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A novel concept for the deflection of rotating asteroids is presented, based on the conversion of the asteroid rotational kinetic energy into translational kinetic energy. Such conversion is achieved using an orbital siphon, a tether-connected chain of masses, arranged vertically from the asteroid surface, which exploits the rotation of the asteroid for the delivery of mass from the asteroid to escape. Under the conditions to be discussed, the siphon can be initiated to ensure self-sustained flow of mass from the asteroid to escape. This mechanism is proposed to use a fraction of the asteroid as reaction mass, with the asteroid rotational kinetic energy leveraged to deliver the mass to escape and hence impart a reaction on the asteroid itself. Key parameters, such as velocity change, deflection duration, tension requirements and siphon length, are discussed. Deflection effectiveness is assessed for different release strategies. It is shown that typical velocity changes on the order of 1 cm/s can be achieved within a time window of a decade.

This paper describes a novel method to compute minimum-time solar-sail trajectories starting from a given low-thrust solution. The method is based on the use homotopy and numerical continuation. Homotopy is used to link the low-thrust with the solar-sail optimal control problem. Numerical continuation is used to compute the optimal solar-sail solution, starting from a given low-thrust planar solution, which is normally easy to find. Planar solar sail trajectories are computed by means of the homotopic approach. These solutions are used to compute, in a single shooting approach, three-dimensional solar-sail trajectories, for transfer scenarios involving a small change of the orbital inclination. The proposed homotopic approach is tested against a conventional approach, based on the use of a genetic algorithm. Numerical test cases are performed both on Earth-Mars and Earth-Apophis rendezvous. The results show that the proposed method is advantageous, in terms of accuracy of the solution and computational time.

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