Near-Earth Objects 285
FIGURE 3 Clearing Out the Solar Nebula: The First Planetesimels. Painting by William K. Hartmann, reprinted with permission.Meteorites], and astronomers find house-sized objects
occasionally passing between Earth and the Moon, knowl-
edge of the near-Earth objects, their locations, and physical
and chemical characteristics is needed to inventory and as-
sess their hazard potential to the Earth.
Disastrous impacts by asteroids and comets have been
the popular subject of Hollywood movies, books, newspaper
articles, and television shows. The recognition that a giant
asteroid or comet perhaps 10 km across most likely caused
the extinction of the dinosaurs in a geological episode known
as the Cretaceous–Tertiary Event has highlighted the po-
tential for destruction should an energetic collision oc-
cur again (Fig. 4). Furthermore, as scientists analyze the
energy involved in collisions, they realize that the impacts
are tremendous and larger than anything created by hu-
man activities (e.g., nuclear weapons) or naturally occur-
ring phenomena on Earth (e.g., volcanoes, earthquakes, or
tsunamis).
Scientists ponder the results of computer simulations
that consider the interactions of colliding objects with vari-
ous Earth systems both natural and civilized. Coupled with
these computer simulations is the very real phenomenon
of the collision of comet Shoemaker–Levy 9 with Jupiter,
which was observed worldwide through telescopes in 1994.
The possibly devastating hazard posed to Earth if hit by a
high-energy asteroid or comet is now well recognized by
scientists and policy makers.
One of the objectives of NASA’s Deep Impact mission,
which sent an impactor spacecraft to collide with comet
9P/Tempel 1 in July 2005, was to study a comet nucleus
and its interior and to assess the hazard to Earth of im-
pact by a comet. When that analysis is complete, addi-
tional basic knowledge of comets will be available to as-
sess what would happen should a new comet be found
on a collision course with Earth. The most hazardous
cometary impact would be one with a large orbital velocity