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The Supreme Canuck
2004-Oct-22, 08:17 PM
Thought about putting this in General Astronomy, but it's not really astronomy, so...

Anyway, is a lunar space elevator possible given the Moon's slow rotation? A geostationary orbit seems hard to achieve. I ask not only out of curiosity, there's an article at Space.com that says it can be done:

http://www.space.com/businesstechnology/technology/space_elevator_020327-1.html

Second page:


Space elevators for use on other worlds, like Mars and the Moon are receiving attention as well.


I have my doubts, but it would be great if they could do it.

ToSeek
2004-Oct-22, 08:53 PM
Pearson thought about building a tower on the Moon. He determined that the center of gravity needed to be at the L1 or L2 Lagrangian points, which are special stable points that exist about any two orbiting bodies where the gravitational forces are balanced. The cable would have to be 291,901 kilometers long for the L1 point and 525,724 kilometers long for the L2 point. Compared to the 351,000 kilometers from the Earth to the Moon, that's a long cable, and the material would have to be gathered and manufactured on the Moon.

- http://www.firstscience.com/site/articles/elevator.asp

The Supreme Canuck
2004-Oct-22, 09:01 PM
Ah, thank you ToSeek. So now that I know it's possible, is it useful? Is it feasible?

Now I'm really curious...

wedgebert
2004-Oct-22, 09:02 PM
It's not the rotational speed of Earth or Luna that makes a space elevator possible or impossible, it's the gravity.

A geostationary orbit over the moon would be inheritantly unstable due to the effects of Earth's gravity, however I'm sure it would be counteractable.

Barring some sort of counter-gravity technology, space elevators look like one of the few realisitic methods we have to lower the cost of getting into orbit. Not only that, but if you don't use a large mass like an asteroid as an anchor, and instead have to extend the cable well outside of geostationary orbit to counter-balance the effects of gravity, you get a lot of "free" velocity if you ride all the way out to the tip of the cable before detaching.

Wow that was a big run-on sentence.

The Supreme Canuck
2004-Oct-22, 09:05 PM
So, assuming they can counteract Earth's gravity, would there be any real issue with constructing the lunar cable, or would it work pretty much the same way as one built on Earth? (Aside from the extra time it takes to ride it)

I can only find stuff about Earth-based space elevators, you see.

Swift
2004-Oct-22, 09:07 PM
Other than cost (yes that is a big other), I think the biggest stumbling block is materials of construction. The cable is going to be under a lot of tension and is beyond the demands of current materials. Things like carbon nanotubes are close theoretically, but there are no current processes to make 100 km of carbon nanotube cable at close to theoretical strength, though people are working on it.

The Supreme Canuck
2004-Oct-22, 09:13 PM
If anyone is interested, I'm currently reading through a NASA paper on space elevators here. (http://flightprojects.msfc.nasa.gov/pdf_files/elevator.pdf)

Caution: PDF.

Kaptain K
2004-Oct-22, 10:04 PM
I think that costs will be the major stumbling block to a "Space Elevator" anywhere.

I think that a solar powered catapult will be the launch method of choice for Luna.

Known engineering - We already know how to build one.

Low amortized costs - Once built, it could last for decades (if not centuries).

Very low "per launch" costs (especially for cargo) - Solar power is, for all practical purposes, "free".

zebo-the-fat
2004-Oct-22, 10:11 PM
Geostationary, should that not be Seleneostationary? :D

wedgebert
2004-Oct-22, 10:48 PM
I actually typed Lunastationary at one point, but decided against it.

The Supreme Canuck
2004-Oct-22, 11:11 PM
Yeah, I couldn't think of a proper word to use.

Edited to add:

Here's another queston: say I wanted to build a space elevator to the L1 point. How close would I have to be to the point? Would there be room for me to build two? Three?

Bob B.
2004-Oct-23, 12:46 AM
A "Lunasynchronous" orbit would have a radius of 88,454 km, which is outside the Moon's gravitational zone of influence. Any object that far away would be dominated by Earth's gravity. Here's the math:

Sidereal rotation period of the Moon = 27.322 days = 27.322*86400 sec
GM of Moon = 4.903E+12 m^3/s^2

P^2 = 4*pi^2*r^3 / GM, or

r = [ P^2*GM / 4*pi^2 ]^1/3

r = [ (27.322*86400)^2*4.903E+12 / 4*pi^2 ]^1/3

r = 88,454,000 m = 88,454 km

The Supreme Canuck
2004-Oct-26, 02:45 AM
Sorry to dredge this up, but I got curious again.

What points on the lunar surface are directly below the L1 and L2 points? (As in the ideal place to anchor a space elevator)

Again, sorry to revive a dead thread, but I couldn't help myself.

