Planets giving satellites a boost

For many years, I've been hearing about satellites being aimed near
planets so that they get a "boost" from passing near them. This concept
has always troubled me, and today I tried to reason it logically.

Correct me if I'm wrong, but I don't believe a planet can increase the
speed of a satellite. Certainly, if an object approaches a planet,
gravity will speed it up. However, to my reasoning, as it blasts on
past, it will lose exactly the amount of kinetic energy it gained (or
maybe even a teeny bit more.) I come to this conclusion in two ways.

First, if you could approach a planet and then speed away faster, then
airplanes would not need to keep their engines going, They could just
dive and gain energy (which they do) and then rise and retain the
energy, which is nonsense.

Second, I think this whole satellite / planet idea violates the old
can't-win, can't-break-even, can't-leave-the-game laws of
thermodynamics.

The only benefit I can see from this procedure is you could use a planet
to make a satellite change course. but even this would seem to have no
useful purpose. Axiomatically, isn't a straight path the shortest
distance between two points.

Well, I know they do it, so it must be good for something. What's the
flaw in my reasoning?

Pete

You missed it.

They aim the probes so they dive into the gravity well of the planet, they are
already hitting up to 24,000 mph, and then as they speed by the planet it does
change the corse their on, making their exit steeper and shorter, so by the time
they've passed the planet, they've picked up maybe another 1,000 mph or more.
The big part is the corse change, it gets done without having to burn any fuel
the probe needs later on. The bit of extra speed is mostly for making sure it
gets away from the planet.

The one that's heading for Saturn, took a swing around Venus and two swings
around Earth itself and had one corse change while using Jupiter as the boost.


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"P T" wrote in message
...
For many years, I've been hearing about satellites being aimed near
planets so that they get a "boost" from passing near them. This concept
has always troubled me, and today I tried to reason it logically.

Correct me if I'm wrong, but I don't believe a planet can increase the
speed of a satellite. Certainly, if an object approaches a planet,
gravity will speed it up. However, to my reasoning, as it blasts on
past, it will lose exactly the amount of kinetic energy it gained (or
maybe even a teeny bit more.) I come to this conclusion in two ways.

First, if you could approach a planet and then speed away faster, then
airplanes would not need to keep their engines going, They could just
dive and gain energy (which they do) and then rise and retain the
energy, which is nonsense.

Second, I think this whole satellite / planet idea violates the old
can't-win, can't-break-even, can't-leave-the-game laws of
thermodynamics.

The only benefit I can see from this procedure is you could use a planet
to make a satellite change course. but even this would seem to have no
useful purpose. Axiomatically, isn't a straight path the shortest
distance between two points.

Well, I know they do it, so it must be good for something. What's the
flaw in my reasoning?

Pete



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That's right, _relative to the planet_ a spacecraft's energy gain going in
will be equal to it's energy gain going out, resulting in no change in
speed. What you're missing is that both bodies are also in orbit around the
Sun, so the planet's gravity can drag the spacecraft along and increase its
speed _relative to the Sun_.


"P T" wrote in message
...
For many years, I've been hearing about satellites being aimed near
planets so that they get a "boost" from passing near them. This concept
has always troubled me, and today I tried to reason it logically.

Correct me if I'm wrong, but I don't believe a planet can increase the
speed of a satellite. Certainly, if an object approaches a planet,
gravity will speed it up. However, to my reasoning, as it blasts on
past, it will lose exactly the amount of kinetic energy it gained (or
maybe even a teeny bit more.) I come to this conclusion in two ways.

First, if you could approach a planet and then speed away faster, then
airplanes would not need to keep their engines going, They could just
dive and gain energy (which they do) and then rise and retain the
energy, which is nonsense.

Second, I think this whole satellite / planet idea violates the old
can't-win, can't-break-even, can't-leave-the-game laws of
thermodynamics.

The only benefit I can see from this procedure is you could use a planet
to make a satellite change course. but even this would seem to have no
useful purpose. Axiomatically, isn't a straight path the shortest
distance between two points.

Well, I know they do it, so it must be good for something. What's the
flaw in my reasoning?

Pete

See http://www2.jpl.nasa.gov/basics/grav/primer.html



"Bill Nunnelee" wrote in message
link.net...
That's right, _relative to the planet_ a spacecraft's energy gain going in
will be equal to it's energy gain going out, resulting in no change in
speed. What you're missing is that both bodies are also in orbit around
the
Sun, so the planet's gravity can drag the spacecraft along and increase
its
speed _relative to the Sun_.


"P T" wrote in message
...
For many years, I've been hearing about satellites being aimed near
planets so that they get a "boost" from passing near them. This concept
has always troubled me, and today I tried to reason it logically.

Correct me if I'm wrong, but I don't believe a planet can increase the
speed of a satellite. Certainly, if an object approaches a planet,
gravity will speed it up. However, to my reasoning, as it blasts on
past, it will lose exactly the amount of kinetic energy it gained (or
maybe even a teeny bit more.) I come to this conclusion in two ways.

