# Thread: What is "Speed increment at infinity" ??

1. How can I understand?

"Speed increment at infinity"

"Infinity" seems to be a speed measurement point, - by space probe trajectories, - but where is "infinity"

http://en.wikipedia.org/wiki/Flyby_anomaly

2.

3. It's the term used for the speed when the probe is "no longer under the gravitational influence of the Sun".
(Although that description too is somewhat nebulous...)

4. Originally Posted by Dywyddyr
It's the term used for the speed when the probe is "no longer under the gravitational influence of the Sun".
(Although that description too is somewhat nebulous...)
Can you explain that further please

5. Originally Posted by Bjarne
Can you explain that further please
It's the speed achieved once "escape velocity" has ensured that the probe is no longer orbiting the Sun - essentially the speed that it will continue to hold until it "reaches infinity".
(If it were orbiting then its speed would vary, unless it were in a perfectly circular orbit).

6. Originally Posted by Dywyddyr
Originally Posted by Bjarne
Can you explain that further please
It's the speed achieved once "escape velocity" has ensured that the probe is no longer orbiting the Sun - essentially the speed that it will continue to hold until it "reaches infinity".
(If it were orbiting then its speed would vary, unless it were in a perfectly circular orbit).
As I understand this, - the Space probe accelerate due to it enters the strong gravity field close to the earth (by swing by).
Due to the acceleration (towards perigee) escape velocity will be reached before the space probe isreaching perigee, - right?

7. Originally Posted by Bjarne
As I understand this, - the Space probe accelerate due to it enters the strong gravity field close to the earth (by swing by).
There's no necessity for swing by to achieve a speed at infinity. Granted we use it because it's practical.

Due to the acceleration (towards perigee) escape velocity will be reached before the space probe isreaching perigee, - right?
Not quite sure what you're getting at here.
How about: the speed at infinity is the speed of the probe on its way "to" infinity - because the apogee is "at" infinity.

8. Originally Posted by Dywyddyr
Originally Posted by Bjarne
As I understand this, - the Space probe accelerate due to it enters the strong gravity field close to the earth (by swing by).
There's no necessity for swing by to achieve a speed at infinity. Granted we use it because it's practical.

Due to the acceleration (towards perigee) escape velocity will be reached before the space probe isreaching perigee, - right?

Not quite sure what you're getting at here.
How about: the speed at infinity is the speed of the probe on its way "to" infinity - because the apogee is "at" infinity.
Ok, - so the measurement point of "speed at infinity" is actuallythe "apogee speed" ?

9. Originally Posted by Bjarne
Ok, - so the measurement point of "speed at infinity" is actuallythe "apogee speed" ?
Essentially yes, but since the probe never reaches apogee then calling it "apogee speed" would be inaccurate.

10. Originally Posted by Dywyddyr
Originally Posted by Bjarne
Ok, - so the measurement point of "speed at infinity" is actuallythe "apogee speed" ?
Essentially yes, but since the probe never reaches apogee then calling it "apogee speed" would be inaccurate.
I think I begin to understand - for example the Space-probe NEAR (andmany other space-probes) was orbiting the Sun not the Earth, so these space-probes will reach aphelion (not apogee) - I understand that, - but still I am confused to where (approximately) at the orbit is then "infinity speed" ?

11. Originally Posted by Bjarne
I think I begin to understand - for example the Space-probe NEAR (andmany other space-probes) was orbiting the Sun not the Earth, so these space-probes will reach aphelion (not apogee) - I understand that, - but still I am confused to where (approximately) at the orbit is then "infinity speed" ?
Well, strictly speaking, they don't "reach" aphelion, since every time you check they're further away from the Sun, and will continue to increase that distance. They're never coming back. (Would that mean that it's not, by definition, an "orbit"?)
As to "where" the speed is reached that would be, at its most basic, when the final burn (or slingshot) is complete.

Hmm, okay, try this one:
As any probe gets further away its speed reduces - that's basic, since, although it's moving away gravity is also "attempting" to drag it back.
The "speed at infinity" is the final velocity achieved, the one which will never reduce (with respect to the Sun), or, to put it another way, its limiting speed.
There's a mathematical treatment of it here.

12. Originally Posted by Dywyddyr
Originally Posted by Bjarne
I think I begin to understand - for example the Space-probe NEAR (andmany other space-probes) was orbiting the Sun not the Earth, so these space-probes will reach aphelion (not apogee) - I understand that, - but still I am confused to where (approximately) at the orbit is then "infinity speed" ?
Well, strictly speaking, they don't "reach" aphelion, since every time you check they're further away from the Sun, and will continue to increase that distance. They're never coming back. (Would that mean that it's not, by definition, an "orbit"?)
As to "where" the speed is reached that would be, at its most basic, when the final burn (or slingshot) is complete.

Hmm, okay, try this one:
As any probe gets further away its speed reduces - that's basic, since, although it's moving away gravity is also "attempting" to drag it back.
The "speed at infinity" is the final velocity achieved, the one which will never reduce (with respect to the Sun), or, to put it another way, its limiting speed.
There's a mathematical treatment of it here.
OK, I think I begin to understand it, we can shortly say it is the last aphelion , right ? (I will look into the math later, thanks for the help).

13. For a given "definition" of aphelion yes.

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