Jpax2003
2004-Oct-26, 03:55 AM
Sorry to dredge this up, but I got curious again.

What points on the lunar surface are directly below the L1 and L2 points? (As in the ideal place to anchor a space elevator)

Again, sorry to revive a dead thread, but I couldn't help myself.wouldn't it be the two spots on the lunar equator at the forward and trailing limbs of the moon? The Lagrange points are ahead of and behind the moon in the same orbit. If you were to build a space elevator, I think it would need to arc along that orbital path due to centripetal motion. But I could be wrong.

The Supreme Canuck
2004-Oct-26, 08:08 PM
Wouldn't the centrifugal force would cause the elevator cable to become tight, straight out from the surface? Think of spinning a stone on the end of a string, it goes straight out without arcing.

One Skunk Todd
2004-Nov-17, 05:57 PM
What would happen if you hung the cable down towards the earth? Could you put it down into the atmosphere at all? How fast would it move in relation to the surface? Could you weight the end to keep the cable taut?

Damburger
2004-Nov-17, 06:10 PM
A "Lunasynchronous" orbit would have a radius of 88,454 km, which is outside the Moon's gravitational zone of influence. Any object that far away would be dominated by Earth's gravity. Here's the math:

Sidereal rotation period of the Moon = 27.322 days = 27.322*86400 sec
GM of Moon = 4.903E+12 m^3/s^2

P^2 = 4*pi^2*r^3 / GM, or

r = [ P^2*GM / 4*pi^2 ]^1/3

r = [ (27.322*86400)^2*4.903E+12 / 4*pi^2 ]^1/3

r = 88,454,000 m = 88,454 km

I'm surprised its that close. I was always under the impression a lunar space elevator would slice the Earth in half on its first orbit.

You're right about the gravity. At 88,454km from the moon your satellite would be 295,949km from Earth.

Its acceleration toward the moon would be 0.0006 m/s^-2

Its acceleration toward the Earth would be 0.0046 m/s^-2

Van Rijn
2004-Nov-17, 09:43 PM
Sorry to dredge this up, but I got curious again.

What points on the lunar surface are directly below the L1 and L2 points? (As in the ideal place to anchor a space elevator)

Again, sorry to revive a dead thread, but I couldn't help myself.wouldn't it be the two spots on the lunar equator at the forward and trailing limbs of the moon? The Lagrange points are ahead of and behind the moon in the same orbit. If you were to build a space elevator, I think it would need to arc along that orbital path due to centripetal motion. But I could be wrong.

Here you can see the L1 and L2 points:

http://www.ottisoft.com/samplact/Lagrange%20point%20L1.htm

So, the elevator would be at the equator along the line to L1 or L2. You would put a "counterweight"/station/etc. slightly past the lagrange point relative to the moon to maintain tension.

One thing that annoys me on this subject is that news stories always focus on the elevator concept and flatly ignore all the other "momentum bank" concepts. A lunar rotovator (or earth rotovator for that matter) would be much shorter and easier to build:

http://members.aol.com/Nathan2go/lunavat.htm

It becomes even easier if the rotovator only does part of the job, meeting a rocket "half way" (from an energy standpoint). The advantage for the moon of some momentum bank scheme is that it would reduce fuel requirements.

And here's a variation that is more rocket then rotovator:

http://www.islandone.org/LEOBiblio/SPBI132.HTM

I fully expect tethers to become a big part of space transportation and eventually progress to advanced lift systems.

ToSeek
2004-Dec-14, 06:46 PM
Going Up, Next Floor
Elevating to the Moon (http://www.astrobio.net/news/modules.php?op=modload&name=News&file=article&sid= 1345&mode=thread&order=0&thold=0)


Science fiction writer Arthur C. Clarke famously predicted that we'd see space elevators 50 years after people stopped laughing at the idea. Jerome Pearson has been thinking about space elevators since the early 1970s, and he's been watching the growing enthusiasm (and fading chuckles) with great interest. But he knows there are significant challenges in engineering and materials that still need to be overcome, so he's suggesting NASA build an elevator on the Moon first. And the agency is taking the idea seriously.

Nicolas
2004-Dec-14, 08:05 PM
So, assuming they can counteract Earth's gravity, would there be any real issue with constructing the lunar cable, or would it work pretty much the same way as one built on Earth? (Aside from the extra time it takes to ride it)


If we would be able to counteract earth's gravity, would there still be a need for a space elevator? Or am I getting the gravity remark wrong?

The Supreme Canuck
2004-Dec-14, 11:53 PM
Yeah, I meant it in response to this:


It's not the rotational speed of Earth or Luna that makes a space elevator possible or impossible, it's the gravity.


Either way, this was before I was told I could just park it at the L1 point.

Nicolas
2004-Dec-14, 11:59 PM
OK that way.