It is well known there is a lot a space junk orbiting the Earth. From previous questions posted on this site it appears that if it were possible to establish a space elevator cable for Earth it would have to be very long. Also, it would need to be placed through the space junk zone orbiting the Earth.
In such a situation could impact by space junk/debris be serious enough to make a space elevator impractical?
The space elevator basically consists of a base station that acts as an anchor, a long cable for movement of 'climbers' and a counterweight to keep the cable in place.
The length of the cable is determined by the intended purpose. Below the altitude of geostationary orbit (~ 35,786 km), the gravity is more than the centrifugal force and above that the centrifugal force is more than the gravity.
Source: Wikimedia Commons: User:Booyabazooka
A space elevator constructed as shown above will pass through the atmosphere, the earth satellite orbits and the Van Allen radiation belts.
The debris in the low earth orbit is but one of the hazards faced by the space elevator. Aircrafts, meteorites and orbiting satellites all can potentially snap the cable.
It is possible to design the satellites so that they avoid the cable(of course you would have to replace every one out there). The orbital debris is another thing.
There are about 6000 tonnes of space junk in orbit and most of them are in the LEO orbit.
Image Source: NASA
There are more than 21,000 orbital debris of diameter 10 cm or more in orbit, which can cause serious damage to the cable as they usually travel in speeds excess of 10 km per second.
Considering the fact that the elevator cable will be practically stationary and the very high number of debris in orbit, it is certain that one of them will hit the elevator cable.
In that case, the cable will snap and more importantly, the counterweight will simply fly out of the earth orbit. Considering the probable impact height (~1000 km) and the total cable height (>36000 km), the entire system will have to be rebuilt again.
This is a serious safety issue for the vehicles using the space elevators during that time as well as they most probably will not have a propulsions stem of their own (this being the most important advantage of space elevator)
In conclusion, the safety of the elevator cable from impacts is one of the most important challenges that has to be solved if the system is to become practical.
I've been thinking about this very thing for some reason. I think, in terms of engineering and feasibility, cleaning up all that debris would be simpler than building a space elevator. So I don't think this is really all that big of a deal.
But, supposing for whatever reason we do not clean up LEO (or to protect it against any case) I'd think large sections of the elevator would have to be radiation shielded, especially where it passes through the van allen. We're also most assuredly needing electricity to course through the thing, so I suppose we could 'kill two stoned birds' by embedding or paralleling the elevator with superconducting material and sending a charge through it. I just don't know how much we'd need produce a magnetic field big enough to, not only shield from rads, but deflect incoming debris as well.
One of the gundam plots was centered around vast photo-voltaic power plants in orbit, connected to the earth by such elevators. This could also necessitate the superconducting core. My point is, I'm sure if we ever got around to building the damn thing, a solution will be generated almost implicitly.
