In our July cover story, “Going Up,” contributing editor Brad Lemley explored maverick aerospace engineer Brad Edwards's wild plans to build an elevator into space. The idea caught the imagination of our readers, who had many questions. Edwards, contributor to several space elevator organizations and president of Carbon Designs, a developer of high-strength materials, has graciously answered a selection.

Full name: David O. Scaer

E-mail: vergerdecedres@yahoo.fr

Location: Roanoke, Virginia

Question: How thoroughly I enjoyed the article on the space elevator [“Going Up,” July]! The sheer whimsy of it, plus the notion that it just might work, combined to make for a great read. One of the places I got stuck on was how all the energies of the various elevator components get managed. If it takes a big rocket to get a spool of ribbon into geosynchronous orbit in the first place, would it not take similar energies to unwind it back toward Earth (and still more energy to unreel a counterweight away from Earth)? Wouldn’t the dangling end of a 22,300-mile-long ribbon have to be deorbited with a big booster just to get it to approach Earth again?


Edwards: From geosynchronous orbit the ribbon would be pulled down by gravity, and the counterweight would be moved upward by an electric propulsion system and the outward acceleration due to Earth’s rotation. No large boosters would be required. The end of the ribbon descending to Earth would need to be slowed, which would be done by brakes on the spacecraft. We should also remember that the energies required here are very manageable when spread out over a week or so. Boosters are large and impressive largely because they are inefficient.

Full name: Jim Chamberlain

E-mail: jfchamberlain@earthlink.net

Location: Thousand Oaks, California

Question: The story about the space elevator only discusses the strength of the cord used to fabricate it. Nowhere does Brad Lemley address the matter of tangential velocity. At Earth’s surface the velocity at the equator is approximately 1,048 miles per hour, and at the top of his elevator the cord would need to have a velocity of about 17,280 mph. Any object traveling up the cord must have its tangential velocity steadily increased to remain geostationary, and the cord could not provide that energy. If you got off the elevator at the space station, you would simply fall right back to Earth without the necessary orbital speed necessary at the 240-mile altitude of the station.

Edwards: The orbital velocity is supplied by the ribbon as the climber ascends. This is easily accomplished if the ribbon bends very slightly (less than a degree). It is true that if you ascended the ribbon to only 240 miles up you would fall back down to Earth if you let go. If you ascend to 25,000 kilometers, you would fall into an elliptical orbit, with its lowest altitude being that of the station. If you ascend it to geosynchronous orbit, however, you would be in orbit when you stepped off. If you ascend even further, you would have enough velocity to escape Earth’s clutches and travel to the moon or to Mars.

Full name: William H. Shallenberger, PE Ret.

E-mail: billshall@juno.com

Location: Oxnard, California

Question: What an interesting article and a fascinating concept. Here is one of my concerns: As Earth rotates, the gravitational forces of the sun and moon cause tidal movements in the oceans. These waves are restricted by the continents and by friction on the seafloor. What effect would these forces have on the space elevator? Would the outer end of the ribbon wave back and forth? In doing so would it be straight, as if hinged from the Earth end, or would it assume some other shape?

Edwards: The gravitational forces of the sun and the moon can move the elevator within a 24-hour period. The frequency of the elevator is 7 hours, so the interaction is poor, and little more than a small deflection would occur. Any such oscillations can also be canceled by an opposite movement of the anchor station.

Full name: Tommy Person

E-mail: tperson@midsouth.rr.com

Location: Memphis, Tennessee

Question: “Going Up” was fascinating. The idea is so simple (if the carbon nanotubes can be mass produced) that it’s brilliant. However, a couple of things are still unresolved in my head. First, why do the space shuttle and other rockets have to reach bone-jarring velocities to get into orbit but the elevator could lollygag along at only 125 miles per hour to go higher than any rocket? Also, would there not be a problem with this ribbon becoming a massive electrical conductor as it spun through the magnetosphere traveling at Earth’s rotational velocity?

Edwards: Since the climber is hanging on a ribbon, it can travel at any speed and can even stop without penalty. Rockets do not have this luxury. A rocket must continue to produce thrust to ascend or maintain its altitude. The faster it ascends, the more efficient it is. Rockets are just barely efficient enough to make orbit; if they were only slightly slower, they wouldn’t make it. The ribbon is conducting, but since the magnetosphere and the ribbon are rotating together, little current is produced. There is some current produced by the ribbon passing through the interplanetary magnetic field, but this is also minimal.

Full name: Julian Kane

E-mail: MURIELKANE@aol.com

Location: Great Neck, New York

Question: Brad Lemley writes that Brad Edwards’s concept of space elevators, moving along fantastically strong nanotube ribbons, could make space voyages as simple and safe as using planes, trains, and cars to travel on Earth, by eliminating the use of huge, dangerous rockets to escape the strong pull of gravity close to Earth. However, an impediment that must be overcome before this bold and innovative plan can succeed involves the conservation of angular momentum. A mass ascending a fixed cable from the 1,140-mile-per-hour, 15-degree-per-hour eastward-rotating equator surface is continually increasing its angular inertia. Therefore, wouldn’t its angular (as well as linear) velocity decrease as it traveled up, farther away from Earth’s surface and its spin axis? Wouldn’t the rising mass continually drift to the west and cause the taut nanotube tether-cable to bend and pull an attached geosynchronously orbiting space platform closer to Earth at increasing velocities until it struck Earth in a spectacular crash?

Edwards: Actually, as the climber ascends, its angular velocity will increase linearly with the distance from Earth’s center. This angular velocity is provided by the ribbon, and yes, the ribbon is slowed slightly by the climber as it ascends. The amount the ribbon is bent is a function of the rate of ascent and the relative masses of the climber , the ribbon, and the counterweight. Since the ascent is relatively slow (taking a week to get to geosynchronous orbit), and the climber’s mass is only 1.5 percent that of the ribbon and the counterweight, the ribbon will be deflected a small fraction of a degree at most. The angular momentum of the ribbon and the counterweight is restored by Earth’s rotation. This has all been modeled carefully, so there is no need to fear a “spectacular crash.”

Full name: William Carpenter

E-mail: Billswmc@aol.com

Age:14

Location: Kennett Square, Pennsylvania

Question: If you were to put a space elevator into orbit, wouldn’t it slow the rotation of the Earth ever so slightly? It seems very similar to a person spinning on a stool. When a person spins with his legs tight to his chest, he spins faster, and when he stretches out his legs, his rate of rotation slows. His mass doesn’t increase or decrease during the spinning, but the placement of the mass changes, and so does the rate of rotation. If we follow this line of thought, is it possible that a space station attached to Earth via a carbon cable (as described in the article) would slow the Earth’s rate of rotation because it would be like Earth sticking out its “legs”?

Edwards: Yes, the space elevator would slow Earth’s rotation. However, we must look at the amount it will slow Earth itself. If we were to launch the maximum from 100 space elevators for 1,000 years, it would lengthen Earth’s day by about 100 nanoseconds, or by a second in a billion years.