292 Encyclopedia of the Solar System
the inventory, but there are inherent limitations in search
techniques. Consider setting out to count the number of
near-Earth objects. First, one can only look for them at
night. At any one time, one can only search half the sky.
Then there are limitations in how much sky one can cover
in one night, controlled by the telescope field of view and
the recording instrumentation. The realities of weather and
equipment performance further hinder the search. The
combination of these factors represents an estimate of what
fraction of an expected population has been found for a
range of size and brightness.
To date, search programs have found more than 4100
near-Earth objects of all sizes. The biggest objects appear
brightest and are most easily found. Searchers know of 30
NEOs as large as 5 km across and believe all objects this
size and larger have been found. Around 300 objects have
been cataloged that are larger than 2 km, and NEO catalogs
are nearly complete at this size. Catalogs are known to be
incomplete for objects smaller than 2 km, but by knowing
how much area of the sky has been searched and how sen-
sitive these searches have been, it is possible to estimate
how many objects are left to find. A recent Ph.D. thesis by
J. Scott Stuart carefully analyzed the search statistics from
the LINEAR program, taking into account the different
colors and reflectivities (albedos) that are typical for NEOs.
Based on Stuart’s work, the best estimate is that there are
about 1100 total NEOs larger than 1 km in diameter and
up to 85,000 NEOs larger than 100 m (Fig. 10).
When considering impact hazards on Earth, most scien-
tists consider 1 km as the size large enough for an impact
to present a global threat to human survival. Thus, current
search efforts have as their most immediate goal to find
all objects larger than 1 km. The good news is that more
FIGURE 10 Estimated number of NEOs as a function of
diameter.
than 870 of all cataloged NEOs are estimated to be 1 km
or larger and thus astronomers are 80% toward complet-
ing the most immediate goal, and that may be reached in
just a few more years. In the process, many smaller ob-
jects are found, and these begin to help bring completeness
to all sizes. Searchers have a long way to go to complete
the survey of all 85,000 objects that may be larger than
100 m; these may be capable of Tunguska-like (or somewhat
greater) amounts of damage. Completing the surveys down
to these sizes will require new, large, specialized telescopes
with huge CCD arrays to scan the skies more frequently
and with greater sensitivity. Another possibility would be to
conduct the search using small telescopes in space.5. Physical Properties
The first physical measurement after the position of a near-
Earth object is established is its brightness measured on the
astronomicalmagnitudescale. The changing cross section
of an object as viewed from Earth affects its brightness
and with time reflects the shape and rotation rate of the ob-
ject. Analysis of this changing brightness, accounting for the
observational geometry, results in constraints on its shape
and the determination of its rotation rate and orientation
in space. From analyses of reflected sunlight off main-belt
asteroid surfaces at different wavelengths, NEO colors are
classified into different taxonomic types. [SeeMain-Belt
Asteroids.] Further analysis can determine surface min-
eralogy, and, from that, constraints on the temperatures at
which these objects formed can be made.
TheNear-Earth Asteroid Rendezvousmission studied
the physical and chemical properties of asteroid 433 Eros
from orbit and at the spacecraft’s landing site. From its
shape and surface morphology, astronomers deduced infor-
mation about its global structure. An X-ray and gamma ray
spectrometer provided information about its surface chem-
istry. See Section 6 for details.5.1 Brightness
The standard asteroid photometric magnitude system com-
pensates for the distance andphase angleat which the
object is observed. The magnitude scales by the inverse
square law. As the distance from both the Sun and the ob-
server increases, the brightness decreases by a factor equal
to the inverse square of those distances. Scattering prop-
erties of the surface are expressed in the phase function,
which is compensated for by extrapolating the magnitude to
0 ◦phase. For comparison purposes, a magnitude measure-
ment is converted to an absolute scale,H, which is defined
as the brightness of an object at a distance of 1.0 AU from
both the Earth and Sun, and viewed at 0◦phase angle. The
measured slope of brightness changes with phase,G, has