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Thread: Solar Polar "Geosynchronous" Orbit Questions

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    Solar Polar "Geosynchronous" Orbit Questions

    Has this ever been tried?

    And wouldn't such a spot allow one to out SOHO SOHO?

    What would be the difficulties of such a mission?
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    Can you explain what "solar polar geosynchronous orbit" means? "Geo" means Earth, so "geosynchronous" is only applicable to objects orbiting the Earth, whereas it seems like you're suggesting some kind of orbit around the sun.

    Do you mean a polar orbit around the sun that precesses in such a way to always be aligned between sun and earth?

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    Quote Originally Posted by Amber Robot View Post
    Do you mean a polar orbit around the sun that precesses in such a way to always be aligned between sun and earth?
    I haven't done the sums, but I suspect the sun is too close to spherical to allow such an orbit to exist.

    Grant Hutchison

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    No, I mean a satellite in a stationary position over the solar north or south poles.
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    Hence the quotations around geosynchronous.
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    You mean a statite, hovering in place with a solar sail to counteract gravity?

    Grant Hutchison

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    Never heard of those Doc.

    Wait, I'm starting to get the suspicion that one doesn't do a stationary orbit over a pole...
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    Quote Originally Posted by BigDon View Post
    Wait, I'm starting to get the suspicion that one doesn't do a stationary orbit over a pole...
    Afraid not. You need to be orbiting the object with its spin to appear to hover in place.

    The best you can do to get plenty of dwell over a pole is to have a highly elliptical orbit. Something like the Molinya orbits used by Russian comms sats. They'd probably be sub-optimal for solar observation as the way they reduce the amount of time spent over areas other than the selected pole is to get really close to the parent body.

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    For something to orbit above the same spot it has to be moving around the object in same direction and around the center of mass.
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    I thought originally this was going to be a question about sun-synchronous orbits around the Earth, which are inclined so as to have a precession period equal to a sidereal year. These are pretty close to polar (actually, highly inclined retrograde), can give you a continuous view of the sun if orientated close to the dawn-dusk plane, and have been used for solar observation satellites.

    Grant Hutchison

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    And spy satellites as well, I think.
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    Quote Originally Posted by Trebuchet View Post
    And spy satellites as well, I think.
    Yes - the constant low angle of the sun is good for photogrammetry.

    Grant Hutchison

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    Sun-synchronous orbits have been used for heaps of satellites.
    I'm almost certain Landsat was Sun-synchronous. I think many
    early LEO sats were Sun-synchronous just to keep them in the
    sunlight so they would always have power.

    Quote Originally Posted by BigDon View Post
    Wait, I'm starting to get the suspicion that one doesn't do a stationary
    orbit over a pole...
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    Quote Originally Posted by Jeff Root View Post
    Sun-synchronous orbits have been used for heaps of satellites. I'm almost certain Landsat was Sun-synchronous. I think many early LEO sats were Sun-synchronous just to keep them in the sunlight so they would always have power.
    LandSat, the Worldview birds, the polar component of the weather satellite network - you are right, there is a huge list. In terms of power though I believe that only the orbits near the dawn to dusk slot offer any significant advantage to power generation over any other LEO slot. Active satellites in particular took advantage of this slot to keep up with their power requirements.

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    Quote Originally Posted by BigDon View Post
    Never heard of those Doc.

    Wait, I'm starting to get the suspicion that one doesn't do a stationary orbit over a pole...
    That's correct. If you are stationary over a pole, you are not orbiting.

    To a close approximation, a satellite must move in an ellipse (or a circle, as a special case) with one of the two foci at the centre of the earth. If the satellite is north of the equator sometimes, it must be south of the equator other times.

    Satellites in geosynchronous orbits move around the earth's centre, always maintaining a position over the equator. They do so at just the right speed, to match the earth's rotation. That way, the satellite moves around the earth, but the earth spins underneath it at the same speed, so the same position on the equator is always directly under the satellite.

    To maintain a position over the south pole is not to orbit; it requires propulsion. You could use the satellites fuel to maintain the position, if the engines are sufficiently powerful, but eventually the fuel will run out (generally, rather quickly). grant hutchinson is describing a different mechanism for providing the propulsion needed to keep the satellite over the south pole. Again, though, it's not an orbit - constant effort is required to keep it there.

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    I have a vague memory (sorry, couldn't find a reference) that people have used polar orbits for spy satellites, and to get near-continuous coverage over the polar region, they have a series of satellites in the same orbit. So, a particular satellite may only be near the pole for a limited period of time, but then the next one in the daisy-chain comes along.
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    Quote Originally Posted by Swift View Post
    I have a vague memory (sorry, couldn't find a reference) that people have used polar orbits for spy satellites, and to get near-continuous coverage over the polar region, they have a series of satellites in the same orbit. So, a particular satellite may only be near the pole for a limited period of time, but then the next one in the daisy-chain comes along.
    The Molniya orbits I mentioned were used by the USSR and later Russia for comms sats, TV satellites and (according to Wikipedia) early warning systems in just such a way. IIRC three or four were used to give continuous coverage.

