730 Encyclopedia of the Solar System
FIGURE 12 Images of Uranus with and without adaptive optics. This is a striking demonstration
of the effectiveness of adaptive optics in removing atmospheric turbulence. One can also see that
the signal-to-noise is greatly enhanced because light is concentrated into a diffraction-limited
image with adaptive optics, thus greatly increasing the ability to detect faint spots and cloud
structure. At a wavelength of 1.6 micrometers, we are seeing reflected light from low-altitude
clouds while at 2.2 micrometers the high-altitude clouds are revealed. The planet is much darker
at 2.2 micrometers due to absorption of methane gas in the atmosphere. This allows a much
longer exposure and for the rings to be seen clearly. The point-like cloud features at 2.2
micrometers show that in certain places turbulence is very strong and is pushing material from
lower altitudes into the stratosphere. (Courtesy of H. B. Hammel, I. de Pater, and the W. M.
Keck Observatory.)A number of programs are underway in the US and
other countries that meet or exceed the requirements set by
Congress. Table 2 shows a partial list of sky survey programs
that are currently in progress or planned. Current produc-
tivity of various programs is shown in Figure 16, which
shows all NEOs discovered irrespective of size. While the
NASA directive is aimed at identifying NEOs larger than 1
km diameter, many NEOs smaller than 1 km are also dis-
covered due to the sensitivity of the search programs and
because small objects that come very close to Earth may
be bright enough to be detected. A recent NEO, 2005 WX,
approached to within 1.3 million km of the Earth and had
an estimated diameter of only 10 m!
The number of known NEOS has been increasing due to
the larger number of funded survey programs and advances
in detector arrays that have allowed much larger areas of sky
to be covered in a single exposure. The number of NEOs
discovered as a function of time is shown in Figure 16.
Note that while the total number of asteroids discovered is
still increasing at a rapid rate, the number of new asteroids
larger than 1 km discovered each year is decreasing. This is
a result of the fact that the remaining unknown NEAs are
intrinsically more difficult to detect. Their size and orbit
distribution is different from the known population due to
observational selection effects in the population of known
objects. It is likely that existing survey programs (see Ta-
ble 2) will just miss the goal of discovering at least 90% of
all near-Earth asteroids larger than 1 km by 2008 as man-
dated by Congress. However, when the next generation sur-
veys (see Table 2) come online within the next decade they
will quickly complete the inventory of NEAs larger than
1 km.