ASI W9800041r1.1

Moon Miners' Manifesto

#97 July 1996

Section the Artemis Data Book

Pluto-Charon Cable Car

Robert Dinkel and Francis Graham

Pluto and its moon, Charon, are both tidally locked bodies, with one face of Charon facing Pluto, as our own Moon does to Earth. But in addition Pluto has one face to Charon. So neither Pluto nor Charon rises or sets in the other's sky.

A straight line could be connected between any two points on the mutually facing hemispheres, and it is tempting to think a cable could be strung as well between the two points. Such a scheme has been proposed for a cable car connecting a point on Earth's surface with a geosynchronous space station 22,000 miles above the Earth. The problem with such a scheme is that the weight of the cable would snap itself after progressing to a height on the order of ten miles, even if the cable were made of continuously cast steel. It doesn't matter if the cable is made thick [ed. note by Mark R. Kaehny -- actually tapering the cable is generally proposed]; the increased thickness adds to the weight. When the weight per unit area equals the tensile strength, the cable snaps.

Is Pluto's case different? On June 15, Francis Graham and Robert Dinkel set out to solve this problem. Off-the-shelf steel is available which has a tensile strength of 311 kpsi, so we assume that with some reasonable engineering advance and special order it could be pushed to 400 kpsi.

The following rough dimensions of Pluto and Charon were assumed:

  • Separation 17,500 km [10,850 mi.]
  • Period 6.39 days
  • Mass of Pluto 0.0018 x Earth [0.146 x Moon]
  • Mass of Charon 0.00018 x Earth [0.015 x Moon]
  • Pluto Radius 1500 km [1463 mi. Diameter]
  • Charon Radius 650 km [807 mi. Diameter]
  • Mass of Earth 6 x 1024 kg

    First, we calculated the libration point L1 by setting the gravitational accelerations of Pluto and Charon equal to each other. Solving the resulting quadratic gives 13,295 km from Pluto's center as the point L1. That will be our upper limit for the weight integral. The lower limit is Pluto's radius.

    The element of weight per unit area is

    Formula to calculate weight 
per unit area of a cable strung between Pluto and Charon

    where p is the density of the steel (~6 x 103 kg/m3).
    Integrating over the entire length of the cable from Pluto to L1:

    Weight integral from Pluto's 
surface to the libration point for a cable strung between Pluto and Charon

    The required tensile strength is thus 370 kpsi. 400 kpsi, our futuristic steel, can go to 2.75 x 109 Nt.-m-2, easily enough. If a cable can be thus strung from the libration point to Pluto surface, it certainly can be strung from the libration point to Charon's surface. Thus we conclude that a future cable car between Charon and Pluto can be built and safe, flightless transport for a Pluto base made. Such a cable car might be valuable at Pluto because of the vastly decreased solar power available for energetic transport by photovoltaically synthesized chemical fuels, and may be worth the use of the iron, (perhaps) a rare substance on Pluto. A Pluto base might be valuable for solar system science of all kinds, a new reference for stellar parallax measurements, and a host of other fundamental science experiments.


  • Zeilik, M., The Evolving Universe, 4th ed. Harper & Row, NY: 1985 (Pluto data).
  • Rothbart, H.A., ed., Mechanical Design and Systems Handbook, McGraw-Hill, NY: 1964 (Steel Data).

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