Hi guys<br /><br />I read today that they fixed some sort of motor/joint for the ISS solar panels. That reminded me about something I've noticed a few times. Often you'll see pictures for the ISS where the solar panels are not all facing the same direction. (OK one thingimajing was broken it seems, but still there were more panels not pointing in the same direction).<br /><br />So I was wondering why this is happening. I thought they should all track the sun which mean they must always face the same direction. <br /><br />This makes me wonder about something else as well. If you need to rotate the panels to follow the sun, it means that the ISS is not standing still relative to the sun. That's very strange to me. Won't that be 'normal'. E.g. If some part of the station is pointing to the earth, then surely after half a rotation it will point to the sun. I get the impression that this is not happening. Which sounds to me that they must continuously use some rockets or something to keep it pointing in the right direction... Is that correct? Sounds like A LOT of fuel.<br /><br />I think I should add at least that my simple serviette logic assumes that the station is orbiting along the orbital plane of earth (ie. ISS, Earth and sun is always on the same plane). So I can see why in reality you still need to rotate panels even if the ISS is not pointing in the same direction relative to the earth.<br /><br />So in summary I have the following questions:<br />1. Why aren't all solar panels of the ISS always facing the same direction.<br />2. Is the ISS's attitude always the same relative towards the earth or to the sun?
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Normally the solar panels track the sun. When the shuttle docks, they orient the panels in a fixed position to minimize impact from the RCS jets. Once docked, the panels go into sun track mode again. When undocking, the fixed positions are resumed until the shuttle has departed. <div class="Discussion_UserSignature"> <p><strong><font color="#ff0000">Vote </font><font color="#3366ff">Libertarian</font></strong></p> </div>
To follow up, of course all the images you see are taken as the Orbiter is approaching or leaving, so we never really get to see the panels as they normally operate. <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>
A few more tidbits:<br /><br />There are quite a few panels that you may be looking at.<br /><br />The really big ones that come out in pairs on either side of the main truss are the main solar arrays (and, as it happens, the biggest solar arrays ever put into space). These can rotate along the axis of the arrays themselves, and the truss itself can also rotate (on that rotary joint that you read about). Until now, only the port side has been free to rotate along the solar alpha rotary joint (SARJ).<br /><br />Extending outward from the same truss are some accordion-shaped panels. These do not track the Sun, but do rotate around since they're attached to the same truss as the big solar panels. These are the main radiators which dump excess heat generated by the station's electronics.<br /><br />There are also some more solar arrays on the Russian segment. These are flat and are rather geometric in appearance. They can rotate along their long axis to track the Sun, but cannot rotate another way. So the American solar arrays track the Sun along two axes, while the Russian ones track along only two.<br /><br />Lastly, at any given time there may be up to three additional pairs of solar arrays. Each Soyuz or Progress docked to the station has a pair of fixed solar array "wings". They are quite small compared to the station's solar arrays.<br /><br />As far as why the station always keeps its nadir pointed towards the Earth even as it goes around the Earth -- I think I'd better let somebody who knows more about celestial mechanics and spacecraft operations answer that one. <img src="/images/icons/wink.gif" /> <div class="Discussion_UserSignature"> <p> </p><p><font color="#666699"><em>"People assume that time is a strict progression of cause to effect, but actually from a non-linear, non-subjective viewpoint it's more like a big ball of wibbly wobbly . . . timey wimey . . . stuff."</em> -- The Tenth Doctor, "Blink"</font></p> </div>
"The ISS is normall kept in the same attitude relative to the Earth. "<br /><br />Gravity is our friend <img src="/images/icons/smile.gif" /> <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>
Hi Shuttle_guy, I forgot about the gyros. of course!<br /><br />Meteor_wayne, the only thing that gravity does is to keep it in orbit but it won't affect the attitude. (at least that's what orbitersim taught me 
<br /><br />Thanks all for the responses on the non-tracking. Makes sense!
