dietz@SLB-DOLL.CSNET (Paul Dietz) (12/13/85)
On recovering material from Martian moons with tethers: Deimos is farther out than Phobos, so less energy is required to lift mass. However, Deimos has less angular velocity, so a longer cable would be needed to sling rocks to the same velocity. Phobos is about 2.8 radii from Mars-center, and goes about 2.2 km/sec. Deimos is about 7 radii out, and goes about 1.4 km/sec. Material at Phobos has to be accelerated by .9 km/sec to escape Mars; material from Deimos by about .6 km/sec. The length of a cable needed to sling a payload to escape velocity is 2^(1/3) - 1 times the radius of the moon's orbit (assuming the orbit to be circular); about 26%. This is around 2300 km for Phobos. At Phobos the acceleration due to martian gravity is about .5 m/sec^2, so the acceleration of a mass on the end of this cable (centripetal acceleration minus gravitational acceleration) is about .3 m/sec^2. A 1000 tonne payload will exert a force equal to that of a 30 tonne mass on Earth's surface. (Question: how thick does a kevlar cable have to be to withstand this tension?) I've ignored cable mass. Just accelerating a mass to escape velocity is not terribly useful. If the cable length equals the radius of Phobos's orbit, it will leave the payload with a velocity (relative to Mars, at infinity) of 3.7 km/sec. The cable will have to be stronger in this case. We don't necessarily need to supply all the velocity change needed to get to Earth just with the tether. It might be possible to sling a payload into a Mars-crossing orbit that encounters Mars again on the next revolution, subtracting more momentum. Similar encounters with Venus and Earth could be used to bring the payload to Earth on a more nearly circular orbit. The payload may take years to get to earth but only limited reaction mass is needed. (Question: what is the velocity change needed to get from a circular orbit at Mars to an elliptical orbit with apohelion at Mars and perihelion at Earth, ignoring the planets' gravity?) This strategem substitutes computing power and precise tracking for brute force, but takes longer. Payloads would be carried in shells made on earth or in earth orbit. The shells would be equiped with limited maneuvering capabilities and would serve as aerodynamic bodies for the final aerobraking maneuver at Earth. The shells could be ferried to Mars (by solar sail?), or sent under their own power. Fuel should be made at Mars; an important question is whether non-cryogenic chemical fuel can be processed from the Martian moons (do they contain nitrogen, for hydrazine, say). If the moons contain nitrogen and carbon then it may be possible to manufacture the cable at Mars. Kevlar might be hard to make; nylon should be easier (what is the tensile strength of nylon?).