286 Encyclopedia of the Solar System
FIGURE 4 “Dinosaur’s Demise.” (Painting by Don Davis. Reprinted with permission.)relative to Earth’s. [SeePlanetaryImpacts;Cometary
Dynamics.]
2.3 Exploration Destinations and Resource Potential
NEOs come closer to Earth than any other planetary bodies.
With low orbital inclinations and small semimajor axes, they
are accessible targets for spacecraft. As humans extend their
activities beyond low Earth orbit, relatively nearby destina-
tions are attractive as training venues for missions to Mars.
Considering the very long-term future in space one real-
izes that launching materials from Earth is expensive. As
civilization moves beyond Earth, knowledge of materials in
space is critical to their efficient use in situ. It is probably
more economical to use space resources than transporting
material from Earth (Fig. 5).
3. Origins
In the widely accepted scenario of the formation of the
solar system, gas and dust collapse into a disk-shaped neb-
ula from which planetesimals and eventually planets form.
Planets grow after seeding conditions begin and molecules
and dust grains form aggregates, which then form clumps
that continue growing into objects large enough to be called
planetesimals. This process starts with dust and ice grains
about 1 mm in diameter. They behave at first as discrete
particles sweeping up smaller grains as they grow. Both
electromagnetic and gravitational forces come into play to
overcome the destructive forces of erosion from particle
collisions. Planet growth is gravitationally controlled and is
called accretion. Asteroids are planetesimals that were pre-
vented from growing to the size of the major planets by
pervasive eroding forces that counteract accretion, the net
effect being to keep the asteroids relatively small.
The formation of Jupiter was a major force in interfer-
ing with the growth of a larger planet between Mars and
Jupiter at∼2.8 AU. The details of the main-belt formation
are not well known because it formed early in the history of
the solar system,∼4.5 billion years ago. Since the earliest
formation times, gravitational interactions between planets
and small objects (asteroids and comets) have resulted in
perturbations of their orbits. These perturbations result inFIGURE 5 Painting showing the beginning of a mission to an
Earth-approaching asteroid (Denise Watt, NASA).