Thinking about orbital mechanics makes my head hurt ...

[larper]

One final way to think of escape velocity....<br /><br />If a small mass started out at some extreme, but finitely large, distance from a large mass, with a zero velocity relative to the large mass, the small mass will fall towards the large mass (yes, they are really falling towards each other) and impact at, you guessed it, escape velocity.<br /><br />So, drop a ball from very far away, it will hit the earth at 25,000 mph. <div class="Discussion_UserSignature"> <p><strong><font color="#ff0000">Vote </font><font color="#3366ff">Libertarian</font></strong></p> </div>

 

[frodo1008]

Of course, long before you reached this infinite distance you would be under the influence of other gravity fields, and therefore affectively out of the Earth's influence. However, theoretically I can indeed see your point.<br /><br />That would then mean that (to varying degrees) we are under the influence of every piece of matter in the universe. Interesting thought!

 

[frodo1008]

So, let me understand this here. The obiter actually fires its OMS engines to come out of orbit in the direction opposite to the then current flight direction to actually increase velocity. This then lowers the current orbit until the obiter then hits the atmosphere in such a manner as to use the thicker atmosphere to slow down below orbital velocity until the orbiter is eventually at zero velocity on the ground.<br /><br />WOW, I can now see why there is a very severe heating problem with the obiter coming back from space! Just how fast is the velocity at which this slowing affect happens?

 

[frodo1008]

OK. That is actually what I originally thought.<br /><br />I thank you very much for your patience and kindness in sharing your considerable knowledge with me. This also goes for the others that have done so!<br /><br />I am also grateful to askold for starting this terrific thread!<br /><br />To me at least this is what this most fantastic of sites is all about!<br />

 

[comga]

"When the astronauts threw the object backwards from the ISS - what is the resultant path of the junk?"<br /><br />There are some simple equations for small changes to orbits. They are called the Clohessey-Wilshire equations. <br /><br />As described by others, if you start with a circular orbit and "throw an object backwards" that is in the anti-velocity direction, the orbit becomes an ellipse with the high point (apogee) at the radius of the original orbit and the low point (perigee) slightly lower. This reduces the orbital period. Therefore, when it gets back to the apogee after one orbit, it is ahead of the larger craft, here the ISS. The thrown object would appear to loop below the ISS and move further ahead of it each orbit.<br /><br />If the object has a lower ballistic coefficient, that is less mass per unit area, it will be decelerated by air drag at a greater rate than the ISS. Therefore its altitude will drop faster, and it will move farther ahead in the orbit. Eventually, the ISS will reboost, (check out the graph of ISS altitude at www.heavens-above.com or http://www.hq.nasa.gov/osf/station/images/issalt.gif<br />as the thrown object gets lower and lower and reenters.

 

[frodo1008]

Thanks! That certainly seems reasonable that they would want to ensure that whatever they were getting rid of stood NO chance at all of ever coming back to in any way endanger the ISS itself!!

 

[racer7]

This is an interesting thread. My thanks to everyone who has contributed. It however has made me wonder about the Moon. Is its orbital velocity constant? How about its velocity in respect to the sun? I think I know how it has to work, but it's kind of interesting to think about.

 

[MeteorWayne]

No, it's orbital velocity is not constant since it is in an elliptical orbit around the earth-moon barycenter. so it moves fastest at perigee (closest to earth) and slowest at apogee, just as the earth moves fastest at perihelion (closest to sun) in January and slowest in July at Perihelion.<br /><br />And of course the moons motion is a combination of the earth-moon barycenter's orbit and it's orbit around that. <div class="Discussion_UserSignature"> <p><font color="#000080"><em><font color="#000000">But the Krell forgot one thing John. Monsters. Monsters from the Id.</font></em> </font></p><p><font color="#000080">I really, really, really, really miss the "first unread post" function</font><font color="#000080"> </font></p> </div>

 

[nrrusher]

