ISS in orbit - does it drift laterally?

brellis · Dec 17, 2008

Had this question in mind for some time now, and it's tough to confirm by google.  Does anyone know if ISS experiences left-right drift in addition to vertical drag?  I would assume so, but I don't see reference to any corrective maneuvers.  I only see that Shuttle and Soyuz (and now Verne) boost altitude. Does ISS keep its 51.6 degree orbit in place with its own lateral thrusters?  edit: does it even have attitude thrusters? <div class="Discussion_UserSignature"> I'm a recovering optimist - things could be better. </div>

brellis · Dec 17, 2008

For instance, if ISS must dodge some debris, does it go left-right or simply up-down? <div class="Discussion_UserSignature"> I'm a recovering optimist - things could be better. </div>

MeteorWayne · Dec 17, 2008

Replying to: <DIV CLASS='Discussion_PostQuote'>For instance, if ISS must dodge some debris, does it go left-right or simply up-down? Posted by brellis</DIV> Good question, I don't know the answer for sure. However, it seems likely that the only maneuvers would be to speed up (most of the time) raising the orbit, or rarely, to slow down (lowering the orbit). From what I recall, the amount of propellant to change the plane of the orbit would make that unlikely to be used unless it was a grade 10 alarm bells ringing emergency.Now I'll be quiet and wait until those that know for sure speak up. <div class="Discussion_UserSignature"> But the Krell forgot one thing John. Monsters. Monsters from the Id. I really, really, really, really miss the "first unread post" function </div>

secondAttempt · Dec 17, 2008

It experiences precession from the buldge of Earth's equator.  This is a lateral drift.  As far as purposful burns, I'm not sure, but MW's posts seems reasonable.  Burning any direction will get you out of the path on a projectile, with your biggest changes coming from prograde and retrograde burns.  Since raising your orbit is something that needs to be performed anyway from time to time, why not go up?

vogon13 · Dec 17, 2008

There would be several effects on the ISS.  Atmospheric drag is the 'biggee' and deflection manuvers to avoid debris would have to be strongly preferred to raise the orbit, since as it was correctly pointed out, it needs to be done anyhow.  There is an effect from the moon to (IIRC) to increase the ellipticity of the ISS orbit, and a smaller term tending to make the ISS orbit more coplanar with the moons orbit.  I bet that effect is tiny.  If one timed the burns to correspond to 'induced' lunar derived apogees, one might enjoy a small blip in efficiency, but, again, this effect assist is tiny.  Solar effects will tend to elipticize the ISS orbit too, but the effect is smaller than the moon's, and will add to the moon's effect at new and full phase, and counteract at first and last quarter.  Earth's 'bulge' will tend to lessen the inclination, but I suspect this is another tiny effect.  Overtaxing the shuttle or soyuz's ability in this regard would be minute, as any planar error would be known at launch, and would not have much time to increase until docking.  Another effect on the ISS orbit would be light pressure on the solar panels, and reaction effects from (hopefully) small air leaks.  You might want to watch the last one for any surprises.  Venting the toilet (if the converter gadget craps out) would be another problem.   <div class="Discussion_UserSignature"> TPTB went to Dallas and all I got was Plucked !!So many people, so few recipes !!Let's clean up this stinkhole !! </div>

BoJangles · Dec 17, 2008

Turds in space...*Goes back to the shadows* <div class="Discussion_UserSignature"> -------------- Let me start out with the standard disclaimer ... I am an idiot, I know almost nothing, I haven’t taken calculus, I don’t work for NASA, and I am one-quarter Bulgarian sheep dog. With that out of the way, I have several stupid questions... *** A few months blogging can save a few hours in research *** </div>

brellis · Dec 19, 2008

Did a little learning:  The ISS has its own Russian-built thrusters, which get fueld by shuttles, Progress/Soyuz, etc. and its has gyroscopes to control its attitude.  Lateral variation in its orbit is very very slight, so miniscule that they don't compensate for it, they just keep track of exactly where it is via 24/7 ground tracking.   When a craft is launched to rendez-vous with ISS, they allow several hundred meters of "where exactly is that thing" built into their calculations -- i.e. they plan to take a good look before they get anywhere near it -- and have automated tracking systems on board each craft establish the precise approach angle once the approaching craft is in the ballpark. The overhwelming effect of the thin atmosphere in LEO is simply to drag ISS straight down towards earth, not to "blow" it laterally.  Similarly, the effect of solar wind is at most miniscule in its random lateral pushes or pulls on ISS.  Any minor nudge to the attitude of the craft is compensated for by the gyroscopes.An interesting concept regarding the positioning of the solar panels is described in this Wiki Article called Night Glider.  "Turds in Space" are doomed to a fiery demise.  No one's gonna keep that crap floating up there! <div class="Discussion_UserSignature"> I'm a recovering optimist - things could be better. </div>

