moon", Physics - Uspekhi, Vol. 63, No. 1a, pages 73–117.

Lagrange L4 and L5: a question

Small masses can orbit around the L4 and L5 Lagrange points. So, being non-technically simplistic, there is, sort of, a virtual mass at those points. Can a spacecraft do a gravity assist around those virtual masses, even if a very weak assist?

If that works, please could that be said explicitly? If that works but it is weaker by a ratio of formula, perhaps that formula could be given, with numerical examples of Sun-Jupiter, Sun-Earth, and Earth-Moon. JDAWiseman (talk) 10:20, 18 November 2013 (UTC)

To add anything to Wikipedia you need a reference that it is treated in that way. It doesn't work in any normal sense, there's no virtual mass there, they are energy maxima rather than the minima as you would get with a mass.GliderMaven (talk) 12:51, 18 November 2013 (UTC)

To add anything to Wikipedia one needs somebody who understands the subject, who, as part of that understanding, would have suitable references. In this subject, that definitely isn’t me — hence the question. Meanwhile, an energy maximum makes sense, as it explains why only stable because of the Coriolis effect. But near an energy maximum is an energy slope. Except that, presumably (“definitely isn’t me”), near a mass the slope is steep, near a non-mass maximum the slope is very gentle. Which might mean no assist. Rightly or wrongly, this might have talked me out of it. (And if wrongly, please don’t be shy about correcting me. I’m writing in the talk page because I know that I don’t understand.)

But since the question is of interest to this non-expert, it might be of interest to other non-experts. Perhaps a one-sentence mention could be considered. JDAWiseman (talk) 19:44, 18 November 2013 (UTC)

Black Hole Slingshots

Kip Thorne writes in his book "The science of Interstellar" page 279 about this wikipedia entry, that the statement "if a spacecraft gets close to the Schwarzschild radius of a black hole, space becomes so curved that slingshot orbits require more energy to escape than the energy that could be added by the black hole's motion" is wrong. Kip seems to be an well known expert on this stuff. I don't understand why he didn't changed the entry himself, but maybe somebody with some understanding of the subject can correct it. — Preceding unsigned comment added by 84.118.138.139 (talk) 19:10, 15 March 2015 (UTC)

Would a 180 degree course change maneuver really work?

There's a picture in the article, showing a spacecraft approaching a planet from head-on, swinging around it, and then emerging going in the same direction as the planet, but with its speed increased by twice the speed of the planet.

Would that really work?

This simulator: http://www.messenger-education.org/Interactives/ANIMATIONS/grav_assist/gravity_assist.html

Shows gravity assist being done by passing behind the planet, allowing it to tug on the probe for a while, increasing its velocity.

I would have guessed that approaching the planet from head-on would rather slow down, than accelerate the probe. My argument is that approaching head on, you would be subjected to accelerating force (approaching the planet) for a short time, but a decelerating force (while leaving the planet), for a longer time.

Could a real physicist shed some light on this? — Preceding unsigned comment added by Avl (talk • contribs) 16:14, 6 August 2014 (UTC)

The picture for the "simplified example" is misleading; I would suggest it be replaced with one that is equally simple, but with a hyperbolic trajectory. A 180 degree turn as shown right now is not possible, nor is the perfect semicircle on the right side. If an object's orbit has a perfect semicircle in it, then (regardless of the side of the planet) the entire orbit must be perfectly circular. Thus it would not fly of and escape. The trajectory will always be hyperbolic too, so nowhere can it be a straight line. LuxArdens (talk) 14:47, 15 April 2016 (UTC)

I think this image is misleading to the point of being unhelpful. I suggest it be removed. Asgrrr (talk) 01:57, 17 June 2016 (UTC)

We've gone back and forth at least a few times on "mass produces" versus "mass-produces"

The mass is spinning and it produces frame-dragging, hence "spinning mass produces frame-dragging." That is very different from spinning, which some would have us believe produces frame-dragging on a massive scale, as in "spinning mass-produces frame-dragging." Let's keep the former, with "mass produces" unhyphenated. Nathaniel3W (talk) 15:22, 3 October 2016 (UTC)

Lunar gravity assists

Only Stereo A/B are noted as using lunar gravity assists (+/- 1.8 km/s). What other craft have used lunar gravity assists ?
Why isn't the moon always used when leaving earth orbit (1.8 km/s seems useful) ? - Rod57 (talk) 14:48, 17 January 2018 (UTC)

[2] says it imposes tight time constraints on launch windows - need to look for RS - Rod57 (talk) 15:20, 17 January 2018 (UTC)

The Cassini probe – multiple gravity assists - Saturn example

The average orbital speed of Saturn is 9.69 km/s which isn't even close to 5 km/s. The craft would have speed equal to the orbital speed only when it closes to Sun (or moves in the oposite direction) and that would be 9.69 km/s. The lowest speed is when the probe is moving in the oposite direction to the Saturn direction on its Sun orbit (Saturn orbital speed - probe speed in relation to Saturn). — Preceding unsigned comment added by 194.66.32.16 (talk) 17:26, 21 June 2011 (UTC)

