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View Full Version : A Space Elevator at Phobos



matthewota
2009-Aug-17, 07:15 PM
Every once in awhile I peruse NASA PDF files for interesting reading. I ran across this (http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20030065879_2003074809.pdf) today.

It amazes me how some people are fortunate enough to make a living just by thinking up stuff like this. I know a fellow in Cleveland that does this at Glenn Research Center.

If it is feasible to do this, it seems a cheaper way to get to the surface of Mars than a direct landing. The delta-V requirement to get to Phobos is much less than Mars.

However, this project would require research into new materials technology to make a cable strong enough to do the job.

mike alexander
2009-Aug-18, 02:50 AM
As I recall, first mentioned in The Fountains of Paradise, Clarke, 1979.

mantiss
2009-Aug-18, 03:55 AM
The delta-V requirement to get to Phobos is much less than Mars.

Is it? Considering that Phobos is so close to Mars I wouldn't think there would be all that much of a difference? But I'm not versed at all in the calculations required but on a hunch, I'd say it shouldn't be that MUCH less?

neilzero
2009-Aug-18, 04:40 AM
To get to Mars in less than a year, we arrive at Mars 10% or 20% faster than Mars. Because Phobos is orbiting fast around Mars, we can approach Phobos at perhaps 1% or 2% over speed, so the deceleration delta v is substantially less. We also gain on the return trip as we can start our trip faster. The gain is however, perhaps less than 1% of the total fuel for the round trip, Earth to Mars. My guess is Kevlar is strong enough for a Phobos space elevator, but getting a few thousand tons of tether (a few hundred tons for 20 ton rating if you can use CNT with great specs/ maybe next year) to Phobos is not a trivial project. Also the end hanging toward Mars is moving fast so we still need to do aerobraking in the Mars atmosphere, which requires an other vehicle. The Mars to end of the tether vehicle is a challenging design problem, so a Phobos space elevator is likely far in our future. None of the above is true unless Phobos keeps the same face toward Mars. Does it?
A different elevator is possible stretching away from Mars (again assuming same face) but I think it has even less utility, except for a colony on Phobos. Neil

matthewota
2009-Aug-18, 04:55 AM
It is amazing how many historical PDF documents that NASA has scanned in and has put online.

The paper mentions carbon nanotubes as the material for the cable. But nobody has fabricated carbon nanotubes yet.

The Russians will soon sent the Phobos Grunt mission to Phobos. It is easier to get there than to land on Mars.

cjameshuff
2009-Aug-18, 05:20 AM
Is it? Considering that Phobos is so close to Mars I wouldn't think there would be all that much of a difference? But I'm not versed at all in the calculations required but on a hunch, I'd say it shouldn't be that MUCH less?

It's because it's close to Mars that there is a difference. Orbital velocities in lower orbits are higher, and depending on the moon's position in its orbit, that velocity either adds to or subtracts from that of the planet itself.



The paper mentions carbon nanotubes as the material for the cable. But nobody has fabricated carbon nanotubes yet.

Er...they've been produced for decades, and are used in several commercial products. We have yet to produce carbon nanotube composites with the strength needed for an Earth-based space elevator, but production of nanotube is fairly commonplace.

Given the shallower gravity well and much shorter length of this elevator, it should be far less demanding in terms of strength, and indeed, the structure described is far less minimal than the proposed Earth-based elevators, and involves larger safety margins and lower tensile strengths than those considered for an Earth elevator (they didn't even consider strengths much more than half of what an Earth-based elevator would need).

Glom
2009-Aug-18, 10:38 AM
Wouldn't Phobos need to be moved in order for it to work as an anchor for an orbital tether?

djellison
2009-Aug-18, 11:32 AM
The Russians will soon sent the Phobos Grunt mission to Phobos. It is easier to get there than to land on Mars.

Define easier. It requires more DeltaV to rendezvous with Phobos than it does to land on Mars, starting from Earth.

Glom
2009-Aug-18, 12:35 PM
Define easier. It requires more DeltaV to rendezvous with Phobos than it does to land on Mars, starting from Earth.

Care to show your work? Mars is a massive gravity well. We're not just talking about achieving orbit, we talking about shedding all that mechanical energy to land on the surface.

neilzero
2009-Aug-18, 01:58 PM
Hi Glom 6:38 AM: Assuming "orbital tether" = an Edwards type space elevator, attached near the equator of Mars, the counter weight needs to orbit above the equator of Mars with a period of about 25 hours in a circular orbit, so either moon of Mars would require large orbit changes to serve as a counterweight. In recent years serious proposals have rarely included using a natural body as a counter weight for various practical reasons.
On the other hand either moon can have an Edwards type space elevator attached near the equator of the moon (but not attached to Mars), except the tether may collide with the other moon, without rare, but lots of movement of the ribbon = tether. Different types of orbiting tethers are also possible, but perhaps not practical. There are lots of details at the forum at www.liftport.com Neil

mike alexander
2009-Aug-19, 04:39 AM
As I recall, Clarke had his Martian space elevator tuned to resonate, so when Phobos went by it would be out of the way. He suggested charging extra for elevator seats to watrch the passage.

mugaliens
2009-Aug-20, 07:48 AM
The paper mentions carbon nanotubes as the material for the cable. But nobody has fabricated carbon nanotubes yet.

Yes they have (http://en.wikipedia.org/wiki/File:Mutr-nanotubes1.jpg). Unfortunately, they're only up to several milimeters in length.

What might work as well, if not better, is Buckypaper, at 1/10th the weight of steel, but with 500 times the strength, when stacked into a composite. Weaving the Buckypaper similar to those Chinese finger cuffs we all enjoyed as kids would yield a pseudo Buckytube of great strength.

cjameshuff
2009-Aug-20, 12:56 PM
Yes they have (http://en.wikipedia.org/wiki/File:Mutr-nanotubes1.jpg). Unfortunately, they're only up to several milimeters in length.

Cotton fibers have dimensions considerably smaller than those of a pair of jeans, and are themselves composed of far shorter cellulose chains. The molecules of Kevlar are only nanometers in length. A cable for a suspension bridge, space elevator, etc does not need to be made from continuous nanotubes extending its full length, or even a significant fraction of its length.

You're talking about molecules multiple millimeters long (multi-cm range now, actually). What do you need? Since they can be made, extremely long fibers will likely find some use, but I suspect such uses will be due more to electrical properties than mechanical ones. There's some mechanical benefit to using longer tubes, sure, but production quantity seems to be a much bigger issue.

timb
2009-Aug-20, 11:12 PM
On the other hand either moon can have an Edwards type space elevator attached near the equator of the moon (but not attached to Mars), except the tether may collide with the other moon

I don't think Deimos is going to be passing under Phobos anytime soon.