coeptus · Dec 20, 2008

Thanx for the update on your research. {Might pass on a caution regarding the parallel universe; TPTB there are extremely  touchy and have long memories for the slightest infractions. Tread lightly.}      <div class="Discussion_UserSignature"> If not for bad Pluck, I'd have no Pluck at all . . . This is your vogon, posting under coeptus, and trying IE and Firefox to see if either is faster with fewer misloads. Erf !! </div>

skeptic · Dec 20, 2008

Replying to: <DIV CLASS='Discussion_PostQuote'>Did a little learning:  The ISS has its own Russian-built thrusters, which get fueld by shuttles, Progress/Soyuz, etc. and its has gyroscopes to control its attitude.  Lateral variation in its orbit is very very slight, so miniscule that they don't compensate for it, they just keep track of exactly where it is via 24/7 ground tracking.   When a craft is launched to rendez-vous with ISS, they allow several hundred meters of "where exactly is that thing" built into their calculations -- i.e. they plan to take a good look before they get anywhere near it -- and have automated tracking systems on board each craft establish the precise approach angle once the approaching craft is in the ballpark. The overhwelming effect of the thin atmosphere in LEO is simply to drag ISS straight down towards earth, not to "blow" it laterally.  Similarly, the effect of solar wind is at most miniscule in its random lateral pushes or pulls on ISS.  Any minor nudge to the attitude of the craft is compensated for by the gyroscopes.An interesting concept regarding the positioning of the solar panels is described in this Wiki Article called Night Glider.  "Turds in Space" are doomed to a fiery demise.  No one's gonna keep that crap floating up there! Posted by brellis</DIV>  Good report.  I half expected them to use waste water (at least until now when they're beginning to recycle it) turned to steam as a propellent.  It seems to me that would be a lot cheaper that hauling up propellent from earth. I remember an experiment of NASA's about 15 years ago in which they tried to generate electricity by dragging a cable through the earth's magnetic field.  The first time they tried it the cable wouldn't unfurl and the second time it worked for a short time before the cable broke.  Does anyone know the purpose of that experiment?  It seems they get enough electricity from the solar panels and this method has the added disadvantage that it would cause their orbit to decay more rapidly.In fact it seems to me there would be some incentive to do the opposite - to use power from the solar collectors to generate a magnetic field that together with earth's magnetic field would compensate for the natural decay in the orbit or even to move the ISS in other directions.  Of course the orientation of the coil would have to be constantly changed as it moves north and south of the magnetic equator but that could easily be handled by computer.  Again it seems that would be a lot cheaper than hauling up propellent from earth.

brellis · Dec 21, 2008

My inital post asked whether or not ISS had its own thrusters to correct any micro-drift, and in fact it does have the ability to keep itself on the 51.6 path - howver, that path is a bit weird.  Because of earth's equatorial bulge, alluded to by vogon13, ISS experiences what is known as nodal regression - its orbital inclination fluctuates quite a bit each time it crosses the equator.  However, this fluctuation corrects itself each time it leaves the equatorial region. So, if you're looking for a consistent 51.6 degree inclination you will not find it at any time during the ISS orbit.  It averages 51.6 degrees, but the actual shape is stretched. I will now click "add Post" and hope for the best, haha <div class="Discussion_UserSignature"> I'm a recovering optimist - things could be better. </div>

vogon13 · Dec 21, 2008

Pretty complicated question you turned out to have posted.  I am sure most of the large effects totally swamp the tiny ones.   I recall the Gravity Probe satellite, however, and that was dedicated to studying a tiny force whilst the craft doing all the measurements would have been experiencing everything discussed here.  I have tremendous awe regarding how successful that mission was despite the plethora of effects to sort out. (feel free to mention Gravity Probe in the parallel universe)    <div class="Discussion_UserSignature"> TPTB went to Dallas and all I got was Plucked !!So many people, so few recipes !!Let's clean up this stinkhole !! </div>

brellis · Dec 21, 2008

Replying to: <DIV CLASS='Discussion_PostQuote'> Pretty complicated question you turned out to have posted.  </DIV> They've conducted several experiments having objects float insideISS for extended periods, and the drift is extremely minor, measured in millimeters and micro-something-or-others.  About ten years ago, NASA studied the plausibility of putting an array of orbiting crafts in an interferometry setting, and their conclusion was that LEO is too chaotic for that level of synchronization, so those missions are planned to operate in much higher orbits, or L-spots. Replying to: <DIV CLASS='Discussion_PostQuote'>(feel free to mention Gravity Probe in the parallel universe)    Posted by vogon13</DIV>Perhaps that is a subject to raise in "Improving the Software", haha <div class="Discussion_UserSignature"> I'm a recovering optimist - things could be better. </div>