Cassini slingshot details

The slingshot effect Johnson says first Venus flyby added 7 km/s and Earth added 5.5 km/s - Rod57 (talk) 21:29, 13 February 2018 (UTC)

Graphics in "Why gravitational slingshots are used"

The section "Why gravitational slingshots are used" shows a plot of voyager heliocentric speed versus distance from the sun, and also shows the escape velocity of the sun. However, in the first section between earth and Jupiter, voyager's speed declines faster than the sun's escape velocity. Shouldn't the difference between speed and escape velocity remain constant, or am i missing something? LuxArdens (talk) 14:47, 15 April 2016 (UTC) — Preceding unsigned comment added by 80.138.234.160 (talk) 21:12, 11 January 2014 (UTC)

Yes it would be great to have that explained. - Rod57 (talk) 21:35, 13 February 2018 (UTC)

Multiple assists from the same object

Cassini used Venus twice. Galileo used Earth twice. Rosetta used Earth three times. Messenger used 3 Mercury flybys before it entered Mercury orbit. What is the practical limit to the number of times or max delta-V that one object can provide ? - Rod57 (talk) 16:46, 17 January 2018 (UTC)

Cassini managed 127 Titan encounters and got a significant gravity assist from all of them (I've just added this.) As long as you leave on a trajectory which will take you back, there isn't any real limit on the number of encounters. But there are limits on what you can do with them. With only two massive bodies involved (e.g. the Sun and the Earth) there is a conserved quantity (combination of orbital elements) which a flyby will not change. It's called the Tisserand invariant (or parameter) and was originally derived for comets having encounters with Jupiter. Changing a spacecraft's Tisserand parameter takes a rocket burn (which is why a deep space maneuver usually shows up between launch and a Earth flyby) or an encounter with a third massive body. I'll add something about this to the article. Fcrary (talk) 22:16, 14 July 2018 (UTC)

Earth gravity assists

Cassini, Galileo and Rosetta have used gravity assists from Earth itself (eg approx a year after launch from earth) - Can we have an explanation of how or to what extent that can help an earth launched craft ? - Rod57 (talk) 15:17, 17 January 2018 (UTC)

The details aren't clear to me, but the key factor here is the sun. The initial launch results in the solar orbital paths of the satellite and earth to differ enough to alter the satellites orbital path relative to the earth, increasing the effect of gravity assist. Rcgldr (talk) 17:32, 29 March 2018 (UTC)

The reason why this works is that "Gravity Assist" does not use gravity. "Gravity Assist" is a bad and very misleading term. "Gravitational Slingshot" is much better. The space vehicle does not use the planet's gravity but its motion around the sun. And it can use it again and again if it is approaching the same planet several times. -- Wassermaus (talk) 20:33, 1 September 2018 (UTC)

There is a subtle point here and the article may not capture it. An Earth encounter changes the direction but not the magnitude of the spacecraft's velocity relative to the Earth. But the spacecraft's initial velocity relative to the Earth is set by the launch trajectory. To first order, that constrains the magnitude of the velocity but the direction is anything you want. And when it comes back for a flyby it would come back with the same velocity it originally left with.

If you want the spacecraft leaving in the direction of the Earth's orbital velocity, launch so that the outbound asymptote is over dawn. If you want it leaving in the opposite direction, you just launch 12 hours later and have the outbound asymptote over dusk. That's functionally equivalent to launching in the wrong direction and the using an Earth flyby to rotate the direction of the relative velocity. So what's the point of the flyby?

In practice, and except for some special cases, spacecraft do something between launch and the Earth flyby. Either an encounter with another planet or a large deep space maneuver. That lets it return with a different speed from its original, post-launch asymptotic speed. And that's where the value of an Earth encounter comes from.

Unfortunately, that description sounds too wordy and too much like original research for an article (even for this one, which has more than its share of stylistic problems.) Can any one think of a good, succinct way to describe this? Fcrary (talk) 21:19, 1 September 2018 (UTC)

Need for super-simple explanation

A while ago I composed a super-simple explanation of gravitational slinghshots ([3]) - the basic explanation is over-simplified, but that version admitted that after explaining the basic principles. I think the explanation produced by recent edits is a lot more difficult for lay readers - it uses two frames of reference and says without further explanation that vector addition shows how the spacecraft gains speed.

I think the fundamental question is, "What is the article's target audience?" IMO, it's readers with minimal scientific or mathematical knowledge, since more advanced readers will go straight to textbooks on orbital mechanics. What does everybody else think? Philcha 12:45, 17 April 2007 (UTC)

That's understandable. I added the 2-D version because the diagram was missing when I came across the page last week, and it was difficult to understand how that simple example relates to the motion of spacecraft (I was similarly confused by my first physics textbook, which had the 1-D version). I suggest that we move the explanation to the end, and add the super-simple 1-D version, but keep the 2D version as it is currently. Also, I would argue that the 1D version isn't much easier than the 2D version, but we should probably add it. I'll get started.Tac2z 20:08, 17 April 2007 (UTC)

The diagram was missing because it was "speedy deleted" despite the fact that I changed the licensing details after uploading and getting a warning. I've re-loaded it at [[4]] and used it in Interplanetary travel.

