The Science of Discworld Revised Edition

on a giant disposable firework.
    Until recently, there has been a general assumption that the energy to get into space has to be carried with the craft. However, we already have the beginnings of one way to get off the Earth that keeps the power source firmly on the ground. This is laser propulsion, in which a powerful beam of coherent light is aimed at a solid object and literally pushes it along. It takes a lot of power, but prototypes invented by Leik Myrabo have already been tested at the High Energy Laser System Test Facility at White Sands. In November 1997 a small projectile reached a height of 50 feet (15 m) in 5.5 seconds; by December this had been improved to 60 feet (20 m) in 4.9 seconds. This may not sound impressive, but compare with Goddard’s first rocket. The method involves spinning the projectile at 6000 revolutions per minute to achieve gyroscopic stability. Then 20 laser pulses per second are directed towards a specially shaped cavity, heating the air beneath the craft and creating a pressure wave of thousands of atmospheres with temperatures up to 30,000° Kelvin – and that’s what propels the projectile. At higher altitudes the air becomes very thin, and a similar craft would need an onboard fuel source. Fuel would be pumped into the cavity to be vapourized by the laser. A megawatt laser could lift a 2-pound (1 kg) craft into orbit.
    It is also a very powerful weapon …
    Another possibility is power beaming. It is possible to ‘beam’ electromagnetic power from the ground in the form of microwaves. This isn’t just fantasy: in 1975 Dick Dickinson and William Brown beamed 30 kilowatts of power – enough for thirty electric fires – over a distance of one mile. James Benford and Myrabo have suggested launching a spacecraft using millimetre range microwaves which are not attenuated by the atmosphere. This is a variation on the laser method and would use the same kind of projectile.
    Both of these methods rely on a lot of raw power, betraying traces of the basic engineering assumption that getting into space needs a lot of energy to overcome the Earth’s gravity. They do have the advantage that the raw power is just sitting on the planet; the 1 ,000 megawatt power station your laser launcher would require could generate for the National Grid when a launch wasn’t going on.
    A method of greater subtlety is the bolas, first proposed in the 1950s. Traditionally, a bolas is a hunting device made by tying three weights to strings and then tying the ends of the strings together. When thrown, it spins, pulling the weights apart, until the strings hit the target, at which point the weights spiral rapidly inwards and deal a killing blow. The same sort of device could be set up in a vertical plane above the equator, a bit like a giant ferris wheel with only three spokes. On the ends of the spokes would be pressurized cabins. The lowest part of the bolas’s swing would be somewhere in the lower atmosphere, the top part way out in space. You would fly up in an aircraft, transfer to the first passing cabin, and be whisked skywards. The biggest obstacle to making such a machine is the cable, which has to be stronger than any known material – but carbon fibre is well on the way to combining enough strength with enough lightness. Friction with the atmosphere would gradually slow the bolas’s rotation down, but that could be compensated for using solar power arrays up in space.
    The most celebrated device of this type, however, is the space elevator. We discussed this earlier, both as a serious technological idea and as a metaphor: here we give a few more details. In essence, the space elevator starts out as a satellite in geosynchronous orbit. Then you drop a cable from it to the ground, and the rest is a matter of building a suitable cabin and, again, finding suitable material for the cable. You get the material up there using rockets or a whole cascade of bolases (and once you’ve got a small cable you can haul up the stuff for the bigger one). You only need to do all this
once
, so the cost is irrelevant over the longer term.
    As we emphasized at the start of the book, once there is as much traffic is coming down as is going up, getting off the ground is essentially free and requires zero energy. At that point you build your interplanetary spacecraft up in space, using raw materials from the Moon or the asteroid belt. So the space elevator gives you
a new place to start from
– which is why