It is tempting to introduce a little science fiction here, and speculate on a few materials that do not yet exist in stable, useful form. The last two items in TABLE 2 both fall into the category of Fictionite (also known as Unobtainium), materials we would love to have available but do not.
A muonium cable would be made of hydrogen in which the electrons in each atom have been replaced by muons. The muon is like an electron, but 207 times as massive, and the resulting atom will be 207 times as small, with correspondingly higher bonding strength. Unfortunately the muonium cable is not without its problems, quite apart from the difficulty of making it in solid form. The muon has a lifetime of only a millionth of a second; and because muons spend a good part of the time close to the proton of the muonium atom, there is a good probability of spontaneous proton-proton fusion.
Time to give up? Not necessarily. It is worth remembering that a free neutron, not forming part of an atom, decays to a proton and an electron with an average lifetime of twelve minutes. Within an atom, however, the neutron is stable for an indefinite period. We look to future science to provide means of stabilizing the muon, perhaps by binding it, as the neutron is bound, within some other structure or material.
Positronium takes the logical final step in getting rid of the wasted mass of the atomic nucleus completely. It replaces the proton of the hydrogen atom with a positron. Positronium has been made in the lab, but it too is highly unstable. It comes in two varieties, depending on spin alignments. Para-positronium decays in a tenth of a nanosecond. Ortho-positronium lasts a thousand times as long — a full tenth of a microsecond.
We are unlikely to have these materials available for some time. Fortunately, we don’t need them. A solid hydrogen cable will suffice to build a beanstalk. Its taper factor is 1.6, from geostationary height to the ground. A cable one centimeter in diameter at its lower end is still only 1.3 centimeters across at geosynchronous altitude. To give an idea just how long this thin cable must be, note that our one-centimeter wire will mass 30,000 tons. And it’s strong. Slender as it is, it will be able to lift payloads of 1,600 tons to orbit.
TABLE 3
Beanstalks around the solar system
Body | Radius of stationary satellite orbit* (kms) | Taper factor (hydrogen cable)
Mercury 239,731 | 1.09
Venus 1,540,746 | 1.72
Earth 42,145 | 1.64
Mars 20,435 | 1.10
Jupiter 159,058 | 842.00
Saturn 109,166 | 5.11
Uranus 60,415 | 2.90
Neptune 82,222 | 6.24
Pluto** 20,024 | 1.01
Luna 88,412 | 1.03
Callisto 63,679 | 1.02
Titan 72,540 | 1.03
* Orbit radius is planetary equatorial radius plus height of a stationary satellite.
** Pluto’s satellite, Charon, is in synchronous orbit. If so, a beanstalk directly connecting the two bodies is possible.
Beanstalks are much easier to build for some other planets. TABLE 3 shows what beanstalks look like around the solar system, assuming we use solid hydrogen as the construction material. As Regulo said, Mars is a snap and we could make a beanstalk there with materials available today. Kim Stanley Robinson included a Mars beanstalk in his Mars Trilogy, Red Mars, Green Mars, Blue Mars. My only objection is that he destroyed the stalk cataclysmically in Red Mars, and in so doing obliterated the town of Sheffield that stood at its tether point.
Building the beanstalk
We cannot build a beanstalk from the ground up. The structure would be in compression, rather than tension, and it would buckle under its own weight long before it reached geostationary height.
We build the beanstalk from the top down. In that way, by extruding cable simultaneously up and down from a production factory in geostationary orbit, we can preserve at all times the balance between outward and inward forces. We also make sure that all the forces we must deal with are tensions, not compressions.
The choice of location for production answers another question raised earlier: Where will we obtain the materials from which to make the beanstalk?
Clearly, it will be more economical to use materials that are already in space, rather than fly them up from Earth’s deep gravity well. There are two main alternatives for their source: the Moon, or an asteroid. My own preference by far is to use an asteroid. Every test shows the Moon to be almost devoid of water or any other ready source of hydrogen. Two of the common forms of asteroid are the carbonaceous and silicaceous types, and coincidentally carbon and silicon fibers are today’s strongest known materials. A small asteroid (a couple of kilometers across) contains enough of these elements to make a substantial beanstalk.
If the solid hydrogen cable proves to be the only acceptable answer, then we need to seek farther afield for construction materials. Hydrogen is readily available in the solar system, but not on small asteroids whose orbits bring them anywhere near the Earth. Their volatile materials have long since boiled off due to solar heating. However, if we look farther out, hydrogen as components of water and methane becomes plentiful. A comet, which is little more than a huge dirty snowball, would serve us very well to make a beanstalk; and quite a small comet, with a head a few kilometers across, is big enough.
We must tether the lower end of the beanstalk cable at the equator. As a fringe benefit of the system, if we send mass all the way to the end of the beanstalk, far beyond geostationary orbit, then we will also have a free launch system. A mass released from 100,000 kilometers out can be thrown to any part of the solar system. The energy for this is, incidentally, free. It is provided by the rotational energy of the Earth itself.
Using the beanstalk
A load-bearing cable is not a transportation system, any more than an isolated elevator cable is an elevator. To make the transportation system, several additional steps are needed. First, we strengthen the tether, down on Earth’s equator, so that it can support a pull of many thousands of tons without coming loose. Next we go out to the far end of the cable and hang a big ballast weight there. The ballast pulls outward, so the whole cable is under an added tension, balancing the pull of the ballast against the tether.
We are going to attach a superconducting drive train to the cable. This will employ linear synchronous motors to move payloads up and down the length of the beanstalk. These motors are well-established in both principles and practice, so we can use off-the-shelf fixtures — except that we will want about 100,000 kilometers of drive ladder, and will need appropriate construction facilities and abundant materials. Here we will find a use for an asteroid of different composition, one high in metallic ores.
The motors will drive cargo cars up and down the beanstalk. Passengers, too, if the traveler is willing to put up with a rather long journey. At a uniform travel speed of 300 kms an hour, a journey to synchronous orbit will take five days. Much slower than a rocket but a lot more restful, and with spectacular scenery, this trip may resemble a leisurely transatlantic crossing on one of the great ocean liners.
The added tension provided by the ballast is very important. Each time a payload is attached to the drive train, the upward force on the tether is reduced by the weight of the payload. However, provided that the payload weighs less than the outward pull of the ballast weight, the whole system is stable. If the payload weighed more than the ballast’s pull, we would be in trouble. The whole beanstalk would be dragged down towards the Earth.