Talk:Gravitational slingshot

From Wikipedia, the free encyclopedia

	This article is within the scope of WikiProject Physics, which collaborates on articles related to physics.	Physics Portal
???	This article has not yet received a rating on the assessment scale. [FAQ]
???	This article has not yet received an importance rating within physics.
Help with this template Please rate this article, and then leave comments here to explain the ratings and/or to identify the strengths and weaknesses of the article.

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. --P3d0 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. --P3d0 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)

Contents 1 Saturn example 2 Limits to slingshot use 3 Speed Relative to a Planet 4 Terminology 5 Merging 6 Powered Slingshot

[edit] Saturn example

For the record, the Saturn example was somewhat imprecise. It claimed the delta-v for the Hohmann transfer from Earth to Saturn is 15.7 km/s. However, this figure disregards two effects that alter the situation drastically:

1. The planets' own gravity. To get from a low-Earth orbit to a low-Saturn orbit actually requires about 18 km/s.
2. Aerobraking. Upon arriving at Saturn, it's only necessary to alter the spacecraft's orbit around Saturn from a hyperbola to an ellipse with a periapsis passing through Saturn's atmosphere; after that, subsequent passes through periapsis will circularize the orbit. Taking this into account leads to a delta-v more like 10 km/s.

I added "disregarding Earth's and Saturn's own gravity wells". (I forgot to mention aerobraking, but I figure that's not worth disrupting the flow of the sentence any more than I already have.) --P3d0 21:09, Jun 18, 2005 (UTC)

For clarity, it ought to be a comparion of the same mission flown with and without the slingshot. What happens at the destination should be the same, whether it's a flyby, braking into orbit with a rocket, or aerobraking. Since the paragraph uses Cassini as an example, what would the total delta-V be, going directly to the same orbit around Saturn? The cited 2 km/s figure obviously doesn't include Cassini's intial boost.

—wwoods 00:04, 19 Jun 2005 (UTC)

[edit] 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. --P3d0 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 atmospher? 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)

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. --P3d0 04:19, 4 November 2005 (UTC)

[edit] 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. --P3d0 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)

[edit] 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)

[edit] 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)

[edit] 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)