Nearly 300 million yrs ago a comet with a diameter of approx.10 km struck the Earth near Chicxulub in present-day Mexico. The impact created a crater nearly 300 km in diameter and drove very large quantities of dust and debris into the atmosphere. The dust prevented much of the Sun's radiation from reaching the Earth's surface and the resulting Ice Age caused the extinction of approx. 50 percent of the existing animal species-including, most famously, the dinosaurs. Approximately 50,000 yrs ago a stony-iron asteroid with a diameter of approx. 150 m struck the Earth in what is now north-central Arizona. This impact created a crater 1 mi in diameter and the resulting shockwave killed every large mammal within a radius of 24 km. In 1908, a small comet or asteroid with a diameter of approx. 50 m entered the Earth's atmosphere over eastern Russia. The extreme heat and aerodynamic pressures generated during entry caused the object to disintegrate explosively at an altitude of approx. 8 km. This explosive burst-since termed an airburst-occurred above Siberia, near the town of Tunguska. The airburst took place very close to the altitude at which the maximum amount of ground damage would result. It left a zone of destruction nearly 40 km in diameter around the point of disintegration. The scientific community now accepts that these events are just major examples of the continuous ongoing bombardment of the Earth by a wide variety of objects, most of them fragments of either asteroids or comets in orbit around the Sun. Many feel that, because there appears to have been no loss of life due to cosmic bombardment, this threat can be ignored. Against this it can be argued that, as our knowledge of the solar system in general-and its minor bodies in particular-is very recent, many unexplained catastrophes in the past may actually be attributable to the impact of asteroids or comets. Additionally, the exponential growth of world population, combined with our increasing technological dependence, makes humanity much more vulnerable to the consequences of a near-Earth object (NEO) impact. Despite these worrying trends, effective means of defense against the NEO threat are available to us. That is the topic of the Marshall Space Flight Center-led study reported in this Technical Publication (TP).