Frontiers
The hazard of near-Earth asteroid impacts on earth

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Abstract

Near-Earth asteroids (NEAs) have struck the Earth throughout its existence. During epochs when life was gaining a foothold ∼4 Ga, the impact rate was thousands of times what it is today. Even during the Phanerozoic, the numbers of NEAs guarantee that there were other impacts, possibly larger than the Chicxulub event, which was responsible for the Cretaceous–Tertiary extinctions. Astronomers have found over 2500 NEAs of all sizes, including well over half of the estimated 1100 NEAs >1 km diameter. NEAs are mostly collisional fragments from the inner half of the asteroid belt and range in composition from porous, carbonaceous-chondrite-like to metallic. Nearly one-fifth of them have satellites or are double bodies. When the international telescopic Spaceguard Survey, which has a goal of discovering 90% of NEAs >1 km diameter, is completed, perhaps as early as 2008, nearly half of the remaining impact hazard will be from land or ocean impacts by bodies 70–600 m diameter. (Comets are expected to contribute only about 1% of the total risk.) The consequences of impacts for civilization are potentially enormous, but impacts are so rare that worldwide mortality from impacts will have dropped to only about 150 per year (averaged over very long durations) after the Spaceguard goal has, presumably, ruled out near-term impacts by 90% of the most dangerous ones; that is, in the mid-range between very serious causes of death (disease, auto accidents) and minor but frightening ones (like shark attacks). Differences in perception concerning this rather newly recognized hazard dominate evaluation of its significance. The most likely type of impact events we face are hyped or misinterpreted predicted impacts or near-misses involving small NEAs.

Section snippets

Why are near-Earth asteroids important?

Although interplanetary space is very empty by human standards, Earth is in a “cosmic shooting gallery”, as anyone looking up into clear, dark skies can witness: several meteors can be seen flashing across the heavens per hour—cometary and asteroidal dust grains disintegrating in the upper atmosphere. They are accompanied by a size spectrum [1] of ever larger, increasingly less common, bodies up to at least several tens of kilometers in diameter. Although occasional recovered meteorites are

Historical recognition of the impact hazard

Ideas that comets might be dangerous date back at least to the 17th century, when Edmond Halley is said to have addressed the Royal Society and speculated that the Caspian Sea might be an impact scar [11]. The physical nature of comets remained poorly understood, however, until the mid-20th century. The first NEA (Eros) was not discovered until 1898 and the first NEA that actually crosses Earth's orbit (Apollo) was not found until 1932. By the 1940s, three Earth-crossing NEAs had been found,

Physical and dynamical properties of NEAs

NEAs are defined, somewhat arbitrarily, as asteroids whose perihelia (closest orbital distance to the Sun) are <1.3 AU (1 AU=the mean distance of Earth from the Sun). About 20% of NEAs are currently in orbits that can approach the Earth's orbit to within <0.05 AU; these are termed potentially hazardous objects (PHOs). In terms of their origin and physical nature, PHOs are no different from other NEAs; they just happen to come close enough to Earth at the present time so that close planetary

Past history of impacts on Earth

I now discuss briefly, from a planetary science perspective, the role of impacts on the geological and biological history of our planet. By considering the past, I set the stage for the modern impact hazard. Clearly, impacts dominated the early geological evolution of the surface of our planet until at least 3.8 Ga. It is almost equally incontrovertible that impacts have continued to interrupt more quiescent evolution of our planet's ecosphere well into the Cenozoic; such impacts will continue,

The impact hazard: consequences for society in the 21st century

Cosmic projectiles rain down on us, ranging from the frequent flashes of meteoroids, to less frequent meteorite-producing bolides, to the even less common A-bomb level upper-atmospheric explosions recorded by Earth-orbiting surveillance satellites, to the historically rare Tunguska-level events, and finally to the still rarer but potentially extremely destructive impacts of bodies >100 m diameter, which must be considered not in terms of their frequency but instead in terms of their low but

Three representative scenarios of NEA consequences

I briefly summarize three scenarios (drawn from many more in [63]), which illustrate the breadth of issues that must be confronted in managing potential consequences of NEA impacts. For each impact disaster scenario, I consider the nature of the devastation, the probability that the event will happen, the likely warning time, the possibilities for post-warning mitigation, the nature of issues to be faced in after-event disaster management, and—of most practical interest—what can be done now to

Evaluation of the modern impact hazard

Unlike other topics in astronomy (except solar flares and coronal mass ejections), only the impact hazard presents serious practical issues for society. Contrasting with most practical issues involving meteorology, geology and geophysics, the impact hazard is both more extreme in potential consequences and yet so rare that it has not even been experienced in more than minor ways in historical times. It has similarities to natural hazards in that its practical manifestations mainly involve

Acknowledgements

I thank Chris Koeberl, Rick Binzel and Larry Nittler for helpful reviews. Preparation of this review was assisted by the Alan Harris, David Morrison and members of the B612 Foundation. [AH]

Dr. Clark R. Chapman is a planetary scientist at the Boulder, CO, office of Southwest Research Institute. He is past chair of the Division for Planetary Sciences of the American Astronomical Society, first editor of Journal of Geophysical Research-Planets, and has been on the science teams of the Galileo, NEAR-Shoemaker and MESSENGER deep space missions. His PhD (1972) is from the Earth and Planetary Sciences Dept. of MIT.

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    Dr. Clark R. Chapman is a planetary scientist at the Boulder, CO, office of Southwest Research Institute. He is past chair of the Division for Planetary Sciences of the American Astronomical Society, first editor of Journal of Geophysical Research-Planets, and has been on the science teams of the Galileo, NEAR-Shoemaker and MESSENGER deep space missions. His PhD (1972) is from the Earth and Planetary Sciences Dept. of MIT.

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