Near-Earth Objects 287
the orbit, over time, evolving into one crossing a planet’s
orbit, the subject of this chapter.
3.1 Relationship to Main Belt Asteroids
Early asteroid studies in the 1940s revealed a range of col-
ors (see Section 5). Techniques to study both reflected and
emitted electromagnetic radiation from the asteroids were
developed and used to derive information about their min-
eral and chemical composition. In the late 1970s, two sci-
entists, Jonathan Gradie and Edward Tedesco, recognized
that there is a relationship between the apparent composi-
tion of the asteroids and their distance from the Sun. This
finding represented observational support for a model pre-
dicted by another astronomer, John Lewis, in which the
solar nebula was in a state of chemical equilibrium when it
formed. Asteroid composition changes as a function of tem-
perature, and hence distance from the Sun. Therefore, one
does not expect all asteroids to have the same composition.
Furthermore, the exact nature of asteroidal material holds
clues to the temperature and location where the material
formed.
This information is valuable as scientists piece together
the scenario leading to the formation of our solar system
and look for evidence of the existence of other solar sys-
tems. Studies of the composition of near-Earth objects led
to the conclusion that NEO composition spans the range
found among the Main Asteroid Belt, thus establishing that
many or most of the NEOs are derived from the main
belt. Follow-on research has confirmed these findings and
identified the proportion that is derived from comets as
∼15%. Furthermore, physical information derived from
NEOs can be reasonably considered to apply to Main Belt
Asteroids.
Statistical analysis of the evolution of many asteroid
orbits over the age of the solar system indicates that the
lifetime of an Earth-crossing body against gravitational per-
turbations is relatively short, on the order of 10 million years
or less. Within this time frame, the bodies will either col-
lide with a planet or be dynamically ejected from the solar
system. This time interval applies to the average of the en-
tire population and does not refer to the exact lifetime of
any particular asteroid. It turns out that the orbital evo-
lution of a specific asteroid or comet cannot actually be
determined very far into the future or the past owing to
the difficulty of knowing the exact starting conditions and
accurately predicting frequent close approaches between
the NEO and the planets. [SeeSolarSystemDynamics:
Regular andChaoticMotion.]
3.2 Relationship to Meteorites
Exploring the relationship between NEOs and meteorites is
motivated by the possibility of making a very rich connection
between the geochemical, isotopic, and structural informa-
tion on meteorites available from laboratory studies and the
near-Earth objects. Meteorites fall to Earth frequently, but
most often land unnoticed in the oceans or in remote areas.
In January 2000, an exceptionally brightbolidewas seen
by eyewitnesses in the Yukon, Northern British Columbia,
parts of Alaska, and the Canadian Northwest Territories.
Nearly 10 kg of precious samples were recovered from the
surface of frozen Tagish Lake. Using eyewitness reports
and the bolide’s detection by military satellites, the orbit of
the impacting body was traced back to the Asteroid Belt
(Fig. 6). Prior to striking the Earth, the body is estimated to
have been about 5 m across with a mass of 150 metric tons.
[SeeMeteorites.]
The determination of meteorite orbits serves as a con-
straint on the mechanisms that result in meteoroid delivery
to Earth. Numerical computer simulations reveal regions
of the Asteroid Belt that act as “escape hatches” for de-
livering material to the terrestrial planets zone. One such
region corresponds to a Kirkwood gap, located where an
asteroid’s orbital period is shorter than Jupiter’s by the ratio
of two small integers, such as 3:1, 5:2, or 2:1. Any aster-
oid or debris that migrates into this gap finds Jupiter to be
especially effective in increasing its orbital eccentricity. As
the orbit becomes increasingly elongated, it can intersect
the orbit of the Earth. In the 1980s, work by Jim Williams,
Jack Wisdom, and others illuminated the importance and
efficiency of resonances in the Asteroid Belt and their role
in supplying meteorites.3.3 Relationship to Comets
Comets are predominantly icy and dusty objects that come
from the outer reaches of the solar system. Their orbital pe-
riods are long, their orbital eccentricities are high, and they
may have large or small orbital inclinations. What is their re-
lationship to near-Earth objects? In the 1950s, ErnstOpik ̈
concluded that comets must be a partial source of near-
Earth objects because he could not produce the number of
observed meteorites from the Asteroid Belt alone via his cal-
culations. Building onOpik’s work, George Wetherill pre- ̈
dicted that 20% of the near-Earth object population consists
of extinct cometary nuclei. Some now find evidence that the
fraction of comets is smaller, closer to 15%. The hypothesis
that NEOs derive from comets continues to merit consid-
eration as knowledge of comets and asteroids increases and
simulations of the dynamical evolution of interacting small
bodies under the gravitational influences of the planets con-
tinues to develop. [SeeCometaryDynamics;Physics
andChemistry ofComets.]
Are there hints that any particular near-Earth object
that looks like an asteroid was once a comet? If an object
sometimes has a tail like a comet and sometimes looks just
like an asteroid (no coma or tail), which is it: asteroid or
comet? There is both dynamical and physical evidence that
addresses this question.