Yeah, basically you throw a SuperBall at 60mph North, toward a truck that’s heading 60mph South, and the SuperBall ends up going 180mph South (if you ignore air resistance — SuperBalls are quite efficient in their bounce).

Where I put:
> I’m not sure where you’re getting
> “the lowest velocity has the highest orbit”-
> who’s suggesting that?

I should have had:
> I’m not sure where you’re getting
> “The higher the orbit, the greater the velocity.”-
> who’s suggesting that?

Turning to the truck analogy… ok, if it’s a nearly-infinitely-massive truck and a neglibly-massive car, and a 100% efficient trampoline, I can see that it would work that way. I had to force myself to make those assumptions before I could see it.

Did you see my reply about the truck? The analogy works just fine, if you assume a (nearly) infinitely massive truck.

Also, you write, “I’m not sure where you’re getting “the lowest velocity has the highest orbit”- who’s suggesting that?”

Why do you say “who’s suggesting that?”? The numbers you just wrote show that the orbital speed goes inversely with orbital radius (actually goes as r^(-1/2), where r is the orbital separation), just as the claim “the lowest velocity has the highest orbit” suggests.

Assuming your “Does this actually need a reply?” refers to my post 34… well, no-one *has* to reply, but surely sharing understanding with others is what this sort of thread is about? If I’m wrong, I would hope that someone would be able to explain why.

“The higher the orbit, the greater the velocity.
Please list the orbital velocity of each of the planets (or even satellites) and see if I am correct.”

I’m not sure if you’re agreeing or disagreeing. But let’s try it:
http://www.windows2universe.org/our_solar_system/planets_table.html
Merc Ven Earth Mars Jup Sat Ura Nep
mean orbital velocity (km/sec) 47.89 35.03 29.79 24.13 13.06 9.64 6.81 5.43

Higher orbit seems to imply slower velocity, much as I thought.

I’m not sure where you’re getting “the lowest velocity has the highest orbit”- who’s suggesting that?

“Wikipedia is saying that a gravity assist adds the velocity of a space probe plus DOUBLE the velocity of the Planet?”

Wikipedia is indeed saying that- see the first two paragraphs and the diagram under the heading “Explanation”. If Wikipedia is wrong, then it would be good if someone were to correct it.

The point of my lecturer’s question is that the mass of the planet doesn’t really matter- any planet is WAY big enough- but the speed does.

The Cassini graph in Wikipedia, near the bottom of the article, and here
http://en.wikipedia.org/wiki/File:Cassini%27s_speed_related_to_Sun.png
is attributed to JPL, and it shows Venus and Earth affecting the spacecraft’s speed far more than Jupiter. Are you saying the graph is wrong?

I’m not sure your analogy with trucks holds, as it doesn’t appear to represent the kinetic energy that the spacecraft can gain by slowing the planet down.

Best
CJ Nerd.

Yes, you would be going 180 mph! Your relative velocity between you and the truck is 120mph. Perhaps it’s a bit easier to imagine if the truck has a giant trampoline attached to the grill, and you drive right into the trampoline. You come in at 120mph relative to the trampoline, so you’ll go back out (the opposite direction) at 120mph relative to the trampoline. Since the trampoline is affixed to the truck and is traveling at 60mph relative to the ground, you’ll be traveling at (60+120)mph or 180mph relative to the ground after the bounce.

The higher the orbit, the greater the velocity.

Please list the orbital velocity of each of the planets (or even satellites) and see if I am correct.

But to follow this logic, the lowest velocity has the highest orbit? Then all we have to do to place a satellite in geosynchronous orbit would be to throw it into the air a couple of feet!

Wikipedia is saying that a gravity assist adds the velocity of a space probe plus DOUBLE the velocity of the Planet?

Giggle, I would love to see a demonstration of those physics!

Passing a heavy truck in the other lane where each has a speed of 60 mph and throwing out a grappling hook with a very strong cable.

After you spin around and are going in the same direction as the truck, your car would now be going 180 mph?

I DO NOT THINK SO!

I recently attended a talk on this, where the lecturer started by asking the audience to guess if Venus, Jupiter or Saturn was best. Most plumped for Jupiter, a few for Venus.

The correct answer, the lecturer said, was Venus- because it moves faster than the other two, so more change in velocity is available.

In the Wikipedia article
http://en.wikipedia.org/wiki/Gravitational_slingshot
a graph shows that Cassini gained ~9km/sec from two passes of Venus, and just ~1.5km/sec from its one pass of Jupiter.

The total gain AFTER two passes of Venus and one of Earth is about 13km/sec- but that’s AFTER those passes, not FROM them, because obviously falling towards the Sun accounts for quite a bit.

I’d love to understand more about what’s going on in that graph- especially on the first Venus pass. Could you possibly comment?