Hypervelocity nuclear interceptors for asteroid disruption

Abstract

A direct intercept mission with nuclear explosives is the only practical mitigation option against the most probable impact threat of near-Earth objects (NEOs) with warning times much shorter than 10 years. However, state-of-the-art penetrating subsurface nuclear explosion technology limits the penetrator's impact velocity to less than approximately 300 m/s because higher impact velocities prematurely destroy the nuclear fusing mechanisms. Therefore, significant advances in hypervelocity nuclear interceptor/penetrator technology are required to enable a last-minute nuclear disruption mission with intercept velocities as high as 30 km/s. This paper briefly describes both the current and planned research activities at the Iowa State Asteroid Deflection Research Center for developing such a game-changing space technology to mitigate the most probable impact threat of NEOs with a short warning time.

Introduction

A growing interest currently exists for developing a plan to protect the Earth from the future possibility of a catastrophic impact by a hazardous asteroid or comet. In a recent letter on NEOs from the White House Office of Science and Technology Policy (OSTP) to the U.S. Senate and Congress, the White House OSTP strongly recommends that NASA takes the lead in conducting research activities for the development of NEO detection, characterization, and deflection technologies [1]. Furthermore, President Obama's new National Space Policy specifically directs NASA to “pursue capabilities, in cooperation with other departments, agencies, and commercial partners, to detect, track, catalog, and characterize NEOs to reduce the risk of harm to humans from an unexpected impact on our planet.” The Planetary Defense Task Force of the NASA Advisory Council also recommended that the NASA Office of the Chief Technologist (OCT) begin efforts to investigate asteroid deflection techniques. With national interest growing in the United States, the NEO threat detection and mitigation problem was recently identified as one of NASA's Space Technology Grand Challenges.

The Asteroid Deflection Research Center (ADRC) at Iowa State University has been developing strategies and technologies for deflection or disruption of hazardous NEOs. As the first university research center in the United States dedicated to such a complex engineering problem, the ADRC was founded in 2008 to address the engineering challenges and technology development critical to NEO impact threat mitigation.

Although various NEO deflection technologies, such as nuclear explosions, kinetic impactors, and slow-pull gravity tractors (GTs), have been proposed during the past two decades, there is no consensus on how to reliably deflect or disrupt hazardous NEOs in a timely manner [2], [3], [4], [5], [6]. Furthermore,due to various uncertainties in asteroid detection and tracking, the warning time before an asteroid impact with the Earth can be very short. All of the non-nuclear techniques, including hypervelocity kinetic impactors and slow-pull GTs, require mission lead times much longer than 10 years, even for a relatively small NEO. However, the most probable mission scenarios will have a warning time much shorter than 10 years, so the use of higher-energy nuclear explosives in space will become inevitable. Staging direct intercept missions with a short warning time will result in arrival velocities of 10–30 km/s with respect to target asteroids. A rendezvous mission to a target asteroid, requiring an extremely large arrival $\mathrm{\Delta }V$ of 10–30 km/s, is totally impractical.

Although a less destructive standoff nuclear explosion can be employed for deflection missions, the momentum/energy transfer created by a shallow subsurface nuclear explosion is at least 100 times larger than that of an optimal standoff nuclear explosion. However, state-of-the-art nuclear subsurface penetrator technology limits the impact velocity to less than about 300 m/s because higher impact velocities prematurely destroy the penetrating fusing mechanisms. An impact speed limit of 1.5 km/s has been cited for nuclear Earth-penetrator weapons (EPWs) in [7]. Neither a precision standoff explosion at an optimal height of burst (HOB) near an irregularly shaped, smaller NEO, with intercept velocities as high as 30 km/s, nor a contact burst is a trivial engineering task.

Consequently, a hypervelocity nuclear interceptor (HNI) system concept is proposed in this paper, which will enable a last-minute, nuclear disruption mission with intercept velocities as high as 30 km/s. The proposed system employs a two-body space vehicle consisting of a fore body (leader) and an aft body (follower), as illustrated in Fig. 1. The leader spacecraft provides proper kinetic-impact crater conditions for the follower spacecraft carrying nuclear explosive devices (NEDs) to make a robust and effective explosion below the surface of a target asteroid body. Surface contact burst or standoff explosion missions may not require such a two-body vehicle configuration. However, for a precision standoff explosion at an optimal HOB, accurate timing of the nuclear explosive detonation will be required during the terminal phase of hypervelocity intercept missions. Robust nuclear disruption strategies and technologies, to be employed in a last-minute, direct intercept mission, should be further studied, developed, and flight tested/validated.