Bringing up Phobos

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A proposal to solve the future Phobos crisis (crash Phobos into Mars) by raising its orbit.

  • If there is any fear of Phobos falling on a Mars colony, the possibility exists to raise Phobos to a higher orbit using its own orbital energy. Glass-glass composite cable manufactured out of the stuff of Phobos should probably be strong enough to hang the cable one thousand miles Marsward from Phobos. Material lowered to the Marsward end of the cable and dropped will leave Phobos in a higher orbit, eliminating the need to worry about Phobos falling on a future Martian settlement.
  • The tether would go up from Phobos in the Marsward direction about 17 kilometers to the Mars-Phobos L1 point and then down in the Marsward direction another 983 kilometers. The tidal force at the lower tether tip would be 0.17 meter per second squared. The cable could vary in cross section area to be close to its design stress over the entire length. Two sheets of glass film could encircle the cable as micrometeoroid protection with at least 75 mm (three inches) between the two sheets and between the inner sheet and the cable. Bogies carrying the wheels and electric motors would be encircled with the cable while the cars would be supported by structural connections to the bogies that would extend through slots in the micrometeoroid shield, one slot for upward bound cars and one for those downward bound. The shield would be attached to the cable by numerous curved glass whiskers. These whiskers would be like a glass slinky stretched out in the space between sheets and welded with a lower melting point glass wherever the slinky touches a sheet or the cable. The slot in the shield would be angled to avoid a straight line path for micrometeoroids to the cable. The cable would be shaped to include two rail-like portions of its cross section so upward bound empty cars could run on one rail and downward bound cars full of Phobos material could run on the other.
  • Regenerative braking would produce electricity from the downward bound cars to drive the upward bound ones. Excess electricity would be used to toss the Phobos material to the rear to raise Phobos' orbit more than would be the case by simply dropping the material. This would be accomplished by an electric accelerator at the Marsward tip of the tether. The direction of the thrust could be varied in phase with Phobos' orbit to reduce the inclination of Phobos to the equator. Its orbit is presently inclined slightly more than one degree to the equator. When Phobos' orbit has been raised to synchronous altitude, high strength to weight ratio materials could be used to extend the length of the cable as far as people find practical at that time.
  • Phobos material dropped by this means would be ground up so that either the particles would burn up on contact with the Mars atmosphere, or the surviving remnant cinder would be so small and light as to cause no unacceptable damage to Mars installations.
  • If a tether from Mars' surface to orbit is someday constructed, it would be better to use Phobos for the low strength requirement central portion of the cable than to crash Phobos into Mars to avoid it bumping into the tether.

Another variation of this idea is to put a small mass driver on Phobos and accelerate it in its orbit by firing rock and ice backwards. This will raise the orbit of Phobos and lower the orbit of the ejected mass (which will burn up in reentry.) The above plan is better, (in that useful work is preformed by the elevator), but this version has a much lower capital cost.

Analysis

In a sense, this is a method of very slowly taking Phobos apart and crashing little bits of it, to avoid crashing the whole thing in a cataclysmic event. A mathematical model would be helpful here, showing the forces on Phobos that are creating the problem in the first place and the mathematics of the mass/energy exchange. The energy required to raise Phobos' orbit to use it for a geostationnary elevator base station should be calculated, to see if there is any economical sense in the idea.