title: Engineering near-Earth asteroid resources using the orbital siphon effect
Creators: name: Viale, Andrea
Publisher: name: University of Glasgow
Year: 2021
Subjects: subject: asteroid engineering, asteroid mining, orbital siphon, space resources.

description: Exploitation of the resources available in space is one of the key challenges for future space exploration. Many of these resources have been recognized as potentially low-cost alternatives to those launched from Earth. In particular, near-Earth asteroids are among the easiest objects to reach and could provide resources such as water, liquid propellants electrolysed form water, semiconductors, and metals. Several studies have shown that a useful quantity of accessible resources may be available to be transferred into Earth orbit with transfer energies lower than that required to exploit material from the Moon. To address this problem, different scenarios can be envisaged to transfer material to Earth orbit or Halo orbits, such as transport of the entire asteroid or transport of mined material, the optimal choice depending on the particular asteroid of interest. A further possibility is in-situ manufacturing using asteroid resources, for example to assemble space-structures directly nearby the asteroid or to process water for propellants or life support. Motivated by this growing interest in asteroid resource exploitation, this thesis investigates a novel strategy to deliver a fraction of the asteroid mass into orbit about the asteroid or to escape. The analysis has its roots in the idea of leveraging the rotational kinetic energy of a rotating body to lift material, for example with the concept of the space elevator. The elevator is envisaged as a tethered structure to connect a mass in synchronous (or higher) orbit and the surface of the body. The tether is in equilibrium by the balance of centripetal and gravitational forces acting on it; the payload, i.e. mass extracted from the asteroid, is then lifted to the desired altitude along the tether and, if synchronous orbit is reached, the payload could increase its altitude without further work required. A direct evolution of the space elevator is the orbital siphon concept which is the foundation of this thesis. In this case, rather than a single payload ascending along the tether, a chain of tether-connected masses is envisaged, where the centrifugal-induced pull due to the body's spin can overcome the gravitational force on the payloads, eventually allowing payloads to escape. A chain of payloads can therefore be envisaged to provide a continuous mass flow from the surface of a rotating asteroid into orbit (siphon effect): new payloads are connected to the chain while the top payloads are removed and released into orbit, without the need for external work to be done. The siphon, as with the space elevator, can in principle be used as a payload-raising mechanism on any rotating body. However, contrary to the space elevator, the siphon does not require external work to lift asteroid material below synchronous altitude. In support of mining operations, the siphon can be used to raise mined material to a collecting/processing station in orbit around the asteroid or directly connected to the siphon. Alternatively, the siphon can be used to release material to escape, without the need to use propellant-based methods. This thesis therefore will investigate the dynamics of an orbital siphon anchored at an asteroid and examine a range of applications in the context of asteroid manipulation and resource exploitation. Long-term effects of the siphon operation are discussed, showing that this device allows a significant quantity of mass to be raised to orbit or to escape. It is shown that an optimal siphon length can be chosen, such that the extracted mass is maximised. Key variables, such as achievable mass flow rates, tension on the tethers, timescales and anchor forces are discussed. It is demonstrated that the oscillations of this device resulting from Coriolis forces are damped and the siphon will eventually align with the local vertical if mass is released to a collecting spacecraft connected at the top of the siphon. Moreover, it is proposed that the siphon dynamics could be leveraged to deliver resource payloads to stable equilibria about the asteroid, with a smaller delta-v than direct transfer from the surface, which may be beneficial in a long-term mining scenario. Effects of an irregular gravity field on the siphon dynamics are also examined, using polyhedral shape models of two candidate asteroids. The siphon effect is still generated for the candidate asteroids analysed, even with motion of the anchoring system on the asteroid surface, thus allowing the mining location to be moved without interrupting the flow of material to the collecting spacecraft. If a large quantity of material is released to escape, the siphon effect may also be exploited to induce a small variation to the heliocentric velocity of a potentially hazardous asteroid for impact risk mitigation. It is shown that typical delta-v on the order of 1 cm/s can be achieved within a time window of a decade. Finally, use of the orbital siphon to generate artificial cavities for habitats or storage of mined material is discussed.