Earth 107

Basic Planetary Data 109 The Astronomical Unit 110 Planetary Data for Earth 112 The Atmosphere 113 The Hydrosphere 116 The Outer Shell 117 The Interior 120

The Geomagnetic Field and Magnetosphere 121 Development of Earth's Structure and Composition 123 Accretion of the Early Earth 123 Effects of Planetesimal Impacts 124 Planetary Differentiation 125

Earth Impacts 126 Objects That Pose a Threat 128 Frequency of Impacts 130 The Tunguska Event 132 NEO Search Programs 132 Determining the Hazard Potential of an NEO 134 Defending Earth from a Colliding Object 135

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