Assessing NEO hazard mitigation in terms of astrodynamics and propulsion systems requirements

Uncertainties associated with assessing valid near-Earth object (NEO) threats and carrying out interception missions place unique and stringent burdens on designing mission architecture, astrodynamics, and spacecraft propulsion systems. A prime uncertainty is associated with the meaning of NEO orbit predictability regarding Earth impact. Analyses of past NEO orbits and impact probabilities indicate uncertainties in determining if a projected NEO threat will actually materialize within a given time frame. Other uncertainties regard estimated mass, composition, and structural integrity of the NEO body. At issue is if one can reliably estimate a NEO threat and its magnitude. Parameters that determine NEO deflection requirements within various time frames, including the terminal orbital pass before impact, and necessary energy payloads, are quantitatively discussed. Propulsion system requirements for extending space capabilities to rapidly interact with NEOs at ranges of up to about 1 AU (astronomical unit) from Earth are outlined. Such missions, without gravitational boosts, are deemed critical for a practical and effective response to mitigation. If an impact threat is confirmed on an immediate orbital pass, the option for interactive reconnaissance, and interception, and subsequent NEO orbit deflection must be promptly carried out. There also must be an option to abort the mitigation mission if the NEO is subsequently found not to be Earth threatening. These options require optimal decision latitude and operational possibilities for NEO threat removal while minimizing alarm. Acting too far in advance of the projected impact could induce perturbations that ultimately exacerbate the threat. Given the dilemmas, uncertainties, and limited options associated with timely NEO mitigation within a decision making framework, currently available propulsion technologies that appear most viable to carry out a NEO interception/mitigation mission within the greatest margin of control and reliability are those based on a combined (bimodal) nuclear thermal/nuclear electric propulsion platform. Elements of required and currently available performance characteristics for nuclear and electric propulsion systems are also discussed.