Physical Characterization Of Sub-Kilometer NEAs: Low-Cost Mission Approaches
Morrison, D.¹; Chartres, J.¹; Coloprete, A.¹; Genova, A.¹; Jaroux, B.¹; Johnson, R.¹; Lemke, L.¹; Williams, B.²; Chesley, S.^3
1NASA Ames Research Center; ²KinetX; ³Jet Propulsion Lab

Introduction: NEAs are important for both science and planetary defense. This discussion focuses on the small (sub-km) NEAs, with Apophis as a specific example. From a science perspective, we want to characterize the NEA population in terms of physical and orbital properties, to learn what they are made of, where they come from, and their role in the history of the inner solar system. For defense, we want to identify which objects may collide with Earth and to learn enough about their properties (size, shape, density, spin vector) to plan ways to deflect them from a collision trajectory. The first step for both science and defense is to survey the population of NEAs to identify suitable targets for follow-up radar studies and characterization missions. This paper discusses low-cost flight missions that could be used to characterize sub-km NEAs, providing critical data for developing defense technologies.

Physical Characterization: Both science and defense require essentially the same core information: size, shape, spin state, and internal structure (e.g., density). For detailed scientific characterization it is also important to determine composition (mineralogy) and geological history as revealed by surface structure and topography. A measurement of thermal inertia provides information on the upper regolith and is an input to models of the Yarkovsky effect. Mapping of slopes and albedo also contributes to understanding the recent history of the object. Ground-based photometry can determine the approximate size, spin rate, and shape (the ratio of major axes, from light-curve amplitude). Multispectral observations (if the NEA is bright enough) further yield a spectral class, which can in many cases be related to composition (mineralogy) by comparison with meteorite types. Ground-based radar (if the NEA comes close enough to Earth) yields improved size, shape, and spin axis, and can distinguish stony from metallic objects by their radar reflectivity. If the NEA is a binary or has a satellite, radar also yields a mass and allows an estimate of density. However, the sub-km NEAs are too small to retain a satellite in a stable orbit. Flybys are of limited value for such small targets, since they cannot determine mass, volume, density, spin state, or surface properties. It is useful to remember that Apophis is a million times smaller (in mass) than Eros, the target of the NEAR-Shoemaker mission. Apophis is to Eros as Eros is to the Moon. To study such small asteroids, a long-lived rendezvous or lander mission is required.

The Low-Cost Mission Study at NASA Ames: For the past year Ames has been studying a minimum cost mission to Apophis that will provide the critical data needed to plan defense technologies. This utilizes a low-mass space-craft that can be launched either as a secondary payload or as a dedicated launch on the new generation of smaller EELVs. For minimum cost and risk, we rendezvous with the asteroid rather than landing. During several months of proximity operations, we can determine an accurate mass and volume, establish a shape model, determine the spin vector, and use high-resolution imaging and laser ranging to study the surface topography and permit inferences about geological history. This paper describes possible instruments and the time-line for operations near the target. The same mission approach also yields more accurate knowledge of the orbit than can be obtained from ground-based optical or radar observations.