First, if you could approach a planet and then speed away faster, then
airplanes would not need to keep their engines going, They could just
dive and gain energy (which they do) and then rise and retain the
energy, which is nonsense.

Second, I think this whole satellite / planet idea violates the old
can't-win, can't-break-even, can't-leave-the-game laws of
thermodynamics.

The only benefit I can see from this procedure is you could use a planet
to make a satellite change course. but even this would seem to have no
useful purpose. Axiomatically, isn't a straight path the shortest
distance between two points.

Well, I know they do it, so it must be good for something. What's the
flaw in my reasoning?

Pete

For many years, I've been hearing about
(spacecraft) being aimed near planets so that they get a "boost" from
passing near them. This concept has always troubled
me, and today I tried to reason it
logically.

To put it succintly, the accelerated
outbound leg of the slingshot effect
acquires its velocity from the slower,
energy-gaining inbound leg. Less time
is spent in the outbound leg than in the
inbound. Thus the 'slingshot' advantage.
oc

(P T) wrote in news:6490-4011F56C-110@storefull-
3293.bay.webtv.net:

For many years, I've been hearing about satellites being aimed near
planets so that they get a "boost" from passing near them. This concept
has always troubled me, and today I tried to reason it logically.

Correct me if I'm wrong, but I don't believe a planet can increase the
speed of a satellite. Certainly, if an object approaches a planet,
gravity will speed it up. However, to my reasoning, as it blasts on
past, it will lose exactly the amount of kinetic energy it gained (or
maybe even a teeny bit more.) I come to this conclusion in two ways.

You're wrong about thinking that it never works, but you're right to
recognize limitations. Relative to the planet, the object can't increase
its speed. However, it can increase its speed relative to something
else- the sun for instance.

For it to work, the planet needs to be moving (relative to that
"something else") more or less in the direction that you want the object
to go in, and the object has to initially be moving in another direction.
The object swings around the planet and changes both course and speed.
The planet ends up moving slightly (very slightly) slower and the object
ends up moving faster in a new direction.

--
Steve Gray

Hi PT Think of Apollo 13 having that big problem half way to the moon.
It would have been quicker getting back to Earth using its retro-rockets
to slow it down make a U turn and come back to Earth.That would take a
lot of fuel(energy) Instead they picked up acceleration by falling into
the moon's gravity than made sure they missed the moon and came around
it and picked up speed so they could get back to the Earth, You are
right in thinking that you lose speed if you come back in exactly the
opposite direction. Say you were using the gravity of Jupiter you let
your rocket ship pick up speed and as you come into Jupiter say on its
right side you only come partially around its back side (slight curve)
and then rocket away at say 25 degrees. If you come in at 180
degrees,and make a 180 degree (U turn) you would just go round and round
Bert

Hi PT,

Others have already pointed out that you pick up speed relative to the sun.
As to nothing being free, you are right. There is a tiny change in the
planet's speed as well. But because of the different masses, the change in
the satellite's velocity is significant while the change in the planet is
not even measurable.

Clear Skies

Chuck Taylor
Do you observe the moon?
Try the Lunar Observing Group
http://groups.yahoo.com/group/lunar-observing/

************************************
"P T" wrote in message
...
For many years, I've been hearing about satellites being aimed near
planets so that they get a "boost" from passing near them. This concept
has always troubled me, and today I tried to reason it logically.

Correct me if I'm wrong, but I don't believe a planet can increase the
speed of a satellite. Certainly, if an object approaches a planet,
gravity will speed it up. However, to my reasoning, as it blasts on
past, it will lose exactly the amount of kinetic energy it gained (or
maybe even a teeny bit more.) I come to this conclusion in two ways.

First, if you could approach a planet and then speed away faster, then
airplanes would not need to keep their engines going, They could just
dive and gain energy (which they do) and then rise and retain the
energy, which is nonsense.

Second, I think this whole satellite / planet idea violates the old
can't-win, can't-break-even, can't-leave-the-game laws of
thermodynamics.

The only benefit I can see from this procedure is you could use a planet
to make a satellite change course. but even this would seem to have no
useful purpose. Axiomatically, isn't a straight path the shortest
distance between two points.

Well, I know they do it, so it must be good for something. What's the
flaw in my reasoning?

Pete

On Fri, 23 Jan 2004 22:32:44 -0600 (CST), P T wrote:

For many years, I've been hearing about satellites being aimed near
planets so that they get a "boost" from passing near them. This concept
has always troubled me, and today I tried to reason it logically.
[snip]
Second, I think this whole satellite / planet idea violates the old
can't-win, can't-break-even, can't-leave-the-game laws of
thermodynamics.
[snip]
Well, I know they do it, so it must be good for something. What's the
flaw in my reasoning?

I wouldn't presume to guess just what the flaw in your reasoning is.

The information you're lacking is that the additional energy comes from
the planet. Basically, the spacecraft uses gravity to convert kinetic
energy of the planet into kinetic energy of the spacecraft.

I once saw a calculation showing that the gravity assists inflicted on
Jupiter will cause it to be off by a few millimeters several billion years
from now, when the sun starts expanding into a red giant.


--
- Mike

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