Space Elevator.....

[All Seeing Eye]

.....for Mars?
Has any proposal been put forward for a Marsian space elevator? it makes more sense to have one there first since the Gravity of Mars is far less than that of Earth and should future missions take place the space shuttle that takes people there could simply dock at the orbital station and then descend to the Martian surface via the elevator. Also any parts for future bases could be assembled on Earth, launched into space, attached to the elevator and lowered to Mars, plus resources could be raised via the elevator and shipped back to Earth.

 

[Christopher]

The idea of a Martian space elevator has not only been proposed before, but has been explored more than once in fiction, notably in Kim Stanley Robinson's Red Mars and (more fancifully) in Larry Niven's Rainbow Mars.

However, I disagree that it makes more sense to have a space elevator on Mars than on Earth. The main reason a space elevator is desirable on Earth is because it's very difficult and/or expensive to get out of Earth's gravity well using rockets, so a more economical alternative such as a space elevator would be an immense boon, indeed a necessity, for making spaceflight practical and routine. Since Mars's gravity is so much lower, it's a lot easier to get to and from its surface with rockets. So while a space elevator would definitely have advantages on Mars, those advantages would not be nearly as great or as critical as they would be for a space elevator on Earth.

 

[Lindley]

The thing about a space elevator that I have trouble with, is what keeps the top of it up?

It's obviously not rigid (thus being held up by Earth itself), because it seems likely something of that scale would be subject to immense strain due to the Earth's rotation if nothing else.

So any time something is raised to the orbital station, a downward force on the station will be exerted. The station could counter with maneuvering jets, but I have to wonder if that would save fuel in the long run.

This, of course, leads back to the question: If a mountain were tall enough, could a satellite in geosynchronous orbit appear to hover just 2-3 feet over the summit?

 

[Christopher]

Lindley said:
The thing about a space elevator that I have trouble with, is what keeps the top of it up?

The center of mass of the elevator is at geosynchronous orbit, 35,000 km up. Above that altitude, there's either another 35,000 km of elevator or a small asteroid serving as a counterweight. So the elevator doesn't need to be "kept up"; it's technically in orbit. The entire elevator moves at the orbital velocity of its center of mass (i.e. 360 degrees in 24 hours). Since orbital velocity decreases with altitude, the part of the elevator above the center of mass is actually moving faster than orbital speed for its altitude, so its momentum is pulling it outwards/upwards. Conversely, the part below the center of mass is moving slower than orbital speed for its altitude, so it gets pulled down by gravity. This keeps the elevator taut and vertical.

So its top doesn't need to be kept up, because it isn't like a skyscraper. It's essentially hanging down from geosynchronous orbit. Cut it free at the bottom, and the tether will just hover above the ground.

It's obviously not rigid (thus being held up by Earth itself), because it seems likely something of that scale would be subject to immense strain due to the Earth's rotation if nothing else.

The elevator cable is under tension, yes, but that tension is desirable for keeping it stable, as with a suspension bridge. It would require a very strong material, but something made with carbon nanotubes should be strong enough.

So any time something is raised to the orbital station, a downward force on the station will be exerted. The station could counter with maneuvering jets, but I have to wonder if that would save fuel in the long run.

Yes, there would be momentum-transfer issues, but these would typically be dealt with by sending down masses equal to what you send up. Alternatively, the counterweight could be adjusted to compensate.

Or: a conductive tether passing through the Earth's magnetic field can use magnetic induction to gain or shed kinetic energy, essentially "pushing" against the field to move into a higher or lower orbit. The elevator tether could maneuver itself in this way.

This, of course, leads back to the question: If a mountain were tall enough, could a satellite in geosynchronous orbit appear to hover just 2-3 feet over the summit?

Sure, in theory, but in this case it's the other way around -- a space elevator is a satellite in geosynchronous orbit, but the satellite is "tall enough" that its lower end can hover just over the Earth's surface, or even be attached to it.

 

[Lindley]

I hadn't considered the possibility of the thing extending "above" its destination. That does seem to solve most of the issues in a rather elegant way.

Which leaves me wondering where the sticking point is. If the design is essentially worked out, and we have materials strong enough, why don't we have one of these things yet?

 

[Christopher]

We don't quite have materials strong enough; we have a material that could be made into materials strong enough if we work the bugs out. Carbon nanotubes can be individually as strong as we need, but manufacturing them in enough lengths and quantities and with enough consistency of strength to make a viable tether are still a bit beyond the state of the art.

