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The Earth is the most geologically active of the terrestrial planets and it has retained the poorest sample of the record of hypervelocity impact by interplanetary bodies throughout geologic time. Although the surviving sample of impact structures is small, the terrestrial impact record has played a major role in understanding and constraining cratering processes, as well as providing important ground-truth information on the three dimensional lithological and structural character of impact structures (). Recently, there has been a growing awareness in the earth-science community that impact is also potentially important as a stochastic driving force for changes to the terrestrial environment. This has stemmed largely from: the discovery of chemical and physical evidence for the involvement of impact at the Cretaceous-Tertiary (K/T) boundary and the associated mass extinction event (e.g. ; ; ), and their relation to the Chicxulub impact structure in the Yucatan Peninsula, Mexico (), the recognition of the resource potential of impact structures, some of which are related to world-class ore deposits, both spatially and genetically (; ), and the recognition of the potentially disastrous consequences of impacts for human civilization ().

Humans and cosmic impacts have had a long and intimate relationship. People live in ancient impact craters, such as at Ries and Steinheim in Germany, and use impact breccias for building material. People historically witnessed and venerated fallen meteorites, in some cases the meteorites becoming among the most sacred of objects — such as that kept in the Kaaba at Mecca. People made tools from meteoritic iron, including certain examples from the objects named the “tent,” “woman,” and “dog” by the Greenland Eskimos. And in one of the more peculiar ironies linking humans and cosmic impacts, people carved a portion of an ancient Ohio impact crater into the shape of a Great Serpent. This act not only created one of the more spectacular archaeological sites in North America, but also depicted a symbol used by a number of cultures to represent comets, the very source of some impact craters on the Earth.

The celestial environment has always played a significant role in the shaping of human culture. Written records spanning thousands of years are replete with examples of the importance of the celestial constants (e.g. the Sun, moon, stars, planets) in the basic ideologies and the everyday lives of peoples around the world. Of equal or greater importance are transient celestial phenomena (e.g. eclipses, meteor storms, asteroids, comets). Because of the infrequency, unpredictability, and often fantastic manifestations that are presented by these transient events, they have been viewed as having much greater import than the much more predictable celestial constants.

Master (2001) discovered a ca. 3.4 km diameter circular structure, in the marshes of southern Iraq, on published satellite imagery (Fig. 4.1, after ), and interpreted it to be a possible meteorite impact crater, based on its morphology (its roughly polygonal outline, an apparent raised rim and a surrounding annulus), which differed greatly from the highly irregular outlines of surrounding lakes. The structure, which is situated in the Al ’Amarah Marshes, near the confluence of the Tigris and the Euphrates Rivers (at 47° 4′ 44.4″ E, 31° 8′ 58.2″ N), was identified by Master (2002) as the Umm al Binni lake, based on a detailed map of the marshes published by Wilfred Thesiger (1964). Following the Gulf War of 1991, Saddam Hussein’s regime embarked on a massive program to drain the Al ’Amarah marshes, by building a huge canal named the “Glory River” parallel to the Tigris River (Fig. 4.3) (, ; ; , ; ; ). After the almost complete draining of the marshes since 1993 (; , ; ) the Umm al Binni Lake has disappeared and in recent Landsat TM and ASTER satellite imagery, it appears as a light colored area, due to surface salt encrustations (Fig. 4.4). Following the Iraq War of 2003, there are moves afoot to re-flood the marshes in an attempt to restore its devastated ecology (; ; ; ; ; ).

The dates of a series of (dates where numbers of long-lived oaks showed catastrophically narrow growth rings at the same time) have been identified in a long Irish oak tree-ring chronology (). The dates were christened ‘marker dates’ because they were immediately noted to fall in clusters of information relating to traumatic happenings in widely separated areas around the world. For example, one of the Irish oak dates was 207 BC. In China events in 208 BC, and the years following, included a dim Sun, crop failures, famine and high death rates; and a new dynasty, the Han, is believed to have started in 206 (). Meanwhile, in Europe, problems in Rome called for consultation of the Sibylline Books resulting in the return of the Goddess Cybele from Asia Minor; Cybele was manifest as a ‘small black meteorite.’ This latter occurrence made sense of a series of references by Livy to ‘stones falling from the sky’ and strange lights in the sky, ‘prodigies of Jupiter’, et cetera (). Clearly, dates around 207 BC might be expected to show up in other records.

