Near-Earth Objects 295
andMpandmsare the mass of the primary and secondary
and are solved for as a sum. Using this expression, the mass
of the binary near-Earth object 2000 DP107, for example,
is calculated to be 4.6±0.5× 1011 kg, a little more than
1/1000ththe mass of all living matter on Earth, estimated
at 3.6× 1014 kg. At least 25 other NEOs are known to be
binaries. From Doppler and range measurements of the
NEAR–Shoemakerspacecraft, the mass of Eros was mea-
sured to be 6.687±0.003× 1015 kg. For comparison, the
Moon’s mass is more than 10 million times greater at 7.348×
1022 kg. While the range of measured NEO masses spans
four orders of magnitude, their total mass is small compared
to the solar system’s total planetary mass.
5.6 Color and Taxonomy
Since the early part of the 20th century, astronomers have
recognized that small bodies come in different colors. As
observational techniques evolved and the ability to investi-
gate them improved, the number of observable character-
istics increased. Sorting objects into meaningful groups is
the process of classification or taxonomy. Asteroid taxonomy
developed in response to advances in observing techniques
and new technology in the field of stellar photometric as-
tronomy. Current taxonomy is based on the application of
statistical clustering techniques to the parameters of color
and albedo. The intention of the classification scheme is
to reflect the compositional variations and thus their origin
and evolution. Astronomers are constantly attempting to
test and refine the asteroid taxonomy by employing new sta-
tistical methods and extending the number of meaningful
parameters that are included in the classification process,
while eliminating meaningless or redundant parameters.
Today, the alphabet soup of asteroid taxonomy extends to
about 12 letters with subtypes numbering up to 26. The tax-
onomy too has evolved, and one has to be aware of which sys-
tem is being used and what the exact definitions are. Bobby
Bus presented a taxonomy in 1999 that has 26 classes. [See
Asteroids.]
Near-Earth objects have representatives from all taxo-
nomic types except one, indicating that many locations in
the Asteroid Belt feed the near-Earth population. Ninety
percent of NEOs fall in the S-, Q-, C-, and X-complexes
(a complex is a grouping of taxa from different instrument
types and different taxonomies combined into a general cat-
egory that can encompass all available observations). Two
thirds of NEOs are bright and members of the S- (40%) or
Q- (25%) complexes. When considering the observed ra-
tio of dark objects to bright, there are almost four times as
many bright objects observed compared to dark ones in the
NEO population. However, darker objects are more diffi-
cult to discover and measure. Accounting for this discovery
bias against darker objects is especially important when es-
timating how many extinct comets may be present in the
near-Earth object population (Section 3.2).
5.7 Mineralogy
By measuring the percentage of reflected sunlight from the
surface of an object, it is possible to constrain its surface
mineralogy. This technique was pioneered by Tom McCord
and his students and colleagues in the 1970s. In 2006, spec-
tral reflectance measurements of over 200 NEOs were avail-
able. The inventory is still growing.
Astronomers find that 65% of near-Earth objects contain
two strong absorption bands, one in the ultraviolet with a
band centered below 0.35μm and the other in the near in-
frared near 1μm. Sometimes a second near-infrared band
is observed at a wavelength of 2μm. Other objects do not
have prominent absorption bands: They are found to be fea-
tureless and either flat or sloped. Most often these feature-
less objects also have a low albedo. Figure 13 shows spec-
tral reflectance measurements of some near-Earth objects.
Three spectra have prominent ultraviolet and near-infraredFIGURE 13 Spectral reflectance measurements of four NEOs.
The range of spectra reflects the range of surface characteristics
including mineralogy and particle sizes of the surface material.