SciFi Question of the Day: Logic dictates that a space elevator be tethered from a point on the equator to a point out in geosynchronous orbit. But if you tethered it someplace that wasn’t on the equator, it would still be “flung” out into a geosynchronous orbit… but it would appear to go up at an angle, “leaning” towards the equator. Would this still work? Would there be any advantage?
Pony Horton I don’t think it would work, because it’s outward force needs to be centrifugal, and that can only best be achieved on the Equator.
It’s much the same principle as the way I fling my bullwhip: use the wrong angle, and the whip’s arc becomes distorted and useless, or worse, unruly.
Matthew B. Tepper Start buying land in Ecuador.
Pony Horton Matthew, THAT’S thinking ahead!!!!!!!
Stephonie K Williams Wow. When dealing with geosynchronous orbit things tend to get into weird mathematical equations.
What I understand is that it wouldn’t actually work with the tether someplace other than the equator because geosynchronous orbit is at the equator. If the elevator leans then it puts a lot more pressure on whatever is in orbit and it would eventually pull it out of orbit. Putting an elevator on the tether would definitely pull it out of orbit.
William J. Teegarden By definition, and orbital path about a gravitational source must be at right angles to the center of that source. Centrifugal force is an illusion; there are two forces acting on any orbiting object, linear force (or velocity) and centripetal force. Centripetal force is the component vector of force exerted towards the rotational center; the linear force or velocity is the component vector acting at a right angle to the centripetal force and tangential to the orbital path.
The space elevator problem comes about when the centripetal force vector and the gravitational force vectors are not aligned, as they would be along the equator.
AmyBeth Fredricksen I’ll have to ask my friends at LiftPortal Space Education what a ribbon would look like if it was tethered someplace like, say, the Nevada desert, with the anchor allowed to naturally “fall” out to wherever it would fall…
Douglas S Caprette No, it wouldn’t work because the tension in the tether would exert a cross-track force on the satellite that would move it out of geosynchronous orbit.
In a satellite body coordinate system the radial direction is toward the center of mass of the central body, the along track direction is the direction of motion and the cross track direction is perpendicular to them both.
Bill more or less stated the same thing above. However there is a correction to be made to his nomenclature:
“there are two forces acting on any orbiting object, linear force (or velocity) and centripetal force. ”
Actually that first ‘force’ is not a force. It is momentum. The centripetal force is the attraction due to gravity, aka weight. That centripetal force is perpendicular to the momentum so it accelerates the satellite in the radial direction which is why the satellite moves in a circle instead of the straight line that would be its path were there no forces acting on it.
Also, and I am sure Bill knew this, there are numerous small forces (e.g. perturbations) that act on satellites in Earth orbit including the gravitational attraction of other bodies like the sun and moon, solar radiation pressure, drag, radiation pressure from the Earth, and variable gravity due to tides and seasonal mass redistribution on the Earth (e.g. rain and snowfall).
William J. Teegarden Thanks Doug, I knew you’d chime in sooner or later to set me straight. I don’t always get the terminology correct, but the understanding of the concepts involved is both immediate and complete.
AmyBeth Fredricksen So… what would happen if a space elevator was tethered on the equator, attached to an ocean rig, and some mad-man from a James Bond film decided to hijack it and move it a few hundred kilometers off the equator? Would it fall? Would it curve?
Douglas S Caprette ” a few hundred kilometers off the equator? ” North or South?
AmyBeth Fredricksen Would it matter? Oh, and he’s going “Mwa Ha ha haaaa!!!” the whole way.
Stephonie K Williams I suspect that the satellite could temporarily compensate for the change until the rig could be moved back to the equator, but I don’t know how long that would take or how long it would take for the satellite to notice the change in position and make the needed changes.
William J. Teegarden I suspect that, given a ribbon material of sufficient excess strength, anchoring a space elevator on a floating platform would be problematic. The forces involved would likely lift it right out of the water unless it were very massive, and tides and storms would keep things interesting too.
Douglas S Caprette ”Would it matter?” Yes. It would change the inclination of the orbit. The difference is which way it would change (ascending node or descending nod) But half a day later, if the ship kept in the same direction, it would be changing the inclination back to what it was before. The result would be an oscillating increase in the tension in the tether. When it breaks, the satellite will be in a slightly inclined orbit.
