298 Encyclopedia of the Solar System
FIGURE 15 Gamma ray spectra of
the surface of 433 Eros. (Credit:
NASA/JHU/APL.)spectrometer, a laser ranging instrument, and an X-ray flu-
orescence spectrometer. The asteroid has major axes of
x=535,y=294,z= 209 ±1 m, which is the same as
0.33×0.18×0.13 miles. The science team considered
this object to be shaped like a sea otter with a head and a
body (Fig. 16). Eighty percent of the surface is considered
to be rough terrain; the smooth areas are called seas. The
surface shows many boulders ranging in size from a few to
50 m, the largest, named Yoshinodai, is 1/10th the size of the
whole asteroid. The boulders are likely relicts from large im-
pact events that produced Itokawa’s current shape. Instead
of counting craters on Itokawa, scientists count boulders
to understand its impact history. Itokawa has experienced
less processing in its rough terrain–including breaking, sort-
ing and transporting of material in its lifetime–than other
FIGURE 16 Global image of the western hemisphere of 25143
Itokawa imaged by theHayabusaimaging camera during science
mode imaging. (Courtesy: JAXA.)
small bodies such as Eros and Phobos, the Martian moon.
Smooth terrain, covering 20% of the body, has few if any
boulders and is featureless. At closest range, the spacecraft
resolved centimeter- to millimeter-sized grains, the size of
pebbles, in the smooth areas. Missing from the asteroid is
a range of crater sizes, though their remnants are barely
visible, almost erased by debris from both impacts and
global shaking, which filled in craters with fine material over
time.
With a mass of 3.510× 1010 ±0.105 kg and a volume
of 1.840× 107 ±0.092 m^3 , Itokawa’s density is 1900± 130
kg/m^3. This is lower than other S-type asteroids. Using the
compositional knowledge from the near-infrared spectra
and the X-ray spectrometer, indicating that the bulk compo-
sition is close to LL ordinary chondrites [seeMeteorites],
and the measured bulk density of those meteorites, the
macroporosity of Itokawa is estimated to be 41%, twice
as high as Eros and, in fact, soils on Earth. The absence of
linear features extending the length of the body, the pres-
ence of local facets (flat areas) 10s of meters long and the
high porosity indicate that Itokawa is a rubble pile with a
composition closest to that of LL ordinary chondrite but
also consistent with some primitive achondrites.
If all goes well with the spacecraft’s return to Earth in
2010, scientists will eagerly open the return capsule and
hopefully find precious samples of the asteroid’s surface.7. Impact Hazards
It does not take much imagination to envision an asteroid
or comet hurtling through space that just happens to be on