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    Quote Originally Posted by Shaula View Post
    The Molniya orbits I mentioned were used by the USSR and later Russia for comms sats, TV satellites and (according to Wikipedia) early warning systems in just such a way. IIRC three or four were used to give continuous coverage.
    The smart thing with Molniya orbits is that they are highly eccentric, and inclined at around 63 degrees. That inclination is at the sweet spot (for a satellite in Earth orbit) where the rotation of the apsides is zero, so the apogee and perigee remain fixed. If the apogee is placed at the north end of the orbit, the satellite spends most of its time over the northern hemisphere.

    Grant Hutchison

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    Quote Originally Posted by BigDon View Post
    No, I mean a satellite in a stationary position over the solar north or south poles.
    You wouldn't be orbiting if you you were hovering over the pole, so you'd need some pretty big rockets to keep yourself from falling straight down into the Sun.

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    The OP's question reminded me of an old story I read years ago. I don't know how plausible it would be in reality, but in this story there was an interesting concept for entering a planet's (Earth-like) atmosphere from space while avoiding the high re-entry speeds associated with re-entry from planetary orbit.

    The story was Torch Of Honor by Roger MacBride Allen. In the story a major plot point hinged around a giant spacecraft designed to enter a planet's atmosphere and then act as an airborne carrier and command base. The quandary was how to get the thing from space into the planets atmosphere without it being ripped apart into myriad fire balls. What the enemy's designers came up with, or rather the story's author, was to enter the planet's atmosphere over a pole from a "solar" orbit rather than entering from a planetary orbit.

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    Quote Originally Posted by Darrell View Post
    The OP's question reminded me of an old story I read years ago. I don't know how plausible it would be in reality, but in this story there was an interesting concept for entering a planet's (Earth-like) atmosphere from space while avoiding the high re-entry speeds associated with re-entry from planetary orbit.

    The story was Torch Of Honor by Roger MacBride Allen. In the story a major plot point hinged around a giant spacecraft designed to enter a planet's atmosphere and then act as an airborne carrier and command base. The quandary was how to get the thing from space into the planets atmosphere without it being ripped apart into myriad fire balls. What the enemy's designers came up with, or rather the story's author, was to enter the planet's atmosphere over a pole from a "solar" orbit rather than entering from a planetary orbit.
    And how exactly was that supposed to help? You'd still have the Earth's orbital velocity to contend with.

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    Quote Originally Posted by Amber Robot View Post
    And how exactly was that supposed to help? You'd still have the Earth's orbital velocity to contend with.
    If they're both in solar orbit, they have nearly the same orbital velocity.

    But, if it can take off from the earth, why can't it decelerate to the earth...

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    Quote Originally Posted by grapes View Post
    If they're both in solar orbit, they have nearly the same orbital velocity.
    Sorry... when I read "over a pole" i imagined coming in from above, orthogonally to the Earth's orbit, but now I see that they somehow want the spacecraft to be orbiting the Sun at the same rate that the Earth is and come in near the pole horizontally. This of course somehow ignores the influence of Earth's gravity, which would prevent them from having "nearly the same orbital velocity". I agree that if you get to ignore Earth's gravity, it's easy to enter Earth's atmosphere without heating up.

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    The author didn't ignore the planet's gravity, but that still doesn't mean that his idea was plausible. From what I recall it was a delicate rendezvous, the ship arriving at the correct point in space timed to coincide with the arrival of the planet at a point in space in close proximity and with a relative velocity between the two that entering the planet's atmosphere, while still an exciting event, was considerably less exciting than entering from planetary orbital velocity, which the ship was not capable of or designed to survive. It was depicted as being a tight window that if missed meant disaster, specifically because of the planet's gravity.

    Grapes,
    The ship did not take off from a planet. If I remember the story correctly it was not capable of doing so under any circumstances. Once it entered a planet's atmosphere there was no getting back into space.
    Last edited by Darrell; 2017-Oct-11 at 05:37 PM. Reason: '

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    Seems like it should make things worse, rather than better. Instead of dropping into the atmosphere with a little more than circular velocity, an approach from solar orbit is going to be like dropping in from a greater distance, accelerated all the way by Earth's gravity, and therefore arriving with something closer to parabolic than circular velocity. Getting from "far away" to "at rest in the atmosphere" involves the same change in potential and kinetic energy, no matter how you chop it.

    Grant Hutchison

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    I agree. It appears that the author either did not understand the facts of orbital ballistics or chose to ignore it for the sake of a story.

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