Ultra Long Orbital Tethers Behave Highly Non-Keplerian and Unstable

摘要

Large twin tethers are investigated as possible competitive-cost tools for non-gasdynamic descent, landing, takeoff and return from target celestial bodies and as passive tools for debris retrieval from orbit. The particular behavior of orbiting bodies connected with long cables is a recent preoccupation in astrodynamics and proves being full of unexpected results. The investigation here presented is focused on the non-Keplerian behavior of such large tether systems, considered in a first approximation as rigid or very stiff and massless. The investigation starts with the feasibility of non-gasdynamic orbital deployment of twin tethers without any involvement of expensive rocket propulsion means. The free tether release systems are associated to a horizontal impulsive separation (HIS) and eventual friction-free deployment to the desired length. This horizontal deployment seems to supply the most productive means of continuous separation and departure of masses in orbit. The relative motion during separation is studied and the observation is made that a considerable kinetic moment of the system preserves during all eventual phases of the flight. After the friction-free deployment the extending cable is instantly immobilized at the so-called connection moment. From here after the tether length remains constant. The evolution of the deployed tether is followed in order to record the specific behavior when the length of the tether is extremely great. The motion of the two connected masses and of the mass center proves completely non-Keplerian, beginning with the libration around local vertical due to the considerable residual kinetic moment at connection. A practical application of the quasi-vertical libration is in orbital passive debris collector, when a sandwich composite large panel is orbited for long periods of time for collecting small mass, high velocity Earth orbit debris. The most promising and controversial application of such long tethers resides in the anchoring technique to achieve the skeleton of a future space elevator. The stability of motion is an important aspect which is approached my numerical simulations.