Aas 14-281 Suborbital Intercept And

AAS 14-281 SUBORBITAL INTERCEPT AND FRAGMENTATION OF ASTEROIDS WITH VERY SHORT WARNING TIMES Ryan Hupp,∗ Spencer Dewald,∗ and Bong Wiey The threat of an asteroid impact with very short warning times (e.g., 1 to 24 hrs) is a very probable, real danger to civilization, yet no viable countermeasures currently exist. The utilization of an upgraded ICBM to deliver a hypervelocity asteroid intercept vehicle (HAIV) carrying a nuclear explosive device (NED) on a suborbital interception trajectory is studied in this paper. Specifically, this paper focuses on determining the trajectory for maximizing the altitude of intercept. A hypothetical asteroid impact scenario is used as an example for determining sim- plified trajectory models. Other issues are also examined, including launch vehicle options, launch site placement, late intercept, and some undesirable side effects. It is shown that silo-based ICBMs with modest burnout velocities can be utilized for a suborbital asteroid intercept mission with an NED explosion at reasonably higher altitudes (> 2,500 km). However, further studies will be required in the following key areas: i) NED sizing for properly fragmenting small (50 to 150 m) asteroids, ii) the side effects caused by an NED explosion at an altitude of 2,500 km or higher, iii) the rapid launch readiness of existing or upgraded ICBMs for a suborbital asteroid intercept with short warning times (e.g., 1 to 24 hrs), and iv) precision ascent guidance and terminal intercept guidance. It is emphasized that if an earlier alert (e.g., > 1 week) can be assured, then an interplanetary intercept/fragmentation may become feasible, which requires an interplanetary launch vehicle. INTRODUCTION On October 7th, 2008, asteroid 2008 TC3 was discovered by the Catalina Sky Survey 20 hours before breaking up above the Saharan desert. It was the first asteroid impact ever to be predicted before reaching the atmosphere. Fortunately, it was only 4 meters in diameter and broke up in the atmosphere over a remote part of the world [1]. As detection efforts become more advanced, it is likely that future impactors will be discovered with similar limited warning times. In recent years, most of the largest asteroids in the neighborhood of Earth have been found and catalogued. These asteroids greater than 1 km diameter would devastate the planet, but such an impact will be a rare event. There are millions more small asteroids (50 to 150 m) that are still undetected. These small, city killer objects strike the Earth much more frequently than the larger ones. The Tunguska event occurred in 1908 was caused by a relatively small (< 50 m) asteroid or comet, but the subsequent airburst released an explosion energy of 10-15 Mt. The impact leveled over 2,000 km2 of Siberian forest, an area equivalent to the size of a large city. If such an asteroid is discovered days or hours before impact, there would be no time to deflect the asteroid or even ∗Research Assistant, Asteroid Deflection Research Center, Department of Aerospace Engineering, Iowa State University yVance Coffman Endowed Chair Professor, Asteroid Deflection Research Center, Department of Aerospace Engineering, Iowa State University, Ames, IA 50011, USA 1 evacuate the impact zone. There is currently no planetary defense planning against such a short- warning event. Despite the lack of a known immediate impact threat from an asteroid or comet, historical sci- entific evidence suggests that the potential for a major catastrophe created by an asteroid impacting Earth is very real, and humankind must be prepared for it. Due to a recent asteroid impact event (now referred to as Chelyabinsk meteor event) that occurred in Russia on February 15, 2013 and a near miss by asteroid 2012 DA14 (45 m diameter) on the same day, there is now a growing national and international interest in developing a global plan to protect the Earth from a catastrophic impact by a hazardous asteroid or comet. Note that the Chelyabinsk event had no warning while asteroid 2012 DA14 was detected about one year before its close flyby of the Earth. To date, planetary defense research efforts by NASA and the planetary defense community have mainly focused on discovering, tracking, and characterizing asteroids to provide situational aware- ness of the threat they pose to Earth. While those efforts are crucial to having advance warning of an asteroid on an Earth-impacting trajectory, now is the time to develop an effective means of responding to an asteroid impact threat with short warning time (<< 1 year). For such a worst-case, yet most probable mission scenario, suborbital intercept and fragmentation of an asteroid using a hypervelocity asteroid intercept vehicle (HAIV) carrying nuclear explosive devices (NEDs) may be the only practically viable option other than civil defense (evacuation). An innovative concept being developed in a NASA Innovative Advanced Concepts (NIAC) Phase 2 study utilizes