I still think my version was simpler, but I like the fact that you've added a "1-D" diagram which illustrates the basic principle better than the "2-D" one.

On reading the article, I also like your decision to move the "Explanation" section so that all the theory is together and is preceded by reasons and examples - I think your arrangement will appeal more to non-specialists .Philcha 10:14, 18 April 2007 (UTC)

I also find the current presentation confusing: the ball-train example appears to be in conflict with the main story. In the 1D example, a head-on collision gives the ball greater kinetic energy and the tail-gate collision less energy. In the 2D or 3D example, the spacecraft is said to gain kinetic energy when travelling with the planet. Inala (talk) 22:57, 30 January 2019 (UTC)

Beresheet does not use a gravity assist.

This is the second time someone has added Beresheet as an example of a spacecraft using a gravity assist. I've reverter the change both times because it isn't true. Beresheet does use a low energy trajectory, but that three-body orbit is not the sort this article describes. If fact, the sort of gravity assist described here can never put a spacecraft in orbit around the second body (e.g. the Moon.) Fcrary (talk) 20:50, 17 March 2019 (UTC)

You are absolutely correct. Perhaps a comment in the wikitext could prevent further additions? — JFG ^talk 10:51, 18 March 2019 (UTC)

Gravity assist "first used" on Luna 3 is a bogus claim

I see no legitimate basis for this claim that Luna 3 was the first use of gravity assist. Gravity assist is a free delta-v maneuver which can be used for the purpose of advantage in trajectory design, typically reducing propellant requirements. What happened with Luna 3, as far as I can tell, is that the mission planners merely accounted for the unavoidable trajectory change that happened because of the probe's encounter with the Moon's gravity well. That is quite a distortion of what the term 'gravity assist' means. I see no indication that the Luna 3 mission used the gravity-induced trajectory change to their advantage. Rather, they merely adapted to the necessary impact that the Moon's gravity well had on their trajectory. And that is clearly not a gravity assist. In order to count as an assist, the delta-v has to actually help something. Not just be a nuisance effect which needs to be countered by proper planning. It has been recommended that this gets removed from the Luna 3 article, and unless anyone can provide support for the view that this is a valid claim, then this likewise needs to be removed from this article here.

Because this issue is specific to the Luna 3 mission, a new Talk section has been posted over on that article:

Luna 3: "First_gravity_assist"_-_a_specious_claim

For anyone who has input to this discussion, it will be helpful if we consolidate all of our discussion over there. -- Tdadamemd19 (talk) 08:14, 14 July 2019 (UTC)

I think Voyager 1 and Cassini are counter examples. Both had encounters with undesirable changes to the trajectory due to flybys, but those encounters are commonly referred to as gravity assists. In the case of Voyager 1, the delta v from Saturn encounter precluded continuing on the Uranus and Neptune. That was deliberate and necessary for a close Titan encounter. In the case of Cassini, encounters the Titan and even Enceladus did not always have beneficial effects on the trajectory. For example, to satisfy science requirements for polar coverage while keeping the spacecraft on a low inclination orbit, they had to design alternating north and south pole encounters. There were other cases where Titan encounters jerked the trajectory around in ways no one particularly wanted or liked. But those were all described as gravity "assists." I've never heard anyone talk about gravity "hinderances."

Also, in the case of Luna 3, I think it could fairly be called an assist. They targeted the encounter so the Moon's gravity would put them on a desirable post-encounter trajectory. Specifically one that facilitated communications. Sure, if the Moon hadn't perturbed the spacecraft's orbit, that might have been just as good. But they did consciously target do the delta v from the encounter would help rather than hurt. Fcrary (talk) 19:20, 14 July 2019 (UTC)

Historical origins

I think the article should mention that in popular culture the slingshot is considered to be Arthur C. Clarke's invention, because he was the first to use the term in a popular medium (2001) and had some influence in several missions that used it.

anonymous, 00:26:03 UTC, 2008/05/19

Michael Minovitch was not the first person to realize that a spacecraft could use the gravitational assist. This link gives some examples that far predate 1960 http://www.daviddarling.info/encyclopedia/G/gravityassist.html

Quorsav (talk) 20:00, 13 February 2009 (UTC)

Minovitch might have been the first to develop practical algorithms and computer codes for gravity assist planning. Heavily used for the Voyager mission planning. - Rod57 (talk) 14:37, 29 November 2019 (UTC)

When

In the text I found this:

"That discovery was made by Michael Minovitch in 1961.[5] The gravity assist was first used in 1959 and relied on research performed"

How it's possible it has be used before discovery? Maybe I miss something.... Sorry for my bad english.

Indaco1 (talk) 03:22, 16 April 2008 (UTC)

Minovitch developed practical computer codes to plan gravity assists, and may have discovered the planetary alignments later used for Voyager. - Rod57 (talk) 14:41, 29 November 2019 (UTC)