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Onwards to Uranus and Neptune!
dvandorn
post Feb 27 2008, 04:05 AM
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My understanding has always been that gravity assists work by robbing a body of a bit of its rotational velocity by flying along with its rotating gravity field. The Sun rotates -- why ought this process not work with the Sun?

-the other Doug


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ermar
post Feb 27 2008, 05:18 AM
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Actually, this is not how gravity assists work - rather, they take advantage of an orbiting body's orbital motion relative to background space. Say a planet is orbiting with a speed V relative to the sun and a spacecraft approaches it (from the planet's point of view) from one side with a speed v. From the planet's perspective, the spacecraft ought to have the same relative magnitude of speed leaving it as approaching it, so the spacecraft leaves with speed v too. From, say, the Sun's point of view, though, the spacecraft approached the planet (which was moving at speed V) with some other speed z; by the time the spacecraft has left the vicinity of the planet, the planet has "dragged it along," and some significant fraction of the planet's orbital speed V is added to the original spacecraft velocity. (This can also be used to decelerate the spacecraft, too, by setting up the initial encounter differently.) The bottom line is that gravity assists depend on the mass and orbital velocity of the "assisting" body alone (robbing it of orbital, not rotational momentum), and have nothing to do with its rotation rate.
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JRehling
post Feb 27 2008, 06:57 AM
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Little physics tidbit: Imagine a spacecraft orbiting the Sun in an elliptical orbit, and imagine that it (for the time being) will not pass closely enough to any planet for them to have a significant pull on it.

The sum of the gravitational potential energy of the craft and its kinetic energy is utterly constant. (This is true of circular, parabolic, and hyperbolic trajectories as well.) What it loses in kinetic energy from perihelion to aphelion (or at any other time) is exactly equal to what it gains in gravitational potential energy. And this will be true forever.

So imagine a spacecraft that has left Earth, and that proceeds in to some closer distance to the Sun, then passes on the other side back to (and then past) the orbit of the Earth. (Again, assume that Venus and Mercury don't get in the way.) Between launch (once it gets sufficiently outside the Earth's sphere of influence) and perihelion, what it gains in kinetic energy will be lost, exactly, in gravitational potential energy. Between perihelion and crossing, again, the Earth's orbit, what it lost in kinetic energy will be gained, exactly, in gravitational potential energy. And moreover, because gravitational potential energy is determined by the distance from the Sun, the kinetic energy of the craft when it crosses the Earth's orbit on the way out will be EXACTLY what it was when it left the Earth in the first place. You gain a net sum of nothing. You may as well have just launched in the outward direction in the first place (at some different launch window).
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Juramike
post Feb 27 2008, 07:18 AM
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OK.

Just to be nitpicky, wouldn't some miniscule amount of energy be imparted on the spacecraft due to the sun's rotation during a "solar dive"?

I would assume this would be similar to tidal effect. In theory, there should be a (teensy) tidal bulge raised on the Sun due to the spacecraft's gravity, and the friction of this bulge on the Sun's surface relative to the rotating surface material should fling the spacecraft forward a teensy bit. [Assuming the spacecraft's approach velocity is smaller than the rotational velocity].
This is similar to the process that is making the Earth slowly fling the Moon to a higher orbit, while the Moon is slowing down Earth's rotation.

(Any math whizzes out there able to calculate the amount of (de/ac)celeration of a typical sized spacecraft from the Sun's rotation field due to tidal effects?)

-Mike


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JRehling
post Feb 27 2008, 09:35 PM
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Tidal effects are harder to model than your typical sophomore calculus class lets you do. I know that in some instances, natural satellites spiral in due to tidal effects and in other cases they spiral out. I guess it's a question of whether the sub-satellite point is tracking on the surface faster or slower than the rotating surface or the speed of a wave through the surface. Something passing at a few tenths of an AU from the Sun would be moving slower, so the tidal bulge could track it fairly well, whereas something speeding over Jupiter's cloudtops would outpace the bulge.

