Expert: Philip Stahl - 12/27/2004

Question
Is it possible to connect a space elevator from Earth directly to a space station if the station is in a geostationary orbit?

Answer
Hello.

Although the "space elevator" has been repeately tossed around in one guise or another, I don't think it's as feasible or practical as its advocates claim. This is apart from the enormous energy expenditures - since work must still be done vs. gravitational field- to haul loads to an orbiting space station.

More important that this is the fact that at the distance of a geostationary orbit -  about 36,000 km (22,400 mi.) above the Earth's surface- one would have to deal with *differential gravitational forces* over the elevator displacement distance.

Thus, the g- value (for acceleration of gravity) underscoring these forces, would range from about 9.8 m/s^2 at Earth's surface to 4.3 m/s^2 at half again Earth's radius up, to g = 0.22 m/s^ at the geostationary altitude itself.

For some displaced mass m on the "elevator", these would create "tidal forces" of the form:

F_T = GMm [ 1/ (r + a)^2  - 1/r^2]

e.g. where r is the Earth's radius (6.4 x 10^6 m), M is its mass (6 x 10^24 kg)  G the Newtonian gravitational constant, and a is some distance between r and R the radius of a geostationary orbit. (One varies a over r < a < R, to see what g values arise, and hence what differential forces)

Most plausibly, shearing would occur along the total length of the "wire" or system used for the elevator - creating major problems in maintaining stability.

All of the above, of course, omits numerous other considerations, including:

i) exposure to cosmic rays, gamma rays (say from solar flares) while using such a device.

ii) Possibility of a class X solar flare obliterating the electronics to run the device - leaving occupants of the "elevator" stranded at some distance between the Earth's surface and the platform at distance R.

iii) Tensile strength of the materials needed to run the device - say in order to withstand shear-tidal forces arising from the differential forces already noted.

All in all, from where I sit, I think the "space elevator" makes a neat fantasy concept - but I suspect it will never manfest in the real world.

If it ever does, I will be most surprised.