Space elevators

[pozorvlak]

Another Important Idea That Everyone Should Know About :-)

Capture a small asteroid. Stick it into geosynchronous orbit above a reasonably stable part of the Equator. Attach it to the ground with a big cable. It will need to be a very strong cable to support its own weight (to reach geosynchronous orbit, it would have to be nearly 36,000 km long), so you'd better make it out of carbon nanotubes. Run a railway line up the side. Ta-da! You've just reduced the cost of reaching geosynchronous orbit by a factor of more than 100.

What you have just built is called a Space Elevator.

This idea isn't actually as daft as I've just made it sound, and some very bright people (including NASA) are working on building one. In fact, if you want to go to Mars, it would probably be cheaper to build a space elevator first. These guys reckon they can do it by 2031. There's lots more information in the Wikipedia article linked; I'd also recommend Arthur C. Clarke's excellent novel The Fountains of Paradise, which is half about the construction of a space elevator and half about his beloved Sri Lanka.

Comments

[elvum.livejournal.com]

A minor correction: the asteroid is the counterweight for the mass of the elevator itself, so it needs to go beyond the altitude of geostationary orbit in order to exert a force on the cable. How far depends on the mass of the asteroid. Then at geostationary orbit you can build a space station. As a bonus, the asteroid has a fictional "gravitational" force that pulls things away from the Earth. I call it "gravitational" because lack of air aside, you could live quite happily on the Earth-facing surface. Walk round it a bit and things can fall off the asteroid and get flung into space. This is a bonus for spacecraft, but not for hand tools, scientific instruments, small children etc. It might get quite spooky living there with an enormous planet looming above your head...

Something I've never seen discussed (probably because I've never read a scientific paper on the subject) is the fact that a space elevator would have to withstand a horizontal force from anything climbing up or down it: an object in geostationary orbit is moving much faster around the centre of the Earth than an object on the ground. I think this would (a) cause the elevator to swing very slightly, akin to giving the string of a pendulum a little push, and (b) increase the tension in the cable slightly.

[pozorvlak.livejournal.com]

You're right, of course: I was oversimplifying. The other option is to park your asteroid in geosynchronous orbit, and then build in both directions: one line down to the planet's surface, and one line up and out, to allow for deep space launches. Pick the right asteroid and you can mine it for raw materials as you build. Or simply do without the asteroid, and build a cable that's 72,000 km long instead. I didn't realise about the "gravitational" force on the asteroid - that's very cool.

Horizontal force: Sounds about right (http://en.wikipedia.org/wiki/Space_Elevator#Angular_momentum.2C_speed_and_cable_lean). You can help somewhat by pairing climbers going up with climbers coming down.

[steerpikelet.livejournal.com]

Can I press the 'up and out' button?

[pozorvlak.livejournal.com]

But of course (http://en.wikipedia.org/wiki/Space_Elevator#Launching_into_outer_space) :-)

[bdunbar.livejournal.com]

One minor correction - you don't need an asteroid. You simply need a mass in orbit above GEO. Brad Edward's insight was that if you extend the ribbon beyond GEO for a total length of 100,000 km then IT can function as the counterweight.

Not that getting an asteroid isn't a nifty idea but it does increase the cost of the thing, just a wee bit.

Also of note you can inject satellites into LEO with a space elevator. Just let go sligtly below GEO and your load will shape an eccentric orbit. Rocket motor to circularize your orbit at LEO and you're set.

[pozorvlak.livejournal.com]

Both true, and good points. I was trying to keep things simple in my explanation, but you're right, those were both worth mentioning.