vogon13 · Dec 21, 2008

Even shielded by the matter making up the structure of the ISS, an object inside in a vacuum cask would still experience lunar and solar tidal effects, gravity variations caused by the earths' inhomogeneousism, etc., in perfect synchronization with the ISS itself.   Intersting to take up some samples with the maximum P-N and N-P ratios possible (consistent with safety) and see how they behave.  I assume it would have to be done in the outer solar system and shielded from light (Poynting -Robertson concerns) but another interesting experiment would be to see if it could be possible to set up some sand grains orbiting a .1 meter sphere of osmium.   <div class="Discussion_UserSignature"> TPTB went to Dallas and all I got was Plucked !!So many people, so few recipes !!Let's clean up this stinkhole !! </div>

brellis · Dec 21, 2008

Things floating inside ISS wouldn't be subject to the thin atmosphere, so they'd hit the ceiling before any remarkable left-right drift would occur (that was what first prompted my query). <div class="Discussion_UserSignature"> I'm a recovering optimist - things could be better. </div>

shuttle_guy · Dec 21, 2008

Replying to: <DIV CLASS='Discussion_PostQuote'>Had this question in mind for some time now, and it's tough to confirm by google.  Does anyone knowNO if ISS experiences left-right drift in addition to vertical drag?  I would assume so, but I don't see reference to any corrective maneuvers.  I only see that Shuttle and Soyuz (and now Verne) boost altitude. Does ISS keep its 51.6 degree orbit in place with its own lateral thrusters?  edit: does it even have attitude thrusters? Posted by brellis</DIV>No, there is no movement of the orbit from 51.6 due drag or Mass Concentrations. No corrections to the "left or right" from 51.6 degrees are performed.   <div class="Discussion_UserSignature"> </div>

shuttle_guy · Dec 21, 2008

Replying to: <DIV CLASS='Discussion_PostQuote'>For instance, if ISS must dodge some debris, does it go left-right or simply up-down? Posted by brellis</DIV>ONLY increase velocity (up movement of the (apogee) or decrease velocity (down (preigee) movement) adjustments are used. <div class="Discussion_UserSignature"> </div>

shuttle_guy · Dec 21, 2008

Replying to: <DIV CLASS='Discussion_PostQuote'>Good question, I don't know the answer for sure. However, it seems likely that the only maneuvers would be to speed up (most of the time) raising the orbit, or rarely, to slow down (lowering the orbit). From what I recall, the amount of propellant to change the plane of the orbit would make that unlikely to be used unless it was a grade 10 alarm bells ringing emergency.Now I'll be quiet and wait until those that know for sure speak up. Posted by MeteorWayne</DIV> You are correct on both points. In fact the ISS does never has the prolellant necessary at any time to make a significant orbital plane change. <div class="Discussion_UserSignature"> </div>

shuttle_guy · Dec 21, 2008

Replying to: <DIV CLASS='Discussion_PostQuote'>Did a little learning:  The ISS has its own Russian-built thrusters, which get fueld by shuttles, Progress/Soyuz, etc. and its has gyroscopes to control its attitude.  Lateral variation in its orbit is very very slight, so miniscule that they don't compensate for it, they just keep track of exactly where it is via 24/7 ground tracking.   When a craft is launched to rendez-vous with ISS, they allow several hundred meters of "where exactly is that thing" built into their calculations -- i.e. they plan to take a good look before they get anywhere near it -- and have automated tracking systems on board each craft establish the precise approach angle once the approaching craft is in the ballpark. The overhwelming effect of the thin atmosphere in LEO is simply to drag ISS straight down towards earth, not to "blow" it laterally.  Similarly, the effect of solar wind is at most miniscule in its random lateral pushes or pulls on ISS.  Any minor nudge to the attitude of the craft is compensated for by the gyroscopes.An interesting concept regarding the positioning of the solar panels is described in this Wiki Article called Night Glider.  "Turds in Space" are doomed to a fiery demise.  No one's gonna keep that crap floating up there! Posted by brellis</DIV>The onboard thrusters can not be re-fueled by the Shuttle. That modification was never performed. <div class="Discussion_UserSignature"> </div>