the concept of blending a hypervelocity kinetic impactor with a subsurface nuclear explosion for optimal fragmentation of an asteroid with short warning time [2–5], The secondary goal of this NIAC study is to develop major technology development programs that will lead to flight-validated planetary defense technologies capable of handling such short warning time scenarios. The objective of this paper is to initiate a technology readiness assessment study in preparation for the threat of an asteroid impact with a very short warning time [6] and present example mission design scenarios, including optimized intercept trajectories. Current anti-ballistic missile (ABM) technology could be adapted for use against asteroids. The United States has deployed Ground Based Interceptor (GBI) missiles that are launched from silos and can intercept an enemy missile in the midcourse phase of flight with an Exoatmospheric Kill Vehicle (EKV). SM-3 missiles have a similar capability and are designed to be launched from ships. The higher altitude and larger payload requirements for a suborbital asteroid intercept are beyond current GBI missiles that are launched from silos and can intercept an enemy missile in the midcourse phase of flight with an EKV. However, ICBMs such as the Minuteman III do have adequate launch mass capability. In this paper, we assume that the Minuteman III based interceptor will be able to deliver a HAIV payload into a precision intercept trajectory against an incoming asteroid. When we don’t have sufficient mission lead times for large orbital dispersion of fragments, full neutralization of the asteroid impact threat is infeasible and most of the fragments will still hit the Earth. However, if the asteroid is fractured or fragmented into sufficiently small pieces prior to reaching the atmosphere, each of those small pieces will break up sooner and the resulting airbursts will occur at safer (higher) altitudes. The goal of a suborbital intercept and fragmentation mission is basically to reduce the probable impact damages of a 50 m asteroid to the damage level of multiple Chelyabinsk events. That is, the mission goal is to transform another future Tunguska-like event into multiple Chelyabinsk-like events in a worst-case situation. 2 Figure 1. ECI Reference Frame IDEAL OPTIMAL INTERCEPT Problem Formulation For a given hyperbolic asteroid orbit that intersects the Earth and a fixed launch site on Earth, the goal is to determine the optimal suborbital intercept trajectory. The performance of the missile is limited to its available total ∆V . The criterion for optimal intercept is defined as the maximum altitude of intercept from a fixed launch position on Earth. Because the asteroid is on a hyperbolic encounter orbit, maximum altitude is equivalent to earliest intercept. Our preliminary conceptual orbital intercept study is characterized as • Orbital elements of the target asteroid at acquisition are assumed to be exactly known. • A few locations of the interceptor missile on Earth’s surface (latitude and longitude relative to the ECI frame) are assumed. • Interceptor missile is simply characterized by its available ∆V . • Earth model is assumed to be a rotating sphere with negligible atmosphere. • Missile launches from Earth’s surface with a single impulse. A multi-stage rocket model will be used in a future study. • Two-body orbital dynamics is assumed. Coordinate System. The orbits of the asteroid and interceptor and positions on Earth’s surface are defined with respect to the Earth-Centered Inertial (ECI) reference frame, shown in Figure1. The time, distance, and speed units used here are seconds, km, and km/s, respectively. For nomenclature purposes, a subscript T refers to the target asteroid, and the M for the missile. 3 Figure 2. Relation of Earth’s Surface to ECI Frame Table 1. Time Frame Definition t0 Time of Target Detection t1 Time of Intercept Missile Launch t2 Time of Intercept t3 Time of Projected Earth Impact Because the vectors are defined with respect to the ECI frame, it is not necessary to correlate the problem to a specific sidereal time. Instead, it is assumed that the prime meridian is aligned with the vernal equinox direction at the moment of interceptor launch. This makes it convenient to map the surface longitude to the ECI frame without having to calculate sidereal time. Figure2 shows the orientation of the Earth’s surface with respect to the ECI frame. The latitude and longitude positions are transformed into ECI position vectors as follows: ~r = R cos θ cos I~ + R sin θ cos J~ + R sin K~ (1) where θ is longitude, is latitude, and R = 6378.15 km is the radius of the Earth. Time Frame. The asteroid intercept scenario is independent of time. The timing of the major events such as target acquisition, missile launch, and intercept are all relative to an arbitrary time of impact. Each point in time is measured in seconds until impact.

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