<br /><br />Thanks all for the responses on the non-tracking. Makes sense!No, gravity definitely affects attitude. Large spacecraft experience a large gravity gradient, which can help maintain attitude. <div class="Discussion_UserSignature"> <p><strong><font color="#ff0000">Vote </font><font color="#3366ff">Libertarian</font></strong></p> </div>
Welcome to Space.com.<br /><br />You may be incorrect.<br /><br />If I am not mistaken, the ISS does use the mass of the ISS to aid in alignment.<br /><br />Left to it's own physics, the ISS (in the absence of aerodynamic effects) will naturally align itself so that the most dense mass will face the earth.<br /><br />I believe that is taken into consideration, since there's not point in fighting Mother Nature <img src="/images/icons/smile.gif" /><br /><br />There's a name for that configuration, but it escapes me at the moment.<br />I'll try and find 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>
Local Vertical Hold <br /><br />At least, that is what we called it on systems I worked on. <div class="Discussion_UserSignature"> <p><strong><font color="#ff0000">Vote </font><font color="#3366ff">Libertarian</font></strong></p> </div>
Hi MeteorWayne,<br /><br />That's true. Many of the solar system's irregularly shaped moons point directly <br />at the parent planet through their longest diameters, such as Amalthea with respect to Jupiter<br />(Saturn's Hyperion is an exception, due to a possible 'recent' impact event, <br />but in time will also obey the same rule).<br /><br />The ISS would behave in exactly the same way, if left to gravity acting on its mass.<br /><br />Andrew Brown. <div class="Discussion_UserSignature"> <p><font color="#000080">"I suddenly noticed an anomaly to the left of Io, just off the rim of that world. It was extremely large with respect to the overall size of Io and crescent shaped. It seemed unbelievable that something that big had not been visible before".</font> <em><strong><font color="#000000">Linda Morabito </font></strong><font color="#800000">on discovering that the Jupiter moon Io was volcanically active. Friday 9th March 1979.</font></em></p><p><font size="1" color="#000080">http://www.launchphotography.com/</font><br /><br /><font size="1" color="#000080">http://anthmartian.googlepages.com/thisislandearth</font></p><p><font size="1" color="#000080">http://web.me.com/meridianijournal</font></p> </div>
I watched the NASA-TV feed of the last EVA, and they rotated the panels while the spacewalkers worked underneath. <img src="/images/icons/cool.gif" /> <div class="Discussion_UserSignature"> <p><font size="2" color="#ff0000"><em><strong>I'm a recovering optimist - things could be better.</strong></em></font> </p> </div>
Ok. so seems like you guys are saying if the centre of gravity is off centre for the orbiting body, then it will tend to keep the heaviest part to point to the source of the gravity.<br /><br />What about our moon? is the same part always facing earth or is the same part always facing the sun? ('im not talking the visible bits that we see)
Actually they are the same. The heaviest part of the moon now faces us all the time. Therefore we never see the other side (except for peeks around the edges due to some wobble.) <br /><br />Since theearth is the closest massive body, that's what it points toward.<br /><br />The sun, is part of what causes the little bit of wobble. <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>
hi MW<br /><br />I hadn't thought about weight displacement; is the moon kind of pear-shaped, or is it more dense on its earth-facing side? <div class="Discussion_UserSignature"> <p><font size="2" color="#ff0000"><em><strong>I'm a recovering optimist - things could be better.</strong></em></font> </p> </div>
<font color="yellow">As far as why the station always keeps its nadir pointed towards the Earth even as it goes around the Earth -- I think I'd better let somebody who knows more about celestial mechanics and spacecraft operations answer that one.</font><br /><br />That's just NASA showing its ass. <div class="Discussion_UserSignature"> <font color="#0000ff"><strong>Just tell the truth and let the chips fall...</strong></font> </div>
<img src="/images/icons/laugh.gif" /> <div class="Discussion_UserSignature"> <p><font size="2" color="#ff0000"><em><strong>I'm a recovering optimist - things could be better.</strong></em></font> </p> </div>
"As far as why the station always keeps its nadir pointed towards the Earth even as it goes around the Earth -- I think I'd better let somebody who knows more about celestial mechanics and spacecraft operations answer that one."<br /><br />Well there are probably lots of factors including symmetry of the mass but the main issue is that for the arrays to work at peak generation, and for the equipment to maintain thermal control, that is the orientation that ISS must have. If we fly other attitudes equipment would fail due to limits. Now I know that doesn't completely answer it because you could have designed the equipment differently. Note also that the MMOD sheilding is desinged for this attitude. Any other attitude and we are in hurt. For Russian vehicle dockings the constraints on the arrays and potential excursions outside of the nominal attitudes is very difficult and we are always on the edge of losing control! <br /><br />P.S. The gravity gradient option was taken into account at 13A should we have had to undock the orbiter without Russian computer control. Grav gradient can be your friend!
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