100-C 25-C 50-C 75-C C+M 50-C+M C+Y 50-C+Y M+Y 50-M+Y 100-M 25-M 50-M 75-M 100-Y 25-Y 50-Y 75-Y 100-K 25-K 25-19-19 50-K50-40-40 75-K 75-64-64CHAPTER 14
Near-Earth Objects
Lucy A. McFadden
University of MarylandRichard P. Binzel
Massachusetts Institute of Technology- Introduction 4. Population 7. Impact Hazards
- Significance 5. Physical Properties Bibliography
- Origins 6. In Situ Studies
1. Introduction
Near-earth objects (NEOs)reside in the vicinity of Earth
near 1.0 AU (the mean distance between Earth and the
Sun). Any object, such as an asteroid or comet, orbiting
the Sun with a perihelion,q< 1 .3 AU, well inside the or-
bit of Mars, is defined as an NEO. Aphelia,Q, of NEOs
generally lie within a sphere of radius 5.2 AU, defined by
Jupiter’s orbit. Among this broad group are four subgroups:
Amors, Apollos, Atens, and interior Earth objects (IEOs).
Comets, releasing gas and dust withq< 1 .3 would be re-
ferred to as near-Earth comets (NECs) if they posed an
impact threat to Earth. Amors approach but don’t cross the
orbit of Earth. They have a semimajor axis,a> 1 .0 AU,
and perihelion 1. 017 ≤q= 1 .3 AU, between the aphelion
of Earth’s orbit and inside the perihelion of Mars (Fig. 1a).
Those that actually cross Earth’s orbit, Apollos, havea> 1. 0
AU andq= 1 .017 AU, Earth’s aphelion distance (Fig. 1b).
Atens havea≥ 1 .0 AU andq> 0 .983 AU, Earth’s perihe-
lion distance. An object with bothaandq< 0 .983 AU, is an
IEO.
The Amor asteroid, 433 Eros, was the first NEO dis-
covered in 1898, by D. Witt of Berlin, Germany, using a
photographic plate to record its position. It is also one of
the largest NEOs, being 33 km in its longest dimension, with
two other axes of 10.2×10.2 km diameter. 1862 Apollo, the
first Earth-crossing asteroid, and 1221 Amor, the namesake
of that group, were both discovered in 1932. It wasn’t until
44 years later that 2062 Aten, the first of the group orbiting
within Earth’s orbit, was discovered by Eleanor Helin, still
using photographic plates for the search. 1998 DK36 was
the first IEO discovered in 1998.
As the dynamical evolution of asteroids and their role in
probably causing biological extinction events on the Earth
was recognized in the 1980s, dedicated searches for NEOs
resulted in increased discovery rates. Due to both increased
sky coverage and availability of sensitive digital detectors,
the known NEOs number>4100 at this writing, compared
to 85 known in early 1980 (Fig. 2). About 25 of the NEOs
found since the 1990s are binary objects orbiting around
a common center of mass; 15% of all NEOs are estimated
to be binaries. 1862 Apollo, an asteroid between 1.2 and
1.5 km in diameter, was reported to be a binary in 2005.
Most of the near-Earth objects originated in the Main
Asteroid Belt, located between Mars and Jupiter, although
some of them probably evolved into their current orbits
from the reservoir of short-period comets extending be-
yond Jupiter and into the outer solar system. The range
of composition and physical characteristics of asteroid-like
near-Earth objects spans those found among the Main Belt,
though 15% of them probably are derived from cometary
reservoirs.
TheNear-Earth Asteroid Rendezvous(NEAR) mission
was the first designed to orbit an asteroid. 433 Eros was