shuttle_guy · Dec 21, 2008

Replying to: <DIV CLASS='Discussion_PostQuote'>My inital post asked whether or not ISS had its own thrusters to correct any micro-drift, and in fact it does have the ability to keep itself on the 51.6 path - howver, that path is a bit weird.  Because of earth's equatorial bulge, alluded to by vogon13, ISS experiences what is known as nodal regression - its orbital inclination fluctuates quite a bit each time it crosses the equator.  However, this fluctuation corrects itself each time it leaves the equatorial region. So, if you're looking for a consistent 51.6 degree inclination you will not find it at any time during the ISS orbit.  It averages 51.6 degrees, but the actual shape is stretched. I will now click "add Post" and hope for the best, haha Posted by brellis</DIV>The "drift" from 51.6 is so very insignificant that you would never notice any change from 51.6 degres. It isvery  very far from "quite a bit". <div class="Discussion_UserSignature"> </div>

brellis · Dec 21, 2008

hi shuttle guy - thanks for your input.  I've still had a little confusion about nodal regression, so I poked around some more.   Got this from a google query:  Replying to: <DIV CLASS='Discussion_PostQuote'>The answer below is from one of our Flight Dynamics Officers in Mission Control: I can see how things may be a little confusing, but what Ed is referring to is called nodal regression and for ISS type orbits the orbit shifts westward about 6 degrees each day. The Earth after all is not perfectly spherical and the resulting gravity field does nasty things to orbits. Ed mentions several of these things in his article and he is absolutely correct. At 6 degrees a day, the orbit rotates around the Earth every two months or so. For the ISS crew members, this means they get to observe different targets on the ground with different lighting conditions over the course of time in orbit. This is significant especially when you consider photography and ground based experiments. You can see this effect when it comes to satellite sightings. ISS sightings at the same observer location come in spurts in the evening followed by some time of no sightings followed by a period of morning sightings followed by no sightings and then back to evening sightings. Guess what? That period of evening sightings to nothing to morning sightings to nothing and back to evening sightings is about two months - all because of nodal regression.</DIV>So, to correct my post from a few days ago, nodal regression doesn't cause the shape of the orbit to change back and forth within a single orbit.  51.6 is 51.6 and that's that! <div class="Discussion_UserSignature"> I'm a recovering optimist - things could be better. </div>

MeteorWayne · Dec 22, 2008

Replying to: <DIV CLASS='Discussion_PostQuote'>hi shuttle guy - thanks for your input.  I've still had a little confusion about nodal regression, so I poked around some more.   Got this from a google query: So, to correct my post from a few days ago, nodal regression doesn't cause the shape of the orbit to change back and forth within a single orbit.  51.6 is 51.6 and that's that! Posted by brellis</DIV> Yeah, there's some confusion in there. The reason the orbit moves west for each orbit/day is because the orbital period multiple does not come out to exactly 24 hours. Every orbit is approximately 90 minutes, during that time the earth rotates approximately 15 degrees, But since those numbers are not exact, the exact track 24 hours later (16 orbits X 1 1/2 hour orbit) there is a slow drift each day. It has nothing particularaly to do with gravitational anomolies, it's just a result of that mismatch in precise orbital times.For example, as of yesteraday an ISS orbit takes about 91 minutes 37 seconds. (15.71677458 orbits per day). If it was exactly 16 orbits for day, then the ISS would pass over the exact path on earth every day. But currently the earth turns a bit further by the time the ISS passes over (about 30 minutes later, or 5 degrees longitude).But the angle of the orbit, 51.6411 degrees doesn't substantially change. I'll check again at the next epoch to see the variations of this value. As the ISS slowly falls due to air resistance, the obital time decreases (i.e. more orbits per day) until it is reboosted. The current orbit is ~349 X 359 km. <div class="Discussion_UserSignature"> But the Krell forgot one thing John. Monsters. Monsters from the Id. I really, really, really, really miss the "first unread post" function </div>

brellis · Dec 22, 2008

Regarding ISS orbital path, the first thing I had to get my brain around was why does it look like a Le Mans track on the map at Mission Control?  I grabbed one of my volleyballs and twirled it while waving my other hand at an angle and *boom* - lesson learned! <div class="Discussion_UserSignature"> I'm a recovering optimist - things could be better. </div>

MeteorWayne · Dec 22, 2008

Replying to: <DIV CLASS='Discussion_PostQuote'>Regarding ISS orbital path, the first thing I had to get my brain around was why does it look like a Le Mans track on the map at Mission Control?  I grabbed one of my volleyballs and twirled it while waving my other hand at an angle and *boom* - lesson learned! Posted by brellis</DIV> Like so many things in astronomy, it's a lot easier to understand when you do it in 3D! <div class="Discussion_UserSignature"> But the Krell forgot one thing John. Monsters. Monsters from the Id. I really, really, really, really miss the "first unread post" function </div>

ISS in orbit - does it drift laterally?

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