The Devil has been on about how cool space elevators are. These are essentially bits of string that connect geostationary satellites (which orbit the earth at the same rate as it spins) with the point on the equator below them. Once you’ve got the string connecting the two, you can set up a lift to raise stuff up from the ground to the satellite. No need for huge noisy rockets spewing out greenhouse gases. That’s it, very roughly.
But I had to comment that, even if a space elevator is pretty cool, an orbital siphon would be even cooler, because it wouldn’t even need a lift motor. Once you’ve got a siphon working, the launch costs are zero. Of course a bloke from Spain immediately said the idea wasn’t workable.
What’s an orbital siphon? Well, if a space elevator is a 35,000 km piece of string attached to a massive satellite, an orbital siphon is the same sort of thing, but much longer. In fact, it’s at least 175,000 km long. And that’s pretty much half way to the moon. The interesting thing is that, once you have a chain of masses extending out from the equator to that sort of distance (or further), the chain pulls upwards. That’s because the net centrifugal forces in the chain pulling upward exceed the net gravitational forces pulling downward. And if you let go of it at the bottom, it’ll go upward. And this means that if you let it go up a bit, and release the topmost mass (which would be some spacecraft or other), and at the same time have the chain pull up a new mass at the bottom, you can keep releasing payloads at the top, and feeding them in at the bottom, like on an escalator.
The diagram at right (which is to scale) shows a chain of masses extending 250,000 km out from the earth’s equator (the earth is the little blue circle, and the geostationary orbit is the grey circle around it). The mass chain hangs back slightly by 10 or 15 degrees from vertical (grey line). The diagram is taken from a simulation model of a siphon in which small masses are raised at 50 metres/sec (180 km/hr) upwards on the siphon mass chain. At this speed, it takes 60 days for a mass placed on the bottom of the siphon ‘escalator’ to be raised from the earth’s surface to 250,000 km out in space. And in this particular case the siphon can pump about 16,000 kg of stuff into space every year. And keep on doing so, year after year.
If you wanted to catch a ride on the siphon, you’d probably have to book months or years in advance. If you were offering, say, honeymoon trips round the solar system, you’d have to provide a spaceship with a few small thrusters on it, and some wheels, and a piece of cable to hook to onto the next payload up (which might be a tractor on its way to Martian mines). At the appointed time, you’d hook your spaceship onto a long line of other payloads at the base of the siphon, and when your turn came, you’d be accelerated vertically upwards onto the rising siphon train. After 60 days, when you’d got to the top, you’d be released, and would fly off towards Venus (appropriately), and with luck your honeymoon couple would enjoy the journey of a lifetime around the sun, and return to the earth a few months later, with a bit of help from the thrusters.
It’s not quite that simple, of course. The tension in the siphon mass chain is pretty low at the bottom, and at the top, but reaches a maximum at geostationary altitude, and the stresses are so great that there’s no known material that can withstand them. It needs a bit more than a piece of string. So nobody’s going to build one any time soon. But if they can build a space elevator, which only extends as far out as the grey circle, they’ll have laid the foundations for a siphon.
The great thing about the siphon, as I said, is that you don’t need any fuel to get into space. When you show up at the base of it, and get hooked onto the upbound train, your spacecraft only needs to be accelerated up to 180 km/hr to join the rising train. And the siphon itself can almost certainly provide the power to do that, because at the base of the siphon there has to be a braking system that stops it from accelerating upwards and going too fast. And if this braking system consisted of a dynamo, it would be producing more than enough electrical power to accelerate your spacecraft, and plenty enough to power the lights on the whole 250,000 km chain as well, and the town around the siphon base as well.
And this super-low cost would make space travel super-cheap, and make it possible to send entire space colonies (like all those gigantic things you see in sci-fi movies) into space. Siphons could also be built on other planets or asteroids around the solar system, if they spin fast enough.
I put together a computer simulation model of a siphon back in 1997, and stuck it on the internet. Some ten years later, a professor of astronautical engineering contacted me to ask how it was supposed to work. I explained, and we ended up presenting the idea together at a spaceflight conference in Japan. It seems that I was the first person to think of one of these things (although I seriously doubt that I was). Anyway, it was great fun to go to Japan and talk about it all, and meet the space elevator people.
So I reckon that if they ever build one of them, maybe in a couple of centuries time, who knows, they might well put a statue of me at the bottom. Most likely just outside the booking office, between KFC and McDonald’s and the left luggage office.
I just hope they make sure I’ve got a cigarette in my mouth. It would make a most convenient perch for the pigeons.