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NEOWatcher
2006-Oct-09, 05:22 PM
I pulled out my copy of Cosmos last night for a relaxing evening. During part 4 ("Heaven and Hell") they were talking about long period comets becoming short period comets.

I'm not sure, but I think I saw a bad animation, and I wanted to know if I was correct.

It showed a long period comet coming close to jupiter and being slowed down into a smaller orbit. The problem that I see is that the comet came toward Jupiter from behind, and departed ahead of it. Wouldn't that have caused the comet to speed up and enter a more elliptical orbit?

hhEb09'1
2006-Oct-09, 05:43 PM
It showed a long period comet coming close to jupiter and being slowed down into a smaller orbit. The problem that I see is that the comet came toward Jupiter from behind, and departed ahead of it. Wouldn't that have caused the comet to speed up and enter a more elliptical orbit?JOOC, how did you know that the comet was approaching from behind? Did it show a graphic of Jupiter's orbit?

NEOWatcher
2006-Oct-09, 05:49 PM
JOOC, how did you know that the comet was approaching from behind? Did it show a graphic of Jupiter's orbit?
Yep;

hhEb09'1
2006-Oct-09, 06:00 PM
Yep;with arrows? :)

grant hutchison
2006-Oct-09, 06:18 PM
On its first encounter with Saturn, you can make out that the comet comes from behind and passes ahead of Saturn in its orbit. So the comet loses energy and angular momentum in the encounter: the opposite of a slingshot manoeuvre.
Sagan also describes the movie as a "computer study" rather than an animation, so I guess there's some simulated physics underlying what we see.

Grant Hutchison

NEOWatcher
2006-Oct-09, 06:51 PM
On its first encounter with Saturn, you can make out that the comet comes from behind and passes ahead of Saturn in its orbit. So the comet loses energy and angular momentum in the encounter: the opposite of a slingshot manoeuvre.
Sagan also describes the movie as a "computer study" rather than an animation, so I guess there's some simulated physics underlying what we see.

Grant Hutchison
Oops, Somehow Jupiter stuck in my head, probably because it's so big, and they reference it so much. :doh: Anyway, to my credit, they do show both. :shifty:

Anyway; I thought I had it right in this discussion. (http://www.bautforum.com/showthread.php?t=45508)
I'm thinking vector math. What am I missing?


with arrows? :)
With motion...

grant hutchison
2006-Oct-09, 07:14 PM
Anyway; I thought I had it right in this discussion. (http://www.bautforum.com/showthread.php?t=45508)
I'm thinking vector math. What am I missing?
I don't actually understand what you wrote in that thread, but you wrote it as an explanation for exactly what you've observed in Sagan's simulation: "... heliocentric velocity .... decreases if the spacecraft [/comet] passes in front of Jupiter [/Saturn]"

The comet is overtaking Saturn, so it's moving faster along Saturn's orbit that Saturn is. Saturn slows the comet down a lot, the comet speeds Saturn up a little.

Grant Hutchison

NEOWatcher
2006-Oct-10, 11:46 AM
I don't actually understand what you wrote in that thread, but you wrote it as an explanation for exactly what you've observed in Sagan's simulation: "... heliocentric velocity .... decreases if the spacecraft [/comet] passes in front of Jupiter [/Saturn]"

So it's not the direction of travel, but whether it passes in front or behind the planet? I'll have to do some re-visualizations.

grant hutchison
2006-Oct-10, 12:41 PM
So it's not the direction of travel, but whether it passes in front or behind the planet?I think so.
A prograde object passing behind a giant planet, as in the slingshot manoeuvres familiar from Voyager, is yanked in the direction it's travelling, gaining energy and angular momentum.
But in the case of Sagan's animation, we have an object being hauled in a direction opposite to its direction of travel.

Bit of a coincidence that you should post when you did, BTW. I'd just watched episodes IV and V a couple of nights previously, so not only remembered the animation but still had the disc in the player!

