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JustAGuy
2006-Jan-17, 09:01 PM
Hey folks, I'm having trouble wrapping my head around something gravity-assist related, and thought I'd ask people far more knowledgable than I.

I understand how GA at Jupiter works for increasing-descreasing Sun-relative energy, but what I really need to know is if you can increase/decrease your Jupiter-relative velocity with a Jupiter GA. If this is possible, how, if not, why not?

Thanks.

Saluki
2006-Jan-17, 09:07 PM
Yes. It is a simple matter of transfer of angular momentum. The angular momentum of the spacecraft increases, while the angular momentum of Jupiter decreases. Since Jupiter weighs so much more than the space craft, the change in relative speed is large for the craft, but undetectable for Jupiter.

You can also use gravity assist to slow down, for example to orbit a planet, or to reduce energy so as to reach an inner planet. It all depends on the direction from which you aproach the planet.

NEOWatcher
2006-Jan-17, 09:12 PM
snip
I understand how GA at Jupiter works for increasing-descreasing Sun-relative energy, but what I really need to know is if you can increase/decrease your Jupiter-relative velocity with a Jupiter GA.
GA always works in relation to the body providing the GA.
Jupiter would pull back when the ship is going way at the same force that it pulled it in in the first place. The assist comes in because Jupiter is moving in relation to the original point of reference (the sun).

JustAGuy
2006-Jan-17, 09:17 PM
GA always works in relation to the body providing the GA.
Jupiter would pull back when the ship is going way at the same force that it pulled it in in the first place. The assist comes in because Jupiter is moving in relation to the original point of reference (the sun).
So... it would *not* be possible to alter your Jupiter-referenced momentum via a Jupter gravity assist? Or, ultimately, enter orbit of Jupiter?

ToSeek
2006-Jan-17, 09:54 PM
So... it would *not* be possible to alter your Jupiter-referenced momentum via a Jupiter gravity assist? Or, ultimately, enter orbit of Jupiter?

That's correct (and Saluki is wrong, or just misunderstood the question). Gravity assist works because the spacecraft arrives and leaves the vicinity of the assisting planet with the same speed relative to the planet. If it's moving in the same direction as the planet, then it is sped up. If it is moving in the opposite direction from the planet, then it is slowed down.

In order to alter its speed relative to the planet, it would have to do some sort of maneuver involving the moons of the planet (which Cassini is in fact doing around Saturn by using Titan, though enough just to change its orbit, not enough to get it into or out of Saturn's orbit).

JustAGuy
2006-Jan-17, 10:29 PM
Thanks ToSeek. I had figured this was the case, but needed some confirmation.

I guess a logical extension of this is that it's very hard for a planet to capture moon #1, but much easier for #2...

2006-Jan-17, 10:47 PM
Just to add another GA question here. Has anyone ever considered using the Moon for a GA? Or is it just too small in mass to make it worthwhile?

Cheers

hhEb09'1
2006-Jan-17, 10:54 PM
Just to add another GA question here. Has anyone ever considered using the Moon for a GA? Or is it just too small in mass to make it worthwhile?Does Apollo 13 count? :)

Glom
2006-Jan-18, 12:02 AM
A first order approximation for interplanetary trajectories is the patched conic approximation. This involves considering the isolated two body motion of the satellite around the Sun and the isolated two body motion of the satellite around Jupiter and deciding at which point to flip between them. Basically, when you begin the Jovian conic, you are on a hyperbolic trajectory. The flyby alters your course because you follow the hyperbola and leave the Jovian conic in a different direction. The beauty of this is that when you leave at the hyperbolic excess speed, you are already further out and so have gained energy in the solar conic.

Of course, the rules of two body motion is that energy and angular momentum are conserved. That means that you won't be able to reduce from a hyperbolic orbit at Jupiter to a closed orbit without a burn (or perhaps gravity assist from one of the moons maybe).

Ilya
2006-Jan-18, 02:17 AM
Just to add another GA question here. Has anyone ever considered using the Moon for a GA? Or is it just too small in mass to make it worthwhile?

Yes

http://www-spof.gsfc.nasa.gov/stargaze/Stostars.htm

See the diagram of ICE's leaving Earth-Moon system. It took SEVEN Moon encounters!

Here is a better picture: http://heasarc.gsfc.nasa.gov/Images/misc_missions/isee3_traj.gif

ToSeek
2006-Jan-18, 03:05 AM
STEREO (http://stereo.jhuapl.edu/mission/overview/overview.html) is going to do likewise:

For the first three months after launch, the observatories will fly in an orbit from a point close to Earth to one that extends just beyond the moon. STEREO Mission Operations personnel at The Johns Hopkins University Applied Physics Laboratory, in Laurel, Md., will synchronize spacecraft orbits so that about two months after launch they encounter the moon, at which time one of them is close enough to use the moon’s gravity to redirect it to a position “behind” the Earth. Approximately one month later, the second observatory will encounter the moon again and be redirected to its orbit “ahead” of Earth.

There aren't any diagrams of this yet because the trajectory depends on just when the mission launches. (When I was at APL, one of my coworkers had the full-time job of calculating all the possible trajectories for each launch date.)

NEOWatcher
2006-Jan-18, 01:57 PM
A first order approximation for interplanetary trajectories is the patched conic approximation. snip
Watch what you say, there's children present.http://www.cosgan.de/images/smilie/konfus/k045.gif Not just children, but most adults wouldn't understand a word you said. http://www.cosgan.de/images/smilie/konfus/g060.gif

:lol:

gwiz
2006-Jan-18, 02:16 PM
Just to add another GA question here. Has anyone ever considered using the Moon for a GA? Or is it just too small in mass to make it worthwhile?

Cheers
A few other missions, including the unfortunate Japanese Nozomi Mars probe, the Geotail and Wind missions deep into the geotail, possibly missions to one of the earth-sun Lagrangean points (Genesis used an earth GA) and one communications satellite - Asiasat 3? - that was launched into the wrong orbit and used moon fly-bys to crank its inclination down to near equatorial.

Edit: also the Japanese Hiten technology mission, which was also the first to use aero-braking.

Maksutov
2006-Jan-18, 02:55 PM
Originally Posted by 777 geek
A first order approximation for interplanetary trajectories is the patched conic approximation. snipWatch what you say, there's children present.http://www.cosgan.de/images/smilie/konfus/k045.gif Not just children, but most adults wouldn't understand a word you said. http://www.cosgan.de/images/smilie/konfus/g060.gif

:lol:Perhaps such stuff belongs in the conic pages? A perfect place for various slices of life...

Meanwhile I don't look forward to the day when the Jovians demand we return all that angular momentum we've stolen the past few decades. :shifty:

ToSeek
2006-Jan-18, 03:36 PM
Perhaps such stuff belongs in the conic pages? A perfect place for various slices of life...

Meanwhile I don't look forward to the day when the Jovians demand we return all that angular momentum we've stolen the past few decades. :shifty:

Tell 'em to sue Shoemaker-Levy 9 first. ;)

hhEb09'1
2006-Jan-19, 04:40 PM
Tell 'em to sue Shoemaker-Levy 9 first. Free the S-L 9!

Maksutov
2006-Jan-19, 05:20 PM
Originally Posted by ToSeek
Tell 'em to sue Shoemaker-Levy 9 first. Free the S-L 9!Now that's an attractive slogan!

Right on, brothers Gene and Dave, and sister Carolyn!

And Shoey, Huey, Dewey, and Louie!

http://img295.imageshack.us/img295/5053/donaldduck7yb.gif