Also there are a variety of other engineering problems to tackle, like devising a type of motor that's able to climb a 35,000-km cable without slipping, or finding a way to deliver power consistently, or finding a way to avoid space debris in orbit, or dealing with the huge electrical voltages that could build up along the thing, etc. etc. We know it's doable, but we're still just starting out at actually figuring out the details of actually doing it. There's an X-Prize-type competition encouraging the development of tether technologies, so the work is underway, but it'll take time.

 

[Candlelight]

Not to mention some space alien coming along and using the planet as a yo-yo.

 

[SamuraiBlue]

Christopher said:
We don't quite have materials strong enough; we have a material that could be made into materials strong enough if we work the bugs out. Carbon nanotubes can be individually as strong as we need, but manufacturing them in enough lengths and quantities and with enough consistency of strength to make a viable tether are still a bit beyond the state of the art.

Not just strength but weight is also a crutial factor since tensile strength is being stressed by it's own weight with that much lengths.
We do have breakthrough in carbon nanotubes though.

http://www.physorg.com/news118920838.html

 

[StarryEyed]

It seems to me that this where we should be focusing all our efforts. Easy and cheap access to orbit will create entire new industries. It'll open the door to exploiting the moon and asteroids for raw materials and eventually, moving the entire industrial base off the planet where it belongs.

 

[Bill Morris]

Aside from that, a circular maglev track 2 km in diameter with a 30-degree exit ramp could fire off a capsule at better than Mach 23, after taking hours and a huge amount of electricity to get it up to speed, imparting about 2000 G on the payload, and requiring a rocket to correct its trajectory to place it in the desired orbit. But with enough usage, it could cut launch cost for supplies by over 90%.

 

[Irishman]

makes a hell of a terrorist target, if you ask me.

 

[Christopher]

Irishman said:
makes a hell of a terrorist target, if you ask me.

Well, as I said, if they severed it at the base, it would just hover loose above the surface, maybe drift around a bit. But if it were severed somewhere high off the ground but below the geosynchronous orbit point, then there would be a problem.

 

[StarryEyed]

So it would need to be protected. Just like our current launch facilities which would also make tempting targets.

 

[SamuraiBlue]

Christopher said:
Also there are a variety of other engineering problems to tackle, like devising a type of motor that's able to climb a 35,000-km cable without slipping, or finding a way to deliver power consistently, or finding a way to avoid space debris in orbit, or dealing with the huge electrical voltages that could build up along the thing, etc. etc. We know it's doable, but we're still just starting out at actually figuring out the details of actually doing it. There's an X-Prize-type competition encouraging the development of tether technologies, so the work is underway, but it'll take time.

In terms of climbing up(and/or down), you can always make it a loop instead of a string and attach gondolas like the ones you see at ski resorts.
A massive pully system in orbit powered by solar panels can rotate the loop and the godolas attached will be carried up as the loop rotates.
the string will not drop all the way even if it is severed and the pully can balance the ends.

 

[trekkiedane]

^Or just build a classic launch loop (pdf file) instead.

 

[All Seeing Eye]

I know Carbon Nanotubes are exceptionally strong and light which is why they're the best bet for building a Tether but is there no other material that could be used? is there no metal alloy that can be created that is both strong and light enough to become a tether?

Coudnt we create a Tether using Silk and coat it in a strong/light Metal alloy.

 

[Christopher]

Nope. Until carbon nanotubes were discovered, space elevators were purely in the realm of fiction because there was no known material with great enough tensile strength and low enough mass (remember, the thing has to support its own weight and it's almost as long as the circumference of the Earth). It would have to be based upon carbon nanotubes or some similar advanced, nanoengineered material.

 

[Lindley]

Christopher said:

Irishman said:
makes a hell of a terrorist target, if you ask me.

Well, as I said, if they severed it at the base, it would just hover loose above the surface, maybe drift around a bit. But if it were severed somewhere high off the ground but below the geosynchronous orbit point, then there would be a problem.

So enact a no-fly zone. We have some of those already, it's nothing new. Even easier if this thing is based somewhere in the Pacific, so you can spot an approaching airplane a long way off.

 

[SamuraiBlue]

Lindley said:

So enact a no-fly zone. We have some of those already, it's nothing new. Even easier if this thing is based somewhere in the Pacific, so you can spot an approaching airplane a long way off.

Well if a bomb was to be smuggled into the freight it will cause the same effect. Also if a short range ballistic missle was to be launched at it I don't think the no fly zone will have much of an effect either.

 

[Lindley]

If terrorists had a ballistic missile, I think they'd have better targets than a wire to shoot at.

As for bomb smuggling----do you honestly think anything is going to get into space that hasn't been examined molecule by molecule? That might be a concern once space travel becomes more common. Not at first.