296 Encyclopedia of the Solar System
absorption bands that are common in silicate minerals. The
broad band at 1μm of asteroid 5641 McCleese is diag-
nostic of a mineral called olivine, which consists of silicon
oxide tetrahedra bound in eightfold symmetry by magne-
sium, calcium, and iron cations. Subtle differences in the
position of the center of the band constrain the chemistry of
the olivine, which can accommodate a range of magnesium
and iron in its mineralogical structure. The presence of a
second absorption near 2μm indicates that a second silicate,
pyroxene, is present.
The spectrum of 433 Eros contains both olivine and two
types of pyroxene. Detailed spectral analysis and modeling
suggest the presence of an additional component that may
be a glassy material, or possibly vapor-deposited coatings of
nanometer size iron grains. They are inferred because the
brightness of the spectrum is lower than mixtures of only
crystalline silicates. These mineral constituents are present
in ordinary chondrite meteorites; the deviation from ordi-
nary chondritic composition and the processes controlling
that have been studied and ascribed tospace weathering
and/or partial melting.
The spectrum of asteroid 3908 Nyx (Fig. 13) is domi-
nated by pyroxene and has the same spectral characteristics
as the basaltic achondrite meteorites. This asteroid may
have traveled to the near-Earth region of space over the
age of the solar system and may be a fragment of the large
main-belt asteroid, 4 Vesta. [SeeMain-BeltAsteroids.]
The lower spectrum in Fig. 13 is characteristic of a sub-
group of C-types, labeled B. There is no UV absorption and
not much of an infrared absorption. Interpretation of this
spectrum is uncertain. This asteroid, 3200 Phaethon, is a
candidate for an extinct comet, though its albedo (9–11%)
is higher than most comets observed to date (∼4%).
Mineralogical studies of near-Earth objects show that
they are not all alike. Nor are they alike in the Main Asteroid
Belt. The range of variations in mineral composition reflects
that seen in the Main Asteroid Belt, indicating that NEOs
are mostly derived from the Main Belt. None of the NEOs
are compositionally similar to any of the major planets be-
cause they do not share any of the spectral reflectance char-
acteristics of the major planets or the Moon. NEOs with
low albedo, featureless spectra with higher IR reflectance
relative to the UV, might be extinct cometary nuclei.
6. In Situ Studies
6.1NEAR
The Near-Earth Asteroid Rendezvous spacecraft was
launched from Cape Canaveral, Florida, on February 16,
1996, on a 3 year journey to asteroid 433 Eros.NEARor-
bited Eros for 1 year in 2000–2001, training its 6 scientific
instruments on the asteroid’s surface. It provided the first
detailed characterization of a NEO’s chemical and physical
properties. The objective was to study Eros’ relationship to
meteorites, the nature of its surface and collisional history
as well as aspects of its interior state and structure.
The spacecraft carried a complement of instruments cov-
ering the electromagnetic spectrum. The magnetometer
measured no magnetic field down to its detection limit of
1–2 nano-Teslas (Earth’s magnetic field measures 50,000
nano-Teslas). A possible explanation for this unexpected
result is that magnetic material within Eros is randomly
oriented to the point of canceling all fields. If this is the
case, then there has been no heating of the asteroid to the
point of producing any preferred orientation of any mag-
netic material.
Orbital imaging of Eros revealed an irregularly shaped
body dominated at the global scale by both convex and con-
cave forms, including a 10 km diameter depression named
Himeros, and a 5.3 km bowl-shaped crater named Psyche
(Fig. 12). At scales of 1 km to 100 m, (Fig. 14) there
are grooves and ridge patterns superimposed on a heavily
cratered surface, mostly covered by overlapping craters. At
the<100 m scale, the surface is dominated by boulders, evi-
dence of down-slope movement, and ponding of material in
crater bottoms, all of which indicate regolith accumulation
and transport. There are not many craters<100 m diame-
ter. Evidence of structural strength on the asteroid includes
chains of craters, sinuous and linear depressions, ridges and
scarps, and rectilinear craters. The multiple orientations of
these features indicate that they were formed in multiple
events. The prominent ridge system, named Rahe Dorsum,
spans the northern hemisphere and defines a plane through
the asteroid. It cuts across Himeros and possibly Psyche, in-
dicating that it predates the formation of these large craters.
Segments form cliffs with slopes above the angle of repose
indicating an interior structure with considerable cohesive
strength.
Eros is not a gravitationally bound rubble pile. Rather,
it is a fragment of a once larger body, possessing cohesive
strength throughout. Both gravitational forces and mechan-
ical strength play a role in the formation and evolution of
Eros. Its density of 2670±30 kg/m^3 is low compared to
ordinary chondrite meteorites of 3400 kg/m^3 that have ap-
proximately the same composition as Eros. By considering
the porosity of each meteorite and its relation to a much
larger asteroid, the macroporosity of Eros is determined
to be 20% and most likely due to collisional fragmentation
throughout Eros’ interior.
The X-ray spectrometer (XRS) onboardNEARprovided
relative abundance ratios of six elements: Mg, Al, Si, S,
Ca, and Fe, from< 100 μm depth. For all these elements,
except sulfur, the ratios are within the range of ordinary
chondrite and some R-type (partially melted) meteorites.
[SeeMeteorites.] The sulfur depletion is most likely due
to a surface phenomenon, micrometeorite-induced, impact