The Moon is also moving at slightly greater than true orbital velocity, as it is moving farther away at a decreasing rate of a few centimeters per year or there abouts, and was once much closer.<br /><br />Has anyone seen/played with those big bowl-like funnels at restaurants that have a "launch" at the top for the quarter that they want you to donate to some cause? When I first tried to figure out orbital mechanics I couldn't help thinking of that. Except, don't think of a quarter, but rather a ball bearing as the mental image is better.<br /><br />If you wanted to keep the bearing from falling into the hole, you would need to keep giving is little boosts as it came around (approximating atmospheric drag with friction) and if you wanted it to fly out of the funnel, just give it to much of one. The closer to the hole, the more boost you need to give it to fly out, and the farther from the hole, the less...etc.<br /><br />Anyway....that always helps me....

 

[MeteorWayne]

Actually, less than it's true orbital velocity, since it is moving away toward a slower orbit.<br /><br />I knew about that, but it is a VERY small effect, so I dedn't mention it. <div class="Discussion_UserSignature"> <p><font color="#000080"><em><font color="#000000">But the Krell forgot one thing John. Monsters. Monsters from the Id.</font></em> </font></p><p><font color="#000080">I really, really, really, really miss the "first unread post" function</font><font color="#000080"> </font></p> </div>

 

[larper]

No, its going slightly faster than true orbital speed, which is raising its orbit, resulting in a slower orbit. <div class="Discussion_UserSignature"> <p><strong><font color="#ff0000">Vote </font><font color="#3366ff">Libertarian</font></strong></p> </div>

 

[racer7]

I probably didn't explain what I was remarking on very well in regards to the Moon. I'll try to explain.<br /><br />The Moon is orbiting Earth, which is orbiting the sun. If we looked at just the Moon's orbital track around the sun, it's very similar to the Earth's as seen here. The Moon doesn't travel backwards or make a loopy track. In respect to the sun, the Moon's velocity must appear to change as it orbits the Earth, since first it trails the Earth and then leads. <br /><br />I'm not sure if I'm explaining it well, but I would say it's about the same thing that happens on a carnival ride called a "scrambler". Anyway, it's simple, but something that I never really thought about before. Like I said before, it's an interesting thread.

 

[jimfromnsf]

"No, its going slightly faster than true orbital speed, which is raising its orbit, resulting in a slower orbit."<br /><br />It is not "going faster:, it is receiving some outside energy (gravitional influences) that is changing the orbit.<br /><br /><br />

 

[jimfromnsf]

"The Moon doesn't travel backwards or make a loopy track."<br /><br />Jupiter's moons do

 

[larper]

hmmm... I would disagree.<br /><br />The moon's gravity exerts a torque on the earth (due to the fact that the earths mantle is fluid, and that we have oceans) slowing us down ever so slightly. Since the Earth/Moon system is closed, there is a corresponding torque applied to the moon, speeding it up. Angular momentum is conserved. The torque applied to the moon is causing it to go slightly faster than its instantaneous orbital velocity. Thus, its orbit is constantly rising. <div class="Discussion_UserSignature"> <p><strong><font color="#ff0000">Vote </font><font color="#3366ff">Libertarian</font></strong></p> </div>

 

[jimfromnsf]

1. the Earth/Moon system is not closed, the sun has influences<br />2. Angular momentum is not conserved, the tidal forces dispate some of it.<br /><br />My post should have said energy being exchanged

 

[larper]

1) Yes, but the sun is not what is causing the moon to receed.<br />2) As the moon exerts tidal forces on the earth, two things happen: Some of the forces cause the earth to slow, which applies a counteracting force on the moon, causing its orbit to rise. Secondly, some of the forces cause the earth to heat up.<br /><br />Thus, for all intents and purposes, the Earth/Moon system is closed. And for simplicity's sake, ignore the heating effect and just look at the problem from a conservation of angular momentum problem, since it is the exchange of angular momentum that is really causing the Moon to raise its orbit while the Earth's rotation slows. <div class="Discussion_UserSignature"> <p><strong><font color="#ff0000">Vote </font><font color="#3366ff">Libertarian</font></strong></p> </div>

