Further Reading

The best popular description of the Pioneer plaques is included in Carl Sagan's Cosmic Connection (Doubleday, Garden City, New York, 1973). To learn more about the Voyager plaques, consult C. Sagan, F.D. Drake, A. Druyan, T. Ferris, j. Lomberg and L.S. Sagan, Murmurs of Earth (Random House, New York, 1978).

Various aspects of the prototype holographic message plaque created for the NASA Marshall Space Flight Center and subsequent research on holographic applications to spacecraft propulsion are reviewed in G.L. Matloffs Deep Space Probes, 2nd edn ( Springer-Praxis, Chichester, UK, 2005). Another source is G.L. Matloff, G. Vulpetti, C Bangs and R. Haggerty, The Interstellar Probe (ISP): Pre-Perihelion Trajectories and Application of Holography. NASA/CR-2002-211730 (NASA Marshall Space Flight Center, Huntsville, AL, 2002).

Higher still and higher

From the earth thou springest Like a cloud of fire;

The blue deep thou wingest

Percy Bysshe Shelley, from To a Skylark

IMAGINE catching a plane to a Pacific island located on the equator, walking from the airplane to an elevator that goes straight up—into space—taking you from the ground to any altitude up to or below geosynchronous orbit (that is, approximately 35,786 kilometers—the altitude at which an Earth-orbiting satellite travels with a velocity that maintains it over a fixed point as the Earth rotates beneath it). If you were to travel the full 35,786 kilometers in the elevator, you too would be in orbit around the Earth—and paying just the cost of the electricity operating the elevator to get you there. And this would all be as effortless on your part, and as reliable, as the flight of a skylark.

It is not a new idea. In fact, a visionary Russian scientist named Konstantin Tsiolkovsky first proposed it over a century ago. He claims to have thought of the idea while looking at the Eiffel Tower. He imagined a cable running from the Earth to space, at the end of which would be a way station for space travelers. Is such a tower possible? If it were, would it be the "best" way to get into space? What would be the implications for our exploration of space? These questions are still being debated today. There is no doubt that it would be a cost-efficient, low-pollution method of space access, so it merits some discussion.

If we assume that a space elevator can be constructed, what would it have to be made of in order to not collapse under its own weight or, alternatively, be pulled apart while it is under construction?

Even if we could develop the super-strong, very low-mass material out of which a ground-to-geosynchronous space elevator could be constructed, there are a number of very major obstacles, one of which is what to do about orbital debris.

Because the top of a space elevator is in geosynchronous orbit, the entire structure will seem to hover over one point on Earth's surface. Satellites in low Earth orbit will whiz past the tower's structure at about 8 kilometers per second. If a small piece of orbital debris struck the elevator, the structure would be severed. The entire 35,000 kilometer length of the severed space elevator might then wind itself around Earth's equator several times, with unfortunate consequences for dwellers of equatorial countries such as Ecuador. So we might initially consider instead some less ambitious applications of long, thin cables in space.

Electrodynamic Tethers: Tapping a Planet's

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