But as for integrating over the density of various layers through the depth of the Sun... sounds like a major research problem.
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Greg Hullender
post Feb 28 2008, 03:22 AM
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Or are you expecting to get a boost from the relativistic frame-dragging effect? That'd be tiny, but compared to the tidal effect caused by the spacecraft on the Sun, . . .

--Greg
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nprev
post Feb 28 2008, 03:34 AM
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Well, clearly, JR answered the question in its fundamental form: there ain't no such thing as a free lunch. We can game planetary orbital motion & rotation only so far; sure looks like genuinely revolutionary propulsion systems are needed (very badly) for outer-system exploration.


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rlorenz
post Feb 29 2008, 02:59 PM
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QUOTE (nprev @ Feb 27 2008, 10:34 PM) *
Well, clearly, JR answered the question in its fundamental form: there ain't no such thing as a free lunch.


I think there has been some confusion here. 2 very different things

1. gravity assist, as people have mentioned, relates to robbing the planets of their angular
momentum (=orbital velocity, since tides are small and the rotational angular momentum of the
planets is tiny anyway)

The way to visualize this is that the planets are big billiard balls, and you can bounce off them (their
gravity fields can be considered the same as an elastic collision). If you bounce off them the right
way, you can get a speed boost at their expense, but it relies on them moving fast.

2. more efficient propulsive manoeuvres deep in the gravity well. If your speed relative to the sun
is A, and your engine gives you a velocity increment B, the energy increment is (ignoring factors
of 2 etc( (A+cool.gif^2 - A^2, or roughly 2AB. So the bigger A is (i.e. the faster you are going
relative to the sun, which is to say, the closer you are to the sun when you make the burn)
the more energy increment your fixed B gives you.

Not a free lunch, but a way of making your lunch more satisfying.
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nprev
post Feb 29 2008, 04:28 PM
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I get it now; thanks very much for that clarification, Ralph! smile.gif

Still sounds like Sun-diving with conventional propulsion systems isn't quite worth it. You'd have to expend a massive amount of fuel just to get into a close solar approach trajectory anyhow, then execute another major maneuver to take advantage of the C/A dynamic.

I don't know; could doing a few pump-down gravity assists with the inner planets make this more feasible? Seems to me that if you can save the maximum amount of thrust for solar C/A you still might be able to make some money here. Not sure if conservation of angular momentum for the entire Solar System wouldn't turn around & bite you in the butt, though.


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punkboi
post Nov 8 2008, 02:56 AM
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Emily posted an interesting blog on Planetary.org regarding a proposed flyby of Neptune that could be done by 2027 if it launched by 2020. Called Argo, it would be the fourth New Frontiers mission if approved it in the next selection process, which is to take place in 2013.

http://www.planetary.org/blog/article/00001729/


The original proposal page:

http://www.lpi.usra.edu/opag/march_08_meet...ions/hammel.pdf


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elakdawalla
post Nov 8 2008, 03:56 AM
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Thanks for pointing to that entry -- but Argo's being discussed actively in this thread.

--Emily


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Guest_Zvezdichko_*
post Jan 7 2009, 10:45 AM
Post #117





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Emily,

You have mentioned that New Horizons may try to photograph Uranus and Neptune. Do you have any information about whether pictures have been taken?
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tedstryk
post Jan 7 2009, 10:52 AM
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http://www.unmannedspaceflight.com/index.p...mp;#entry133721

Alan Stern has stated that they have indeed been taken.


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Guest_Zvezdichko_*
post Jan 7 2009, 11:25 AM
Post #119





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Thanks, Ted.
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briv1016
post Aug 14 2009, 10:21 PM
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I've been following the Review of U.S. Human Space Flight Plans Committee meetings the past few weeks and one of the presentations had a relevant chart regarding the lift capability of the Ares V in regards to unmanned planetary missions. Without going into the taboo discussion of human spaceflight or the likelihood of the Ares V being built in this configuration [or at all], I thought that it would be interesting to include the chart.

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