Xjyuk

I've heard of the hypothetical space elevator. But how about space ziplines. Have the idea been explored if so what are the challenges? By zipline I mean giant long cables connecting space station to earth that astronauts can use to travel to and from space station.

Unless the zipline stretches all the way to geosynchronous orbit (GSO), it will wrap around the earth and be destroyed. Remember that a typical satellite orbits the earth in about 90 minutes. Your zipline, being attached to the ground, will be dragged around by its top end, until the whole thing slams into the ground.

Kim Stanley Robinson has a great description of this effect in his Mars Trilogy, where a space elevator cable gets cut, so it is no longer balanced and crashes.

If the line is not attached to the ground, then the whole thing will move through the atmosphere at the speed of the satellite and burn up through friction. If it doesn't burn up, the bottom end will whip past at the speed of the satellite, i.e. 7 km/s. So you'd still need a rocket to catch it - and at 7 km/s friction will destroy that rocket, too.

If the zipline does go all the way to GSO and beyond, then you are back to having a space elevator.

Note that a space elevator is not a tower as, given its height of 36,000 km, no known materials could possibly stand up under their own weight. Instead, the elevator will be a cable under tension. To be tensioned it needs to reach beyond the GSO height of 36,000 km, and possibly with a counterweight such as a small asteroid. As any parts of the cable beyond GSO will travel faster than the local orbital speed, that excess speed can be used to launch spacecraft into deep space.

Such a zipline from a space station in a low orbit of about 400 km would be destroyed by the heat of the compressed atmosphere passed in hypersonic speed.

When most people talk about building a space elevator, they are mainly focusing on the hard part, which is building the cable from earth to geosynchronous orbit. It actually has to extend beyond it so that the center of mass will be at the geosynchronous orbit distance. This is hard because most of the cable is not in free fall. The part farthest from the earth will be straining to fly off into space, while the bottom is pulled more strongly by the stronger gravity, and that balance of forces keeps the whole thing in place. The tension in the cable would be immense, and it is not clear that large scale materials can be made that would be strong enough.

So, just building the cable would be hard, and it would have to be widest at the balance point where the tension was greatest. How wide it would have to be depends on the strength, but we are not talking about something you could wrap your hands around. It would be something rather giant.

You could still run track down the outside that you could slide down sort of like a zipline, but you would be travelling in vacuum for over 20,000 miles. You could write a good science fiction story about adventurers who wear space suits and zip along a maglev track at 2000 miles an hour, but if you can build the cable at all, most people figure the practical way to go up and down it is some kind of elevator.