The threat posed to our planet and our civilization by future comet and asteroid impacts (CAIs) is now widely recognized and is becoming increasingly well constrained. Recent studies have provided tighter estimates of the numbers of potentially-threatening objects, particularly within the near-Earth space (), better approximations of likely frequencies of collision with objects of various diameters (e.g. ), and a more realistic appreciation of the effects of CAIs on society and the environment (e.g. ; ). In this regard, the hazard and risk associated with CAIs are now far better comprehended than those linked with other geological and geophysical phenomena capable of affecting the entire planet or impinging in some detrimental way upon the global community. Such form a compendium of low frequency-high magnitude phenomena of which CAIs are just a single element. While far less well understood, and therefore scientifically much more controversial, currently appear at least as hazardous as impacts of kilometer-sized and larger bolides, and to have frequencies that are considerably shorter than CAIs capable of comparable levels of destruction and disruption (Tables 6.1 and 6.2). A miniscule glimpse of this capability was provided by the December 26, 2004 Asian earthquake and tsunami, which claimed an estimated 250 000 lives (including 100 000 children), destroyed close to half a million buildings, and led to eight million people being made homeless, impoverished, displaced or unemployed.

Sometime in the foreseeable future, perhaps during this decade or maybe not until our great-great-grandchildren are adults, an asteroid the size of a large building will crash into the Earth’s atmosphere, exploding in an air-burst with the force of megatons or more of TNT. Most likely, such an event will happen over an ocean or sparsely populated desert; but, if it occurs over an urban area, the consequences could be very destructive and deadly. Actually, small strikes by cosmic grains of sand happen all the time (witness meteors or “shooting stars”, visible in a dark, clear sky several times an hour) and every year many large rocks, called “meteorites”, survive their atmospheric plunge to be collected and exhibited in museums.

The Earth is immersed in a swarm of Near Earth Asteroids (NEAs) capable of colliding with our planet, a fact that has become widely recognized within the past decade. The first comprehensive modern analysis of the impact hazard resulted from a NASA study requested by the United States Congress. This () provided a quantitative estimate of the impact hazard as a function of impactor size (or energy) and advocated a strategy to deal with such a threat.

Over the last several decades, evidence has steadily mounted that asteroids and comets have impacted the Earth over solar system history. This population is commonly referred to as “near-Earth objects” (NEOs). By convention, NEOs have perihelion distances ≤ 1.3AU and aphelion distances ≥ 0.983AU (e.g. ). Subcategories of the NEO population include the Apollos ( ≥ 1.0AU; ≤ 1.0167AU) and Atens ( < 1.0AU; ≥ 0.983AU), which are on Earth-crossing orbits, and the Amors (1.0167AU < ≤ 1.3AU) that are on nearly-Earth-crossing orbits and can become Earthcrossers over relatively short timescales. Another group of related objects that have not yet been considered part of the “formal” NEO population are the IEOs, or those objects located inside Earth’s orbit ( < 0.983AU). To avoid confusion with standard conventions, I treat the IEOs here as a population distinct from the NEOs. The combined NEO and IEO populations are comprised of bodies ranging in size from dustsized fragments to objects tens of kilometers in diameter ().

Someday, in a not too far away future ... A potentially hazardous astronomical object, with an estimated size significantly above 10 meters, is just detected. Quite soon, the probability of its impact with the Earth in, again, a not too far away future, is found to be close to 1. We certainly want to predict with a decent accuracy the effects of the impact and, even better, to tentatively initiate a mitigation strategy.