Science Forums 2

[CraigD]

Originally Posted by Eclipse Now

What was the Kim Stanley Robinson propulsion system using materials gathered from the gas giants themselves? Are we saying that harvesting the gas from Jupiter is now out because the equipment required to do that would be "nuked" by radiation?

No.

I’d describe the vicinity of Jupiter is better described as “high wear”, rather than “out”, for spacecraft equipment. A case in point, the Galileo spacecraft, which was designed in anticipation of about 1/3 the radiation it actually encountered, operated successfully there for 7 years, from Dec 1995 to Jan 2002, and well enough to respond to commands and deorbit into Jupiter’s atmosphere Sep 2003.

Originally Posted by Eclipse Now

I'm not sure about Earth's Van Allen belts affecting the space elevator, unless there were people required to stay in those Belts. How high are the belts? Surely not high enough to affect the station at the end of the 36k km long elevator?

The inner Van Allen belt, which contains the highest energy particles, extends from roughly 700,000 to 10,000,000 m altitude, so is well below geostationary orbit at about 36,000,000 m (GEO).

Though many descriptions of space elevators assume them to be about 36,000,000 m long, with a massive countermass “station” slightly above GEO, and attached to the ground, it’s worth noting, I think, that this isn’t the only possible design, or even necessarily the best. The advantages of having the bottom of the elevator above the ground, at an altitude easy to reach but above the thickest part of the atmosphere (say, 30,000 m altitude) has been discussed in previous threads. The advantage of having it extend far above GEO is that an object (eg: a spacecraft) released at that altitude would have more speed than required for orbit. For instance, at about 10,000,000 above GEO, an object released would escape the Earth. At about 51,000,000 above GEO, an object released at the right time would be given the extra about 2929 m/s needed for a transfer orbit to Mars. Extending above GEO is easier than extending below it, as the acceleration away from GEO is smaller than for extensions below. At 10,000,000 above GEO, it’s about 0.14 m/s/s (about 0.014 g), at 51,000,000, about 0.45 m/s/s.

To have 1 g (about 9.8 m/s/s) acceleration, the elevator would have to extend 1,800,000,000 m above GEO, 50 times the distance from GEO to the Earth’s surface, and almost 5 times the distance to the Moon. At about 499,000,000 m, about at the Moon’s orbit, the acceleration’s about 2.65 m/s/s (0.27 g), and an object released there would exit the solar system.

Originally Posted by Eclipse Now

But in the KSR books the Martian space elevator came down …

That was a great moment in storytelling, but also a rather horrible idea when considering the potential impact on earth.

Another common depiction of space elevators is that they are very massive. However, calculation like those in “Space elevator cable mass & size calculating program” suggest that only a small difference in material strength results in space elevators being either impossible (as is the case with ordinary high strength steel), massive (as with Kevlar, with a mass of about 3.5e15 kg and a maximum effective circular cross section diameter of about 520 m), or nearly microscopic (as with hypothetical graphite “buckytube” material, with a mass of about 26200 kg and an effective diameter less than 0.001 m).

If materials much stronger than present days one are used to build a space elevator – which, given the difficulties bordering on impossibility otherwise, is IMHO likely to be the case if they’re every built - it’s quite possible the resulting structures will actually be lighter than air. In this case, broken cables, of which there are likely to be many, would be cause for concern about a bizarre kind of air pollution, rather than a catastrophic surface impact. Depending on its specific characteristics, this could more hazardous than a massive surface strike – imagine the effect on a jet engine of sucking in a hair-thin, kilometers long, super-strong piece of lighter than air broken space elevator!