 

[kosmonavtkaa]

<blockquote><font class="small">In reply to:</font><hr /><p>Or the ISS, it maintains a LHLV (local horizontal local vertical) attitude, which means the same side faces the earth. This means that the station is actually doing one "flip" per orbit wrt to inertial space. That is why the ISS solar arrays have rotating joints<p><hr /></p></p></blockquote><br /><br />"What are the ISS attitudes?" NASA Flash animation <div class="Discussion_UserSignature"> </div>

 

[MeteorWayne]

That's a great educational tool, thanx very much for bringing it to our attention. It answered a lot of my questions!!<br /><br />Wayne <div class="Discussion_UserSignature"> <p><font color="#000080"><em><font color="#000000">But the Krell forgot one thing John. Monsters. Monsters from the Id.</font></em> </font></p><p><font color="#000080">I really, really, really, really miss the "first unread post" function</font><font color="#000080"> </font></p> </div>

 

[askold]

<b>"When the astronauts threw the object backwards from the ISS - what is the resultant path of the junk?" <br /><br />There are some simple equations for small changes to orbits. They are called the Clohessey-Wilshire equations. <br /><br />As described by others, if you start with a circular orbit and "throw an object backwards" that is in the anti-velocity direction, the orbit becomes an ellipse with the high point (apogee) at the radius of the original orbit and the low point (perigee) slightly lower. This reduces the orbital period. Therefore, when it gets back to the apogee after one orbit, it is ahead of the larger craft, here the ISS. The thrown object would appear to loop below the ISS and move further ahead of it each orbit. </b><br /><br />That's exactly what I was trying to reconcile. I heard 2 things on the news:<br /><br />1) The object can be of no danger to the ISS. Yet, you say the object's orbit will intersect the circular orbit of the ISS at the object's apogee. Sounds like they can meet again.<br /><br />2) The object will burn up on reentry fairly soon - a year or so. But, if the object's perigee is lowered only a little, it's not going to see the effects of the atmosphere.<br /><br />My head hurts.

 

[comga]

"hat's exactly what I was trying to reconcile. I heard 2 things on the news:<br /><br />1) The object can be of no danger to the ISS. Yet, you say the object's orbit will intersect the circular orbit of the ISS at the object's apogee. Sounds like they can meet again.<br /><br />2) The object will burn up on reentry fairly soon - a year or so. But, if the object's perigee is lowered only a little, it's not going to see the effects of the atmosphere. "<br /><br />Yes, the object returns to the altitude of the ISS, but after each orbit it will be a short distance "ahead" of the ISS. Therefore, there is no immediate danger of contact. The trick for intercepting is getting to the right place at the right time.<br /><br />If there was no air drag and the orbits stayed constant, it could "lap" the ISS, i.e., make N+1 orbits while the ISS makes N orbits, and come close. (We could calculate how long that would take if it leaves at say one mile per per hour, but it would take some doing.) However, within hours, the ISS was boosted to a higher orbit. <br /><br />As to your second question, there is always air drag at these altitudes. Did you check out the links I posted? (An image is attached.) You will see that the ISS drops 2 to 3 km per month from air drag. It must be routinely reboosted to maintain altitude and avoid being dragged into the lower atmosphere. The space junk will have no boosting and will burn up in a predicted 300 to 330 days. <br /><br />Give you head a rest. Rotating coordinate systems are confusing, but given time, you can get a good feel for them.

 

[jimfromnsf]

"1) The object can be of no danger to the ISS. Yet, you say the object's orbit will intersect the circular orbit of the ISS at the object's apogee. Sounds like they can meet again.<br /><br />2) The object will burn up on reentry fairly soon - a year or so. But, if the object's perigee is lowered only a little, it's not going to see the effects of the atmosphere. "<br /><br />1. The ISS also did an orbit raising burn<br />2. There are atmosphere effects at the ISS orbit. The ISS make periodic burns to compensate for atmospheric drag