A space elevator passes through the atmosphere, unlike a space tether. If it is hit by lightning it will be destroyed. Nanotubes conduct electricity well. A conductor going all the way through the atmosphere would be the ultimate lightning rod. The space elevator people say they will put it where there is not much lightning. The investment in dollars and the risk to humans riding the elevator makes even a chance of lightning a serious issue. A space tether does not have a lightning problem at all.
A geo-stationary tether station is about 36,000 km away, and you have to climb uphill the whole way. If the climber averages 100 km/hour it takes 15 days to climb up. Solar powered cars can do this kind of speed on flat ground, but straight up it is not easy. At this rate it would take a long time to bring up cargo equal to the initial weight of the tether (and this assumes you had materials we don't have so that you could even build it). Some places advocate beaming power to the elevator to get around this. Even with plenty of power, there are limits to how fast wheels can climb a rope. And this beaming power is a futeristic concept that makes it seem like tethers are not practical today. In fact, a NASA study estimates it will be 50 to 100 years before this type is practical. This is not what is called "near term technology".
The reliability of elevator climber is a problem. I doubt that any car I have owned has ever gone 22,000 miles without needed some repair, and I am mostly going on flat ground. To go straight up for 22,000 miles without having to have a mechanic fix something is really high reliability for a wheeled vehicle. So this will be a high priced vehicle since high reliability takes high priced engineering and high priced materials. A rotating tether does not need a climber.
A rotating tether is practical with with existing materials and technology. It needs much less mass and so it is cheaper to build and launch. A rotating tether can also lift payloads much faster (like 10 minutes) and more often. Using cheap existing ropes, less mass in space to get started, and more frequent payloads, makes them far more financially interesting.
If it takes 15 days to climb, you have a radiation risk. You are passing through the Van Allen belts. This is not a good place to stay for days even with a couple centimeters of shielding. Your GEO hotel/space-station is going to have plenty of shielding and is only on the far edge of the Van Allen belts. Taking a long time to get there also increases your chance of getting caught by a a solar partical event (aka solar flare). A rotovator can toss to GEO in 4 hours, which greatly reduces the radiation risk. The space elevator approach would require a lot more shielding to get the radiation risk down to the level of the rotovator. The shielding is dead weight. It could take several times the mass of the people to shield the people, which raises the cost of the elavator approach.
The Van Allen belts are also going to be hard on the nanotubes used to make the Space Elevator. The radiation will weaken them rapidly. A rotating space tether does not need to be in the Van Allen belts.
The kind of rotating tether SpaceTethers.com advocates can grab a payload and then toss it about 10 minutes later. With one launch site we could do a payload every orbit, or about every 100 minutes. This is like 14 times per day, instead of every few days as a Space Elevator would be.
The main advantage of space elevators over rotating tethers seems to be that it is easier to explain to someone.
Note that at least one tether has actually already been in orbit for years without breaking.
More on the Space Elevator at spaceelevator.com.
Click for SpaceTethers.com
Copyright (c) 2002, 2003 by Vincent Cate. All rights reserved.