” Oh, and he’s going “Mwa Ha ha haaaa!!!” the whole way.” That would not matter.
Michael Laine @pony horton – Sorry, but you’re wrong. The Earth is still spinning at the same speed, and the elevator will fling it outwards. It will be very controllable. BUT you are RIGHT that the angle will be weird. AmyBeth Fredricksen – Imagine a hockey stick, where the tip is Seattle/Arizona whatever, and the main handle of the the string sits over the equator – going straight up, and out to space. The angle will be acute, but this shift will take place of thousands of miles. regardless of where the base is located, the main length will rise over the equator and out.
Michael Laine @douglas s caprette – You’re mostly right with your explanations. But you’re mistaken when it comes to ‘being pulled out of geosynch’. It won’t be. (Assuming we reach the baseline of 90-100GPA in strength) the Ribbon and countermass will remain in geosynch orbit. The bulk of the Ribbon and the countermass are designed to be substantially beyond “geo” in order to keep this careful balance. Some of the math for this can be found here: http://gassend.net/elevator/
Thoughts on Space Elevators gassend.net
Michael Laine AmyBeth Fredricksen – I really hate the “007 Scenario”. Moving the Ribbon is what the LiftPort Ship is SUPPOSED to do. So a couple hundred miles, (even a thousand+ ) is probably o.k… Bringing it back to LAND would be a big problem. But forget about the ship for a moment, or the hockey-stick angle of the Ribbon… think about all the ‘stuff’ whizzing by in LEO. If you move the ship out of it’s primary location, you’re MUCH more likely to be hit by a satellite in orbit. That would be bad. One of the main concerns of building this thing is the international regulatory environment that manages satellite slots. look up the International Telecommunications Union someday if you want some nightmares…
AmyBeth Fredricksen This same discussion on G+ suggested that having a tether on a sea vessel might be in danger of yanking the tether and ship right up out of the water! Maybe you should chime in there too?
Michael Laine The ship is big (think 3 aircraft carriers, side by side). That’s a lot of MASS. The counterweight is ‘small’, think 5 story building. No contest – the ship is wagging the tail.
Douglas S Caprette ”But you’re mistaken when it comes to ‘being pulled out of geosynch’. ”
What do you think will be the reaction to the cross track force exerted by the tether?
Douglas S Caprette Actually, the author at http://gassend.net/elevator/ says that moving the tether anchor off-equator would move the satellite out of geosynchronous orbit. But he predicts that it would be stable, with the y-component of the tether tension counteracting the y component of its weight, even though the satellite would no longer be in *any* orbit. He illustrates that here: http://gassend.net/elevator/non-equatorial/index.html
Thoughts on Space Elevators gassend.net
Google Plus Answers:
Sally Morem I don’t think so.
Christopher Clark work, yes, but not all that well…and with the greater stress from the angle likely not for long
Jim Hanson Biggest advantage I could think of would be ease-of-access and proprietary rights to the use of the elevator. Equatorial nations are not the most developed in the world, and though they might be physically located in prime locations the amount of on-the-ground development that would need to take place to leverage their locations would be insane.
Sally Morem I wonder if it would work to anchor the bottom of the elevator on the seafloor in international waters so you don’t have that problem dealing with proprietary rights for third world tropical nations.
Sally Morem Very cool.
+Christopher Clark – Actually, the Ribbon is more than strong enough to manage the minor additional stress load of the imposed angle. (Assuming the baseline Ribbon of a 90-100 GPA in strength.)
President, LiftPort Group
p.s. For more information on the Space Elevator (or to get involved!) take a look at our NEW website. www.liftport.com It’s not complete, but I’m scrambling to finish it before the International Space Elevator Conference that starts Friday! www.isec.org/sec And if you’re really enthusiastic, we’ll launch our Kickstarter campaign today! (It’s pending the KS approval process right now.. I’m waiting impatiently.)
I would love to hear what you think! Even if you are reading this post a year or more after publishing, I hope you will leave a comment with your own ideas on this topic.
The previous SciFi Q of the Day is Dark Side Vs Dork Side
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The next SciFi Q of the Day is Two Claudias