Encyclopedia of the Solar System 2nd ed

284 Encyclopedia of the Solar System

Semimajor Axis < 1.0 AU

Perihelion < 1.02 AU

Earth Crossing

Apollo

Semimajor Axis < 1.0 AU

Perihelion < 1.0167 AU

Earth Crossing

Aten

Apohelion < 0.983 AU

Always inside Earth’s orbit

(aka Apohele)

Type Population

Apollo 62% of known

Aten 6% of known

Amor 32% of known

Aphohel Unknown

e number

Inner Earth Objects (IEOs)

1.02 AU < Perihelion < 1.3 AU

Amor FIGURE^1 (a) Amors approach Earth but do not cross its orbit.

(b) Apollo orbits cross that of Earth.

its target, and the spacecraft remained in orbit from 2000
to 2001, ending its mission with a controlled descent and
successfully becoming the first spacecraft to land on an
asteroid.NEARaccomplished the first detailed in situ mea-
surements of an asteroid’s surface morphology, mineral-
ogy, chemistry, internal state, and magnetic properties. The
Japanese-ledHayabusamission was launched on May 9,
2003, on a 4 year mission to investigate asteroid 25143
Itokawa and to demonstrate the technology necessary to
return samples to Earth. The spacecraft went into orbit
around Itokawa in September 2005, performing remote
sensing measurements for 3 months. The shape and sur-
face morphology of this small near-Earth object is unlike
any seen before. In November 2005, there were two sched-
uled touchdowns in which some surface material may have
been collected. The return capsule is scheduled for a June

FIGURE 2 Cumulative total of discovered near-Earth objects
versus time. Large NEOs are defined as those with an absolute
magnitude (H) of 18 or brighter. (Data compiled by Alan
Chamberlin, NASA/JPL.)

2010 return to Earth and will hopefully contain some sur-

face material collected from Itokawa.

2. Significance

2.1 Remnants of the Early Solar System

From a scientific point of view, near-Earth objects are stud-

ied for the same reason as comets and main-belt asteroids:

They are remnants of the early solar system (Fig. 3). As

such, they contain information that has been lost in the

planets through large-scale, planetary processes such as ac-

cretion, tectonism, volcanism, and metamorphism. Knowl-

edge of the asteroids and comets as less processed material

from the early solar nebula, studied together with direct

samples in the form of meteorites, is critical to piecing to-

gether a scenario for the formation of the solar system. [See

TheOrigin of theSolarSystem.]

Most near-Earth objects are asteroid-like in their nature,

being derived from the Main Belt. This region is a dividing

point in the solar system, where the planets that formed

closer to the Sun, the terrestrial planets, are dominated by

rocky,lithophilematerial. Beyond the Asteroid Belt, the

planets are composed predominately of nebula gases. Per-

haps 10–20% of all near-Earth objects originated elsewhere

in the solar system, such as the cometary reservoirs lying at

great distances from the Sun, beyond the gaseous planets.

Knowing about material from these reservoirs reveals in-

formation about both chemical and physical processes that

were active in the outer regions of the solar system, both in

the near and distant past. The objective of scientific study of

the near-Earth objects is to determine which of them might

be derived from which regions of asteroidal and cometary

reservoirs.

2.2 Hazard Assessment

Although chips, hand-sized rocks, and large boulders, all

called meteorites, are continually landing on Earth [See