Grant Hutchison

01101001
2006-Oct-10, 01:39 PM
So it's not the direction of travel, but whether it passes in front or behind the planet? I'll have to do some re-visualizations.

Could one of you who has it fresh in mind, please define "in front of" (and/or "behind")? Are they not relative to the position of an observer? I understand the physics, but I don't understand the word pictures.

grant hutchison
2006-Oct-10, 02:40 PM
Could one of you who has it fresh in mind, please define "in front of" (and/or "behind")? Are they not relative to the position of an observer? I understand the physics, but I don't understand the word pictures.In relation to the planet's direction of travel: "ahead" of refers to the leading side, "behind" to the trailing side.

Grant Hutchison

Ken G
2006-Oct-10, 02:52 PM
If the interaction takes a long time, there might be a role to the Sun's gravity during the interaction (i.e. the curvature of Saturn's motion), and that might be complicated, but if the comet is moving fast enough you might want to just imagine that Saturn is moving in a straight line during the encounter. If you do that, then the easiest way to think about the "ahead" or "behind" business is simply in terms of the change in the comet velocity. If that change, which is a vector, lies generally along the direction of Saturn's motion (as when it comes from in front and passes behind Saturn), then it's like a tennis ball bouncing off a tennis racket-- increasing speed. If from behind and passing in front, that's like "catching" the tennis ball on the racket. It all comes from changing the reference frame into Saturn's frame, than back to the Sun frame.

NEOWatcher
2006-Oct-11, 06:50 PM
Obviously I need some fine tuning on my thinking.
Since visuals help me alot...can someone help me with the following image?
speed up? slow down? any specifics?
3719
There are 14 situations here
The green (unumbered) arrow is the planet.
Cosmos illustrated number 2.
In case it's hard to determine which goes with which
(1-3 red)(4 dark blue)(5-6 blue)(7 gold)
I don't even know if 4 is possible since it is not orbiting the sun.

Ken G
2006-Oct-12, 02:29 AM
Situation 2 is a slowdown. Think of Saturn as the tennis racket, the ball comes from behind, bounces off the racket, and glances to the side. That will slow down the ball-- it is being "caught" on the racket. None of those arrows are "speed up" situations, the slingshot effect Grant was talking about, unless there's something subtle about Saturn's deflection around the Sun during a longer-lasting encounter.

NEOWatcher
2006-Oct-12, 02:03 PM
Situation 2 is a slowdown. Think of Saturn as the tennis racket, ...
Interesting... I studied the JPL descriptions again. (http://www2.jpl.nasa.gov/basics/grav/primer.html)
And that's where I'm getting confused. If I take the vector diagrams an turn the planet's orbit so it's going up in the diagram instead of left. Then the vector additions don't make sense to me.

Now if I take the analogies (tennis and train) I picture a line going from the vertex of the path to the focus of the parabolic of the path.
Now if that line is <90 degrees to the planet's path, it gains speed, if that is >90 it loses speed.
3722

(My terminology is rusty, so please forgive)

Ken G
2006-Oct-12, 03:42 PM
You have to look at both the incident and outgoing path, as what really counts is the (vector) change in the velocity of the comet. If that aligns at all with the direction of the planet motion, you get a speedup.

NEOWatcher
2006-Oct-12, 04:13 PM
You have to look at both the incident and outgoing path, as what really counts is the (vector) change in the velocity of the comet. If that aligns at all with the direction of the planet motion, you get a speedup.

I figure I have a heck of a lot of refinement to go, especially considering how different the two speeds may be. But in a very crude sense, am I getting warmer?

Added:
How about this?
If I look at the path of the comet from the point of view of the planet, then I will see a parabola with respect to my view. Therefore I have a parabola that is moving and slightly changing in direction (of its vertex to focus). If the average direction of that line is with the planet, then it speeds up.
(It's hard to put what I'm thinking into actual words)

Ken G
2006-Oct-12, 05:38 PM
Now I see what you mean now about the focus of the parabolic. Yes, I think your understanding in post #15 is right on target.